A Universally Unique Identifier ( UUID ) is a 128-bit label used to uniquely identify objects in computer systems. The term Globally Unique Identifier ( GUID ) is also used, mostly in Microsoft systems.
115-411: When generated according to the standard methods, UUIDs are, for practical purposes, unique. Their uniqueness does not depend on a central registration authority or coordination between the parties generating them, unlike most other numbering schemes. While the probability that a UUID will be duplicated is not zero, it is generally considered close enough to zero to be negligible. Thus, anyone can create
230-504: A witness in a legal case in Europe, and often correlated with the witness's nobility . In a sense, this differs much from the modern meaning of probability , which in contrast is a measure of the weight of empirical evidence , and is arrived at from inductive reasoning and statistical inference . The scientific study of probability is a modern development of mathematics. Gambling shows that there has been an interest in quantifying
345-425: A σ-algebra of such events (such as those arising from a continuous random variable ). For example, in a bag of 2 red balls and 2 blue balls (4 balls in total), the probability of taking a red ball is 1 / 2 ; {\displaystyle 1/2;} however, when taking a second ball, the probability of it being either a red ball or a blue ball depends on the ball previously taken. For example, if
460-623: A 128 bit integer. For example, the UUID 550e8400-e29b-41d4-a716-446655440000 can also be represented as 113059749145936325402354257176981405696. Note that it is possible to have both signed and unsigned values if the first bit of the UUID is set to 1. A UUID can be represented as a 128 bit binary number . For example, the UUID 550e8400-e29b-41d4-a716-446655440000 can also be represented as 01010101000011101000010000000000111000101001101101000001110101001010011100010110010001000110011001010101010001000000000000000000. RFC 9562 registers
575-450: A 40-bit domain/identifier in the UUID comes with a tradeoff. On the one hand, 40 bits allow about 1 trillion domain/identifier values per node ID. On the other hand, with the clock value truncated to the 28 most significant bits, compared to 60 bits in version 1, the clock in a version 2 UUID will "tick" only once every 429.49 seconds, a little more than 7 minutes, as opposed to every 100 nanoseconds for version 1. And with
690-519: A UUID and use it to identify something with near certainty that the identifier does not duplicate one that has already been, or will be, created to identify something else. Information labeled with UUIDs by independent parties can therefore be later combined into a single database or transmitted on the same channel, with a negligible probability of duplication. Adoption of UUIDs is widespread, with many computing platforms providing support for generating them and for parsing their textual representation. In
805-421: A card from a deck of cards, the chance of getting a heart or a face card (J, Q, K) (or both) is 13 52 + 12 52 − 3 52 = 11 26 , {\displaystyle {\tfrac {13}{52}}+{\tfrac {12}{52}}-{\tfrac {3}{52}}={\tfrac {11}{26}},} since among the 52 cards of a deck, 13 are hearts, 12 are face cards, and 3 are both: here
920-413: A clock sequence of only 6 bits, compared to 14 bits in version 1, only 64 unique UUIDs per node/domain/identifier can be generated per 7-minute clock tick, compared to 16,384 clock sequence values for version 1. Version-3 and version-5 UUIDs are generated by hashing a namespace identifier and name. Version 3 uses MD5 as the hashing algorithm, and version 5 uses SHA-1 . The namespace identifier
1035-478: A manufacturer's decisions on a product's warranty . The cache language model and other statistical language models that are used in natural language processing are also examples of applications of probability theory. Consider an experiment that can produce a number of results. The collection of all possible results is called the sample space of the experiment, sometimes denoted as Ω {\displaystyle \Omega } . The power set of
1150-492: A memoir prepared by Thomas Simpson in 1755 (printed 1756) first applied the theory to the discussion of errors of observation. The reprint (1757) of this memoir lays down the axioms that positive and negative errors are equally probable, and that certain assignable limits define the range of all errors. Simpson also discusses continuous errors and describes a probability curve. The first two laws of error that were proposed both originated with Pierre-Simon Laplace . The first law
1265-784: A particular wireless MAC address. Randomized MAC addresses can be identified by the "locally administered" bit described above. Using wireless access points in SSID -hidden mode ( network cloaking ), a mobile wireless device may not only disclose its own MAC address when traveling, but even the MAC addresses associated to SSIDs the device has already connected to, if they are configured to send these as part of probe request packets. Alternative modes to prevent this include configuring access points to be either in beacon-broadcasting mode or probe-response with SSID mode. In these modes, probe requests may be unnecessary or sent in broadcast mode without disclosing
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#17327874822131380-511: A probability so small that it can normally be ignored. This probability can be computed precisely based on analysis of the birthday problem . For example, the number of random version-4 UUIDs which need to be generated in order to have a 50% probability of at least one collision is 2.71 quintillion, computed as follows: Probability Probability is the branch of mathematics concerning events and numerical descriptions of how likely they are to occur. The probability of an event
1495-491: A purely theoretical setting (like tossing a coin), probabilities can be numerically described by the number of desired outcomes, divided by the total number of all outcomes. This is referred to as theoretical probability (in contrast to empirical probability , dealing with probabilities in the context of real experiments). For example, tossing a coin twice will yield "head-head", "head-tail", "tail-head", and "tail-tail" outcomes. The probability of getting an outcome of "head-head"
1610-426: A red ball was taken, then the probability of picking a red ball again would be 1 / 3 , {\displaystyle 1/3,} since only 1 red and 2 blue balls would have been remaining. And if a blue ball was taken previously, the probability of taking a red ball will be 2 / 3. {\displaystyle 2/3.} In probability theory and applications, Bayes' rule relates
1725-642: A string. For example, 00112233-4455-6677-8899-aabbccddeeff is encoded as the bytes 33 22 11 00 55 44 77 66 88 99 aa bb cc dd ee ff . In most cases, UUIDs are represented as hexadecimal values. The most used format is the 8-4-4-4-12 format, xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx , where every x represents 4 bits. Other well-known formats are the 8-4-4-4-12 format with braces, {xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx} , like in Microsoft's systems, e.g. Windows, or xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx , where all hyphens are removed. In some cases, it
1840-405: A target lifetime of 100 years (until 2080) for applications using EUI-48 space and restricts applications accordingly. The IEEE encourages adoption of the more plentiful EUI-64 for non-Ethernet applications. The distinctions between EUI-48 and MAC-48 identifiers are in name and application only. MAC-48 was used to address hardware interfaces within existing 802-based networking applications; EUI-48
1955-455: Is 0 (zero), the frame is meant to reach only one receiving network interface . This type of transmission is called unicast . A unicast frame is transmitted to all nodes within the collision domain . In a modern wired setting (i.e. with switches , not simple hubs ) the collision domain usually is the length of the Ethernet cabling between two network interfaces. In a wireless setting,
2070-455: Is 1 out of 4 outcomes, or, in numerical terms, 1/4, 0.25 or 25%. However, when it comes to practical application, there are two major competing categories of probability interpretations, whose adherents hold different views about the fundamental nature of probability: The word probability derives from the Latin probabilitas , which can also mean " probity ", a measure of the authority of
2185-483: Is a unique identifier assigned to a network interface controller (NIC) for use as a network address in communications within a network segment . This use is common in most IEEE 802 networking technologies, including Ethernet , Wi-Fi , and Bluetooth . Within the Open Systems Interconnection (OSI) network model , MAC addresses are used in the medium access control protocol sublayer of
2300-704: Is a constant depending on precision of observation, and c {\displaystyle c} is a scale factor ensuring that the area under the curve equals 1. He gave two proofs, the second being essentially the same as John Herschel 's (1850). Gauss gave the first proof that seems to have been known in Europe (the third after Adrain's) in 1809. Further proofs were given by Laplace (1810, 1812), Gauss (1823), James Ivory (1825, 1826), Hagen (1837), Friedrich Bessel (1838), W.F. Donkin (1844, 1856), and Morgan Crofton (1870). Other contributors were Ellis (1844), De Morgan (1864), Glaisher (1872), and Giovanni Schiaparelli (1875). Peters 's (1856) formula for r ,
2415-555: Is a number between 0 and 1; the larger the probability, the more likely an event is to occur. A simple example is the tossing of a fair (unbiased) coin. Since the coin is fair, the two outcomes ("heads" and "tails") are both equally probable; the probability of "heads" equals the probability of "tails"; and since no other outcomes are possible, the probability of either "heads" or "tails" is 1/2 (which could also be written as 0.5 or 50%). These concepts have been given an axiomatic mathematical formalization in probability theory , which
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#17327874822132530-447: Is also possible to have xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx with the "0x" prefix or the "h" suffix to indicate hexadecimal values. The format with hyphens was introduced with the newer variant system. Before that, the legacy Apollo format used a slightly different format: 34dc23469000.0d.00.00.7c.5f.00.00.00 . The first part is the time (time_high and time_low combined). The reserved field is skipped. The family field comes directly after
2645-698: Is an inactive registry which has been replaced by the MA-S ( MAC address block, small ), previously named OUI-36 , and has no overlaps in addresses with the IAB registry product as of January 1, 2014. The IAB uses an OUI from the MA-L ( MAC address block, large ) registry, previously called the OUI registry. The term OUI is still in use, but the IEEE Registration Authority does not administer them. An OUI
2760-722: Is called multicast addressing. The IEEE has built in several special address types to allow more than one network interface card to be addressed at one time: These are all examples of group addresses , as opposed to individual addresses ; the least significant bit of the first octet of a MAC address distinguishes individual addresses from group addresses. That bit is set to 0 in individual addresses and set to 1 in group addresses. Group addresses, like individual addresses, can be universally administered or locally administered. The U/L and I/G bits are handled independently, and there are instances of all four possibilities. IPv6 multicast uses locally administered, multicast MAC addresses in
2875-408: Is concatenated with 12 additional IEEE-provided bits (for a total of 36 bits), leaving only 12 bits for the organisation owning the IAB to assign to its (up to 4096) individual devices. An IAB is ideal for organizations requiring not more than 4096 unique 48-bit numbers (EUI-48). Unlike an OUI, which allows the assignee to assign values in various different number spaces (for example, EUI-48, EUI-64, and
2990-544: Is denoted as P ( A ∪ B ) {\displaystyle P(A\cup B)} and P ( A or B ) = P ( A ∪ B ) = P ( A ) + P ( B ) − P ( A ∩ B ) = P ( A ) + P ( B ) − 0 = P ( A ) + P ( B ) {\displaystyle P(A{\mbox{ or }}B)=P(A\cup B)=P(A)+P(B)-P(A\cap B)=P(A)+P(B)-0=P(A)+P(B)} For example,
3105-453: Is encoded as the bytes 00 11 22 33 44 55 66 77 88 99 aa bb cc dd ee ff . An exception to this are Microsoft's variant 2 UUIDs ("GUID"): historically used in COM/OLE libraries , they use a little-endian format, but appear mixed-endian with the first three components of the UUID as little-endian and last two big-endian , due to the missing byte dashes when formatted as
3220-446: Is formally undefined by this expression. In this case A {\displaystyle A} and B {\displaystyle B} are independent, since P ( A ∩ B ) = P ( A ) P ( B ) = 0. {\displaystyle P(A\cap B)=P(A)P(B)=0.} However, it is possible to define a conditional probability for some zero-probability events, for example by using
3335-579: Is given by P (not A ) = 1 − P ( A ) . As an example, the chance of not rolling a six on a six-sided die is 1 – (chance of rolling a six) = 1 − 1 / 6 = 5 / 6 . For a more comprehensive treatment, see Complementary event . If two events A and B occur on a single performance of an experiment, this is called the intersection or joint probability of A and B , denoted as P ( A ∩ B ) . {\displaystyle P(A\cap B).} If two events, A and B are independent then
3450-399: Is itself a UUID. The specification provides UUIDs to represent the namespaces for URLs , fully qualified domain names , object identifiers , and X.500 distinguished names ; but any desired UUID may be used as a namespace designator. To determine the version-3 UUID corresponding to a given namespace and name, the UUID of the namespace is transformed to a string of bytes, concatenated with
3565-457: Is not possible with version 1. RFC 9562 reserves version 2 for "DCE security" UUIDs; but it does not provide any details. For this reason, many UUID implementations omit version 2. However, the specification of version-2 UUIDs is provided by the DCE 1.1 Authentication and Security Services specification. Version-2 UUIDs are similar to version 1, except that the least significant 8 bits of
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3680-489: Is now used for 802-based networking and is also used to identify other devices and software, for example Bluetooth . The IEEE now considers MAC-48 to be an obsolete term. EUI-48 is now used in all cases. In addition, the EUI-64 numbering system originally encompassed both MAC-48 and EUI-48 identifiers by a simple translation mechanism. These translations have since been deprecated. The Individual Address Block (IAB)
3795-507: Is one less random bit available, 3 bits being consumed for the variant. Per RFC 9562 , the seventh octet's most significant 4 bits indicate which version the UUID adheres to. This means that the first hexadecimal digit in the third group always starts with a 4 in UUIDv4s. Visually, this looks like this xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx , where M is the UUID version field. The upper two or three bits of digit N encode
3910-561: Is optional. The following network technologies use the EUI-48 identifier format: Every device that connects to an IEEE 802 network (such as Ethernet and Wi-Fi) has an EUI-48 address. Common networked consumer devices such as PCs, smartphones and tablet computers use EUI-48 addresses. EUI-64 identifiers are used in: On broadcast networks, such as Ethernet, the MAC address is expected to uniquely identify each node on that segment and allows frames to be marked for specific hosts. It thus forms
4025-526: Is randomly generated. As in other UUIDs, 4 bits are used to indicate version 4, and 2 or 3 bits to indicate the variant (10 2 or 110 2 for variants 1 and 2 respectively). Thus, for variant 1 (that is, most UUIDs) a random version 4 UUID will have 6 predetermined variant and version bits, leaving 122 bits for the randomly generated part, for a total of 2, or 5.3 × 10 (5.3 undecillion ) possible version-4 variant-1 UUIDs. There are half as many possible version 4, variant 2 UUIDs (legacy GUIDs) because there
4140-420: Is simply the ratio of the probabilities of the two events. When arbitrarily many events A {\displaystyle A} are of interest, not just two, the rule can be rephrased as posterior is proportional to prior times likelihood , P ( A | B ) ∝ P ( A ) P ( B | A ) {\displaystyle P(A|B)\propto P(A)P(B|A)} where
4255-524: Is started with a MAC address set by assigning the last three bytes to be unique on the local network. While this is local administration of MAC addresses, it is not an LAA in the IEEE sense. A historical example of this hybrid situation is the DECnet protocol, where the universal MAC address (OUI AA-00-04, Digital Equipment Corporation) is administered locally. The DECnet software assigns the last three bytes for
4370-508: Is the UUID FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF ; that is, all bits set to one. Initially, Apollo Computer designed the UUID with the following wire format: Later, the UUID was extended by combining the legacy family field with the new variant field. Because the family field only had used the values ranging from 0 to 13 in the past, it was decided that a UUID with the most significant bit set to 0
4485-640: Is the probability of some event A , given the occurrence of some other event B . Conditional probability is written P ( A ∣ B ) {\displaystyle P(A\mid B)} , and is read "the probability of A , given B ". It is defined by P ( A ∣ B ) = P ( A ∩ B ) P ( B ) {\displaystyle P(A\mid B)={\frac {P(A\cap B)}{P(B)}}\,} If P ( B ) = 0 {\displaystyle P(B)=0} then P ( A ∣ B ) {\displaystyle P(A\mid B)}
4600-560: Is used instead of MD5. Since SHA-1 generates 160-bit digests, the digest is truncated to 128 bits before the version and variant bits are replaced. Version-3 and version-5 UUIDs have the property that the same namespace and name will map to the same UUID. However, neither the namespace nor name can be determined from the UUID, even if one of them is specified, except by brute-force search. RFC 4122 recommends version 5 (SHA-1) over version 3 (MD5), and warns against use of UUIDs of either version as security credentials. A version 4 UUID
4715-443: Is used to design games of chance so that casinos can make a guaranteed profit, yet provide payouts to players that are frequent enough to encourage continued play. Another significant application of probability theory in everyday life is reliability . Many consumer products, such as automobiles and consumer electronics, use reliability theory in product design to reduce the probability of failure. Failure probability may influence
Universally unique identifier - Misplaced Pages Continue
4830-491: Is used widely in areas of study such as statistics , mathematics , science , finance , gambling , artificial intelligence , machine learning , computer science , game theory , and philosophy to, for example, draw inferences about the expected frequency of events. Probability theory is also used to describe the underlying mechanics and regularities of complex systems . When dealing with random experiments – i.e., experiments that are random and well-defined – in
4945-442: Is written in transmission order with the least significant bit of each byte transmitted first, and is used in the output of the ifconfig , ip address , and ipconfig commands, for example. However, since IEEE 802.3 (Ethernet) and IEEE 802.4 (Token Bus) send the bytes (octets) over the wire, left-to-right, with the least significant bit in each byte first, while IEEE 802.5 (Token Ring) and IEEE 802.6 (FDDI) send
5060-550: The Copenhagen interpretation , it deals with probabilities of observing, the outcome being explained by a wave function collapse when an observation is made. However, the loss of determinism for the sake of instrumentalism did not meet with universal approval. Albert Einstein famously remarked in a letter to Max Born : "I am convinced that God does not play dice". Like Einstein, Erwin Schrödinger , who discovered
5175-507: The Kolmogorov formulation and the Cox formulation. In Kolmogorov's formulation (see also probability space ), sets are interpreted as events and probability as a measure on a class of sets. In Cox's theorem , probability is taken as a primitive (i.e., not further analyzed), and the emphasis is on constructing a consistent assignment of probability values to propositions. In both cases,
5290-640: The Microsoft Windows platforms adopted the DCE design as "Globally Unique IDentifiers" (GUIDs). RFC 4122 registered a URN namespace for UUIDs and recapitulated the earlier specifications, with the same technical content. When in July 2005 RFC 4122 was published as a proposed IETF standard, the ITU had also standardized UUIDs, based on the previous standards and early versions of RFC 4122 . On May 7, 2024, RFC 9562
5405-415: The data link layer . As typically represented, MAC addresses are recognizable as six groups of two hexadecimal digits, separated by hyphens, colons, or without a separator. MAC addresses are primarily assigned by device manufacturers, and are therefore often referred to as the burned-in address , or as an Ethernet hardware address , hardware address , or physical address . Each address can be stored in
5520-680: The laws of probability are the same, except for technical details. There are other methods for quantifying uncertainty, such as the Dempster–Shafer theory or possibility theory , but those are essentially different and not compatible with the usually-understood laws of probability. Probability theory is applied in everyday life in risk assessment and modeling . The insurance industry and markets use actuarial science to determine pricing and make trading decisions. Governments apply probabilistic methods in environmental regulation , entitlement analysis, and financial regulation . An example of
5635-443: The multicast bit in MAC addresses, and setting it serves to differentiate UUIDs where the node ID is randomly generated from UUIDs based on MAC addresses from network cards, which typically have unicast MAC addresses. Version 6 is the same as version 1 except all timestamp bits are ordered from most significant to least significant. This allows systems to sort UUIDs in order of creation simply by sorting them lexically, whereas this
5750-426: The odds of event A 1 {\displaystyle A_{1}} to event A 2 , {\displaystyle A_{2},} before (prior to) and after (posterior to) conditioning on another event B . {\displaystyle B.} The odds on A 1 {\displaystyle A_{1}} to event A 2 {\displaystyle A_{2}}
5865-431: The probable error of a single observation, is well known. In the nineteenth century, authors on the general theory included Laplace , Sylvestre Lacroix (1816), Littrow (1833), Adolphe Quetelet (1853), Richard Dedekind (1860), Helmert (1872), Hermann Laurent (1873), Liagre, Didion and Karl Pearson . Augustus De Morgan and George Boole improved the exposition of the theory. In 1906, Andrey Markov introduced
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#17327874822135980-402: The theory of probability is a representation of its concepts in formal terms – that is, in terms that can be considered separately from their meaning. These formal terms are manipulated by the rules of mathematics and logic, and any results are interpreted or translated back into the problem domain. There have been at least two successful attempts to formalize probability, namely
6095-420: The "uuid" namespace. This makes it possible to make URNs out of UUIDs, like urn:uuid:550e8400-e29b-41d4-a716-446655440000 . The normal 8-4-4-4-12 format is used for this. It is also possible to make a OID URN out of UUIDs, like urn:oid:2.25.113059749145936325402354257176981405696 . In that case, the unsigned decimal format is used. The "uuid" URN is recommended over the "oid" URN. Collision occurs when
6210-758: The 1980s, Apollo Computer originally used UUIDs in the Network Computing System (NCS). Later, the Open Software Foundation (OSF) used UUIDs for their Distributed Computing Environment (DCE). The design of the DCE UUIDs was partly based on the NCS UUIDs, whose design was in turn inspired by the ( 64-bit ) unique identifiers defined and used pervasively in Domain/OS , an operating system designed by Apollo Computer. Later,
6325-564: The Gauss law. "It is difficult historically to attribute that law to Gauss, who in spite of his well-known precocity had probably not made this discovery before he was two years old." Daniel Bernoulli (1778) introduced the principle of the maximum product of the probabilities of a system of concurrent errors. Adrien-Marie Legendre (1805) developed the method of least squares , and introduced it in his Nouvelles méthodes pour la détermination des orbites des comètes ( New Methods for Determining
6440-399: The MAC address randomization technique vary largely in different devices. Moreover, various flaws and shortcomings in these implementations may allow an attacker to track a device even if its MAC address is changed, for instance its probe requests' other elements, or their timing. If random MAC addresses are not used, researchers have confirmed that it is possible to link a real identity to
6555-457: The MAC address, which is issued by the IEEE to manufacturers of networking equipment. The uniqueness of version-1 and version-2 UUIDs based on network-card MAC addresses also depends on network-card manufacturers properly assigning unique MAC addresses to their cards, which like other manufacturing processes is subject to error. Virtual machines receive a MAC address in a range that is configurable in
6670-450: The Orbits of Comets ). In ignorance of Legendre's contribution, an Irish-American writer, Robert Adrain , editor of "The Analyst" (1808), first deduced the law of facility of error, ϕ ( x ) = c e − h 2 x 2 {\displaystyle \phi (x)=ce^{-h^{2}x^{2}}} where h {\displaystyle h}
6785-528: The Standards-Track RFC 9562 from the "Revise Universally Unique Identifier Definitions Working Group" as revision for RFC 4122 . RFC 4122 is technically equivalent to ITU-T Rec. X.667 | ISO/IEC 9834-8, but is now obsolete. A UUID is 128 bits in size, in which 2 to 4 bits are used to indicate the format's variant. The most common variant in use today, OSF DCE, additionally defines 4 bits for its version. The use of
6900-574: The US National Security Agency has a system that tracks the movements of mobile devices in a city by monitoring MAC addresses. To avert this practice, Apple has started using random MAC addresses in iOS devices while scanning for networks. Other vendors followed quickly. MAC address randomization during scanning was added in Android starting from version 6.0, Windows 10, and Linux kernel 3.18. The actual implementations of
7015-529: The UUID), with a 60-bit timestamp, being the number of 100- nanosecond intervals since midnight 15 October 1582 Coordinated Universal Time (UTC), the date on which the Gregorian calendar was first adopted by the bulk of Europe. RFC 4122 states that the time value rolls over around 3400 AD, depending on the algorithm used, which implies that the 60-bit timestamp is a signed quantity. However some software, such as
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#17327874822137130-602: The assignment of a MAC address to network interface when scanning for wireless access points to avert tracking systems. In Internet Protocol (IP) networks, the MAC address of an interface corresponding to an IP address may be queried with the Address Resolution Protocol (ARP) for IPv4 and the Neighbor Discovery Protocol (NDP) for IPv6, relating OSI layer 3 addresses with layer 2 addresses. According to Edward Snowden ,
7245-482: The attached addresses to activate. Hence, various configuration scripts and utilities permit the randomization of the MAC address at the time of booting or before establishing a network connection. Changing MAC addresses is necessary in network virtualization . In MAC spoofing , this is practiced in exploiting security vulnerabilities of a computer system. Some modern operating systems, such as Apple iOS and Android, especially in mobile devices, are designed to randomize
7360-399: The ball, variations in hand speed during the turning, and so forth. A probabilistic description can thus be more useful than Newtonian mechanics for analyzing the pattern of outcomes of repeated rolls of a roulette wheel. Physicists face the same situation in the kinetic theory of gases , where the system, while deterministic in principle , is so complex (with the number of molecules typically
7475-408: The basis of most of the link layer (OSI layer 2 ) networking upon which upper-layer protocols rely to produce complex, functioning networks. Many network interfaces support changing their MAC address. On most Unix -like systems, the command utility ifconfig may be used to remove and add link address aliases. For instance, the active ifconfig directive may be used on NetBSD to specify which of
7590-650: The bytes over the wire with the most significant bit first, confusion may arise when an address in the latter scenario is represented with bits reversed from the canonical representation. For example, an address in canonical form 12-34-56-78-9A-BC would be transmitted over the wire as bits 01001000 00101100 01101010 00011110 01011001 00111101 in the standard transmission order (least significant bit first). But for Token Ring networks, it would be transmitted as bits 00010010 00110100 01010110 01111000 10011010 10111100 in most-significant-bit first order. The latter might be incorrectly displayed as 48-2C-6A-1E-59-3D . This
7705-428: The case of a roulette wheel, if the force of the hand and the period of that force are known, the number on which the ball will stop would be a certainty (though as a practical matter, this would likely be true only of a roulette wheel that had not been exactly levelled – as Thomas A. Bass' Newtonian Casino revealed). This also assumes knowledge of inertia and friction of the wheel, weight, smoothness, and roundness of
7820-1116: The chance of rolling a 1 or 2 on a six-sided die is P ( 1 or 2 ) = P ( 1 ) + P ( 2 ) = 1 6 + 1 6 = 1 3 . {\displaystyle P(1{\mbox{ or }}2)=P(1)+P(2)={\tfrac {1}{6}}+{\tfrac {1}{6}}={\tfrac {1}{3}}.} If the events are not (necessarily) mutually exclusive then P ( A or B ) = P ( A ∪ B ) = P ( A ) + P ( B ) − P ( A and B ) . {\displaystyle P\left(A{\hbox{ or }}B\right)=P(A\cup B)=P\left(A\right)+P\left(B\right)-P\left(A{\mbox{ and }}B\right).} Rewritten, P ( A ∪ B ) = P ( A ) + P ( B ) − P ( A ∩ B ) {\displaystyle P\left(A\cup B\right)=P\left(A\right)+P\left(B\right)-P\left(A\cap B\right)} For example, when drawing
7935-445: The clock sequence are replaced by a "local domain" number, and the least significant 32 bits of the timestamp are replaced by an integer identifier meaningful within the specified local domain. On POSIX systems, local-domain numbers 0 and 1 are for user ids ( UIDs ) and group ids ( GIDs ) respectively, and other local-domain numbers are site-defined. On non-POSIX systems, all local domain numbers are site-defined. The ability to include
8050-421: The collision domain is all receivers that can detect a given wireless signal. If a switch does not know which port leads to a given MAC address, the switch will forward a unicast frame to all of its ports (except the originating port), an action known as unicast flood . Only the node with the matching hardware MAC address will (normally) accept the frame; network interfaces with non-matching MAC-addresses ignore
8165-522: The complete MAC address to be AA-00-04-00-XX-YY where XX-YY reflects the DECnet network address xx.yy of the host. This eliminates the need for DECnet to have an address resolution protocol since the MAC address for any DECnet host can be determined from its DECnet address. The least significant bit of an address's first octet is referred to as the I/G , or Individual/Group , bit. When this bit
8280-500: The concept of a measure. The opposite or complement of an event A is the event [not A ] (that is, the event of A not occurring), often denoted as A ′ , A c {\displaystyle A',A^{c}} , A ¯ , A ∁ , ¬ A {\displaystyle {\overline {A}},A^{\complement },\neg A} , or ∼ A {\displaystyle {\sim }A} ; its probability
8395-520: The earliest known scientific treatment of the subject. Jakob Bernoulli 's Ars Conjectandi (posthumous, 1713) and Abraham de Moivre 's Doctrine of Chances (1718) treated the subject as a branch of mathematics. See Ian Hacking 's The Emergence of Probability and James Franklin's The Science of Conjecture for histories of the early development of the very concept of mathematical probability. The theory of errors may be traced back to Roger Cotes 's Opera Miscellanea (posthumous, 1722), but
8510-480: The effect of such groupthink on pricing, on policy, and on peace and conflict. In addition to financial assessment, probability can be used to analyze trends in biology (e.g., disease spread) as well as ecology (e.g., biological Punnett squares ). As with finance, risk assessment can be used as a statistical tool to calculate the likelihood of undesirable events occurring, and can assist with implementing protocols to avoid encountering such circumstances. Probability
8625-404: The efficacy of defining odds as the ratio of favourable to unfavourable outcomes (which implies that the probability of an event is given by the ratio of favourable outcomes to the total number of possible outcomes ). Aside from the elementary work by Cardano, the doctrine of probabilities dates to the correspondence of Pierre de Fermat and Blaise Pascal (1654). Christiaan Huygens (1657) gave
8740-554: The events {1,6}, {3}, and {2,4}), the probability that at least one of the events will occur is given by the sum of the probabilities of all the individual events. The probability of an event A is written as P ( A ) {\displaystyle P(A)} , p ( A ) {\displaystyle p(A)} , or Pr ( A ) {\displaystyle {\text{Pr}}(A)} . This mathematical definition of probability can extend to infinite sample spaces, and even uncountable sample spaces, using
8855-501: The first 28 bits being assigned by IEEE. The first 24 bits of the assigned MA-M block are an OUI assigned to IEEE that will not be reassigned, so the MA-M does not include assignment of an OUI. Addresses can either be universally administered addresses (UAA) or locally administered addresses (LAA). A universally administered address is uniquely assigned to a device by its manufacturer. The first three octets (in transmission order) identify
8970-502: The first dot, so in this case 0d (13 in decimal) for DDS (Data Distribution Service) . The remaining parts, each separated with a dot, are the node bytes. The lowercase form of the hexadecimal values is the generally preferred format. Specifically in some contexts such as those defined in ITU-T Rec. X.667, lowercase is required when the text is generated, but the uppercase version must also be accepted. A UUID can be represented as
9085-485: The first octet is 06 (hexadecimal), the binary form of which is 000001 1 0, where the second-least-significant bit is 1. Therefore, it is a locally administered address. Even though many hypervisors manage dynamic MAC addresses within their own OUI , often it is useful to create an entire unique MAC within the LAA range. In virtualisation , hypervisors such as QEMU and Xen have their own OUIs. Each new virtual machine
9200-428: The frame unless they are in promiscuous mode . If the least significant bit of the first octet is set to 1 (i.e. the second hexadecimal digit is odd) the frame will still be sent only once; however, network interface controllers will choose to accept or ignore it based on criteria other than the matching of their individual MAC addresses: for example, based on a configurable list of accepted multicast MAC addresses. This
9315-433: The hypervisor. Additionally some operating systems permit the end user to customise the MAC address, notably OpenWRT . Usage of the node's network card MAC address for the node ID means that a version-1 UUID can be tracked back to the computer that created it. Documents can sometimes be traced to the computers where they were created or edited through UUIDs embedded into them by word processing software. This privacy hole
9430-404: The ideas of probability throughout history, but exact mathematical descriptions arose much later. There are reasons for the slow development of the mathematics of probability. Whereas games of chance provided the impetus for the mathematical study of probability, fundamental issues are still obscured by superstitions. According to Richard Jeffrey , "Before the middle of the seventeenth century,
9545-653: The identity of previously known networks. The standard ( IEEE 802 ) format for printing EUI-48 addresses in human-friendly form is six groups of two hexadecimal digits, separated by hyphens ( - ) in transmission order (e.g. 01-23-45-67-89-AB ). This form is also commonly used for EUI-64 (e.g. 01-23-45-67-89-AB-CD-EF ). Other conventions include six groups of two hexadecimal digits separated by colons (:) (e.g. 01:23:45:67:89:AB ), and three groups of four hexadecimal digits separated by dots (.) (e.g. 0123.4567.89AB ); again in transmission order. The standard notation, also called canonical format, for MAC addresses
9660-439: The input name, then hashed with MD5, yielding 128 bits. Then 6 or 7 bits are replaced by fixed values, the 4-bit version (e.g. 0011 2 for version 3), and the 2- or 3-bit UUID "variant" (e.g. 10 2 indicating a RFC 9562 UUIDs, or 110 2 indicating a legacy Microsoft GUID). Since 6 or 7 bits are thus predetermined, only 121 or 122 bits contribute to the uniqueness of the UUID. Version-5 UUIDs are similar, but SHA-1
9775-525: The interface hardware, such as its read-only memory , or by a firmware mechanism. Many network interfaces, however, support changing their MAC addresses. The address typically includes a manufacturer's organizationally unique identifier (OUI). MAC addresses are formed according to the principles of two numbering spaces based on extended unique identifiers (EUIs) managed by the Institute of Electrical and Electronics Engineers (IEEE): EUI-48 —which replaces
9890-690: The joint probability is P ( A and B ) = P ( A ∩ B ) = P ( A ) P ( B ) . {\displaystyle P(A{\mbox{ and }}B)=P(A\cap B)=P(A)P(B).} For example, if two coins are flipped, then the chance of both being heads is 1 2 × 1 2 = 1 4 . {\displaystyle {\tfrac {1}{2}}\times {\tfrac {1}{2}}={\tfrac {1}{4}}.} If either event A or event B can occur but never both simultaneously, then they are called mutually exclusive events. If two events are mutually exclusive , then
10005-412: The libuuid library, treats the timestamp as unsigned, putting the rollover time in 5623 AD. The rollover time as defined by ITU-T Rec. X.667 is 3603 AD. A 13-bit or 14-bit "uniquifying" clock sequence extends the timestamp in order to handle cases where the processor clock does not advance fast enough, or where there are multiple processors and UUID generators per node. When UUIDs are generated faster than
10120-437: The notion of Markov chains , which played an important role in stochastic processes theory and its applications. The modern theory of probability based on measure theory was developed by Andrey Kolmogorov in 1931. On the geometric side, contributors to The Educational Times included Miller, Crofton, McColl, Wolstenholme, Watson, and Artemas Martin . See integral geometry for more information. Like other theories ,
10235-742: The obsolete term MAC-48 —and EUI-64 . Network nodes with multiple network interfaces, such as routers and multilayer switches , must have a unique MAC address for each network interface in the same network. However, two network interfaces connected to two different networks can share the same MAC address. The IEEE 802 MAC address originally comes from the Xerox Network Systems Ethernet addressing scheme. This 48-bit address space contains potentially 2 (over 281 trillion) possible MAC addresses. The IEEE manages allocation of MAC addresses, originally known as MAC-48 and which it now refers to as EUI-48 identifiers. The IEEE has
10350-521: The order of magnitude of the Avogadro constant 6.02 × 10 ) that only a statistical description of its properties is feasible. Probability theory is required to describe quantum phenomena. A revolutionary discovery of early 20th century physics was the random character of all physical processes that occur at sub-atomic scales and are governed by the laws of quantum mechanics . The objective wave function evolves deterministically but, according to
10465-555: The organization that issued the identifier and are known as the organizationally unique identifier (OUI). The remainder of the address (three octets for EUI-48 or five for EUI-64) are assigned by that organization in nearly any manner they please, subject to the constraint of uniqueness. A locally administered address is assigned to a device by software or a network administrator, overriding the burned-in address for physical devices. Locally administered addresses are distinguished from universally administered addresses by setting (assigning
10580-2599: The possibilities included in the "3 that are both" are included in each of the "13 hearts" and the "12 face cards", but should only be counted once. This can be expanded further for multiple not (necessarily) mutually exclusive events. For three events, this proceeds as follows: P ( A ∪ B ∪ C ) = P ( ( A ∪ B ) ∪ C ) = P ( A ∪ B ) + P ( C ) − P ( ( A ∪ B ) ∩ C ) = P ( A ) + P ( B ) − P ( A ∩ B ) + P ( C ) − P ( ( A ∩ C ) ∪ ( B ∩ C ) ) = P ( A ) + P ( B ) + P ( C ) − P ( A ∩ B ) − ( P ( A ∩ C ) + P ( B ∩ C ) − P ( ( A ∩ C ) ∩ ( B ∩ C ) ) ) P ( A ∪ B ∪ C ) = P ( A ) + P ( B ) + P ( C ) − P ( A ∩ B ) − P ( A ∩ C ) − P ( B ∩ C ) + P ( A ∩ B ∩ C ) {\displaystyle {\begin{aligned}P\left(A\cup B\cup C\right)=&P\left(\left(A\cup B\right)\cup C\right)\\=&P\left(A\cup B\right)+P\left(C\right)-P\left(\left(A\cup B\right)\cap C\right)\\=&P\left(A\right)+P\left(B\right)-P\left(A\cap B\right)+P\left(C\right)-P\left(\left(A\cap C\right)\cup \left(B\cap C\right)\right)\\=&P\left(A\right)+P\left(B\right)+P\left(C\right)-P\left(A\cap B\right)-\left(P\left(A\cap C\right)+P\left(B\cap C\right)-P\left(\left(A\cap C\right)\cap \left(B\cap C\right)\right)\right)\\P\left(A\cup B\cup C\right)=&P\left(A\right)+P\left(B\right)+P\left(C\right)-P\left(A\cap B\right)-P\left(A\cap C\right)-P\left(B\cap C\right)+P\left(A\cap B\cap C\right)\end{aligned}}} It can be seen, then, that this pattern can be repeated for any number of events. Conditional probability
10695-399: The probability of both occurring is denoted as P ( A ∩ B ) {\displaystyle P(A\cap B)} and P ( A and B ) = P ( A ∩ B ) = 0 {\displaystyle P(A{\mbox{ and }}B)=P(A\cap B)=0} If two events are mutually exclusive , then the probability of either occurring
10810-646: The proportionality symbol means that the left hand side is proportional to (i.e., equals a constant times) the right hand side as A {\displaystyle A} varies, for fixed or given B {\displaystyle B} (Lee, 2012; Bertsch McGrayne, 2012). In this form it goes back to Laplace (1774) and to Cournot (1843); see Fienberg (2005). In a deterministic universe, based on Newtonian concepts, there would be no probability if all conditions were known ( Laplace's demon ) (but there are situations in which sensitivity to initial conditions exceeds our ability to measure them, i.e. know them). In
10925-453: The range 3 3 -33-XX-XX-XX-XX (with both bits set). Given the locations of the U/L and I/G bits, they can be discerned in a single digit in common MAC address notation as shown in the following table: IEEE standard 802c further divides the locally administered MAC address block into four quadrants. This additional partitioning is called Structured Local Address Plan (SLAP) and its usage
11040-408: The remaining bits is governed by the variant/version selected. The variant field indicates the format of the UUID (and in case of the legacy UUID also the address family used for the node field). The following variants are defined: The OSF DCE variant defines eight "versions" in the standard, and each version may be more appropriate than the others in specific use cases. The version is indicated by
11155-486: The results that actually occur fall in a given event, the event is said to have occurred. A probability is a way of assigning every event a value between zero and one, with the requirement that the event made up of all possible results (in our example, the event {1,2,3,4,5,6}) is assigned a value of one. To qualify as a probability, the assignment of values must satisfy the requirement that for any collection of mutually exclusive events (events with no common results, such as
11270-623: The same UUID is generated more than once and assigned to different referents. In the case of standard version-1 and version-2 UUIDs using unique MAC addresses from network cards, collisions are unlikely to occur, with an increased possibility only when an implementation varies from the standards, either inadvertently or intentionally. In contrast to version-1 and version-2 UUIDs generated using MAC addresses, with version-1 and -2 UUIDs which use randomly generated node ids, hash-based version-3 and version-5 UUIDs, and random version-4 UUIDs, collisions can occur even without implementation problems, albeit with
11385-425: The sample space is formed by considering all different collections of possible results. For example, rolling a die can produce six possible results. One collection of possible results gives an odd number on the die. Thus, the subset {1,3,5} is an element of the power set of the sample space of dice rolls. These collections are called "events". In this case, {1,3,5} is the event that the die falls on some odd number. If
11500-415: The system clock could advance, the lower bits of the timestamp fields can be generated by incrementing it every time a UUID is being generated, to simulate a high-resolution timestamp. With each version 1 UUID corresponding to a single point in space (the node) and time (intervals and clock sequence), the chance of two properly generated version-1 UUIDs being unintentionally the same is practically nil. Since
11615-408: The system that it is a version 8 UUID. The remaining 122 bits are up to the vendor to customize. The difference with version 4 is that those 122 bits are random, but the 122 bits in UUID version 8 are not, because they follow vendor specific rules. The "nil" UUID is the UUID 00000000-0000-0000-0000-000000000000 ; that is, all bits set to zero. The "max" UUID, sometimes also called the "omni" UUID,
11730-433: The term 'probable' (Latin probabilis ) meant approvable , and was applied in that sense, univocally, to opinion and to action. A probable action or opinion was one such as sensible people would undertake or hold, in the circumstances." However, in legal contexts especially, 'probable' could also apply to propositions for which there was good evidence. The sixteenth-century Italian polymath Gerolamo Cardano demonstrated
11845-536: The time and clock sequence total 74 bits, 2 (1.8 × 10, or 18 sextillion) version-1 UUIDs can be generated per node ID, at a maximal average rate of 163 billion per second per node ID. In contrast to other UUID versions, version-1 and -2 UUIDs based on MAC addresses from network cards rely for their uniqueness in part on an identifier issued by a central registration authority, namely the Organizationally Unique Identifier (OUI) part of
11960-519: The timestamp follows the version nibble, that must have a value of 7. The variant bits have to be 10x . Remaining 74 bits are random seeded counter (optional, at least 12 bits but no longer than 42 bits) and random. Two counter rollover handling methods can be used together: In DBMS UUIDv7 generator can be shared between threads (tied to a table or to a DBMS instance) or can be thread-local (with worse monotonicity, locality and performance). Version 8 only has two requirements: Those requirements tell
12075-496: The use of probability theory in equity trading is the effect of the perceived probability of any widespread Middle East conflict on oil prices, which have ripple effects in the economy as a whole. An assessment by a commodity trader that a war is more likely can send that commodity's prices up or down, and signals other traders of that opinion. Accordingly, the probabilities are neither assessed independently nor necessarily rationally. The theory of behavioral finance emerged to describe
12190-522: The value 40:D8:55 was used. The owners of an already assigned IAB may continue to use the assignment. The MA-S registry includes, for each registrant, both a 36-bit unique number used in some standards and a block of EUI-48 and EUI-64 identifiers (while the registrant of an IAB cannot assign an EUI-64). MA-S does not include assignment of an OUI. Additionally, the MA-M ( MAC address block, medium ) provides both 2 EUI-48 identifiers and 2 EUI-64 identifiers,
12305-456: The value of 1 to) the second- least-significant bit of the first octet of the address. This bit is also referred to as the U/L bit, short for Universal/Local , which identifies how the address is administered. If the bit is 0, the address is universally administered, which is why this bit is 0 in all UAAs. If it is 1, the address is locally administered. In the example address 06-00-00-00-00-00
12420-407: The value of the higher nibble (higher 4 bits, or higher hexadecimal digit) of the 7th byte of the UUID. In hex, this is the character after the second dash. For example, the UUID 9c5b94b1-35ad- 4 9bb-b118-8e8fc24abf80 is version 4, because of the digit after the second dash is 4 in ...- 4 9bb-... . Version 1 concatenates the 48-bit MAC address of the "node" (that is, the computer generating
12535-498: The variant. Values are 8 , 9 , A or B for the 2 bit indication, values C or D for the 3 bit indication. For example, a random UUID version 4, variant 2 could be 8D8AC610-566D-4EF0-9C22-186B2A5ED793 . Version 7 UUIDs (UUIDv7) are designed for keys in high-load databases and distributed systems. UUIDv7 begins with a 48 bit big-endian Unix Epoch timestamp with approximately millisecond granularity. The timestamp can be shifted by any time shift value. Directly after
12650-595: The various context-dependent identifier number spaces, like for SNAP or EDID ), the Individual Address Block could only be used to assign EUI-48 identifiers. All other potential uses based on the OUI from which the IABs are allocated are reserved and remain the property of the IEEE Registration Authority. Between 2007 and September 2012, the OUI value 00:50:C2 was used for IAB assignments. After September 2012,
12765-430: The wave function, believed quantum mechanics is a statistical approximation of an underlying deterministic reality . In some modern interpretations of the statistical mechanics of measurement, quantum decoherence is invoked to account for the appearance of subjectively probabilistic experimental outcomes. MAC address A MAC address (short for medium access control address or media access control address )
12880-482: Was a legacy UUID. This gives the following table for the family group: The legacy Apollo NCS UUID has the format described in the previous table. The OSF DCE UUID variant is described in RFC 9562 . The Microsoft COM / DCOM UUID has its variant described in the Microsoft documentation. When saving UUIDs to binary format, they are sequentially encoded in big-endian . For example, 00112233-4455-6677-8899-aabbccddeeff
12995-403: Was published in 1774, and stated that the frequency of an error could be expressed as an exponential function of the numerical magnitude of the error – disregarding sign. The second law of error was proposed in 1778 by Laplace, and stated that the frequency of the error is an exponential function of the square of the error. The second law of error is called the normal distribution or
13110-631: Was published, introducing 3 new "versions" and clarifying some ambiguities. UUIDs are standardized by the Open Software Foundation (OSF) as part of the Distributed Computing Environment (DCE). UUIDs are documented as part of ISO / IEC 11578:1996 " Information technology – Open Systems Interconnection – Remote Procedure Call (RPC)" and more recently in ITU-T Rec. X.667 | ISO / IEC 9834-8:2014. The Internet Engineering Task Force (IETF) published
13225-515: Was used when locating the creator of the Melissa virus . RFC 9562 does allow the MAC address in a version-1 (or 2) UUID to be replaced by a random 48-bit node ID, either because the node does not have a MAC address, or because it is not desirable to expose it. In that case, the RFC requires that the least significant bit of the first octet of the node ID should be set to 1. This corresponds to
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