Dot-decimal notation is a presentation format for numerical data. It consists of a string of decimal numbers, each pair separated by a full stop (dot).
A common use of dot-decimal notation is in information technology where it is a method of writing numbers in octet-grouped base-10 (decimal) numbers separated by dots (full stops). In computer networking, Internet Protocol Version 4 addresses (IPv4 addresses) are commonly written using the quad-dotted notation of four decimal integers, ranging from 0 to 255 each.
Definition and use
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.
Object identifiers use a style of dot-decimal notation to represent an arbitrarily deep hierarchy of objects identified by decimal numbers.
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.
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 overall 32-bit number is represented in hexadecimal notation as 0x7F000001.
No formal specification of this textual IP address representation exists. The first mention of this format in 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 32bit 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. RFC 1123 (Requirements for Internet Hosts – Application and Support) from October 1989 mentions a requirement on 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”. A separate IETF draft to define textual representation of IP addresses expired.
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 full 32-bit addresses, it also supported intermediate syntax forms of octet.24bits (e.g. 10.1234567; for 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, by prefixing them with 0x and 0, respectively. These features continue to be supported by software until today, even though they are seen as non-standard. But this also means addresses where an IP address component is written with a leading zero digit may be interpreted differently by different programs: some will ignore the leading zero, some will interpret the number as octal.
IP addresses in the 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 8-bit routing prefix, and therefore the subnet mask 255.0.0.0.
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