Editing Dot-decimal notation (section)

==IPv4 address==
[[Image:IPv4 address structure and writing systems-en.svg|right|300px|thumb|An IP address (version 4) in both dot-decimal notation and [[binary code]]]]

In computer networking, the notation is associated with the specific use of ''quad-dotted notation'' to represent IPv4 addresses<ref>{{cite web
|url=http://searchsmb.techtarget.com/sDefinition/0,,sid44_gci211994,00.html
|title=Dot address
|publisher=TechTarget
|access-date=2010-11-18
}}</ref> and used as a synonym for ''dotted-quad notation''.<ref>{{cite web
|url=http://www.encyclopedia.com/doc/1O12-dotteddecimalnotation.html
|title=Dotted Decimal Notation
|publisher=encyclopedia.com
}}{{dl|fix-attempted=yes|date=August 2021}}</ref> Dot-decimal notation is a presentation format for numerical data expressed as a string of decimal numbers each separated by a full stop. For example, the [[hexadecimal number]] ''0xFF000000'' may be expressed in dot-decimal notation as ''255.0.0.0''.

An IPv4 address has 32 bits. For purposes of representation, the bits may be divided into four octets written in decimal numbers, ranging from 0 to 255, concatenated as a character string with full stop delimiters between each number.<ref name=IBM>{{cite web |title=IPv4 and IPv6 address formats |url=https://www.ibm.com/docs/en/ts3500-tape-library?topic=functionality-ipv4-ipv6-address-formats |website=www.ibm.com |language=en-us|quote=An IPv4 address has the following format: x . x . x . x where x is called an octet and must be a decimal value between 0 and 255. Octets are separated by periods. An IPv4 address must contain three periods and four octets. The following examples are valid IPv4 addresses:{{br}}    1 . 2 . 3 . 4{{br}}    01 . 102 . 103 . 104}}</ref> This octet-grouped dotted-decimal format may more specifically be called "dotted octet" format,<ref name="ipaddr-text-rep" /> or a "dotted quad address".<ref>{{cite web |title=Definition of dot address |url=https://www.pcmag.com/encyclopedia/term/dot-address |website=PCMAG |language=en}}</ref>

For example, the address of the [[loopback]] interface, usually assigned the host name [[localhost]], is 127.0.0.1. It consists of the four octets, written in binary notation: ''01111111'', ''00000000'', ''00000000'', and ''00000001''. The 32-bit number is represented in hexadecimal notation as ''0x7F000001''.

No formal specification of this textual IP address representation exists.<ref name="ipaddr-text-rep" /> The first mention of this format in [[Request for Comments|RFC]] documents was in RFC 780 for the [[Mail Transfer Protocol]] published May 1981, in which the IP address was supposed to be enclosed in brackets or represented as a 32-bit decimal integer prefixed by a pound sign. A table in RFC 790 (''Assigned Numbers'') used the dotted decimal format, zero-padding each number to three digits.<ref name="ipaddr-text-rep">{{cite IETF
 |title= Textual Representation of IPv4 and IPv6 Addresses
 |draft= draft-main-ipaddr-text-rep-02
 |last= Main
 |first= Andrew
 |date=23 February 2005
 |publisher=[[Internet Engineering Task Force|IETF]]
}}</ref> RFC 1123 (''Requirements for Internet Hosts – Application and Support'') of October 1989 mentions a requirement for host software to accept "IP address in dotted-decimal ("#.#.#.#") form", although it notes "[t]his last requirement is not intended to specify the complete syntactic form for entering a dotted-decimal host number".<ref name="host-requirements">{{cite IETF
 |title= Requirements for Internet Hosts -- Application and Support
 |url=https://tools.ietf.org/html/rfc1123
 |last= Braden
 |first= Robert
 |date=1 October 1989
 |publisher=[[Internet Engineering Task Force|IETF]]
 |section=2.1
}}</ref> An IETF draft intended to define textual representation of IP addresses expired without further activity.<ref name="ipaddr-text-rep" />

A popular implementation of IP networking, originating in [[4.2BSD]], contains a function ''inet_aton()'' for converting IP addresses in character string representation to internal binary storage. In addition to the basic four-decimals format and 32-bit numbers, it also supported intermediate syntax forms of ''octet.24bits'' (e.g. 10.1234567; for [[Classful network|Class A]] addresses) and ''octet.octet.16bits'' (e.g. 172.16.12345; for Class B addresses). It also allowed the numbers to be written in [[hexadecimal]] and [[octal]] representations, by prefixing them with 0x and 0, respectively. These features continue to be supported in some software, even though they are considered as non-standard.<ref name="ipaddr-text-rep" /> This means addresses with a component written with a leading zero digit may be interpreted differently in programs that do or do not recognize such formats.<ref>{{cite web
|url = http://support.microsoft.com/kb/115388
|title = Ping and FTP resolve IP address with leading zero as octal
|publisher = Microsoft Support
|archive-url = https://web.archive.org/web/20061206211851/http://support.microsoft.com/kb/115388
|archive-date = 2006-12-06
}}</ref>

A [[POSIX]]-conforming variant of ''inet_aton'', the ''inet_pton()'' function, supports only the four-decimal variant of IP addresses.<ref>{{man|3|inet_pton|die.net}}</ref>

IP addresses in dot-decimal notation are also presented in [[CIDR notation]], in which the IP address is suffixed with a slash and a number, used to specify the length of the associated routing prefix. For example, 127.0.0.1/8 specifies that the IP address has an eight-bit routing prefix, and therefore the subnet mask ''255.0.0.0''.