Editing IP address (section)

==IPv4 addresses==
{{Main|IPv4#Addressing}}
[[Image:IPv4 address structure and writing systems-en.svg|right|300px|thumb|Decomposition of an IPv4 address from [[dot-decimal notation]] to its binary value]]
An IPv4 address has a size of 32 bits, which limits the [[address space]] to {{gaps|4|294|967|296}} (2<sup>32</sup>) addresses. Of this number, some addresses are reserved for special purposes such as [[private network]]s (≈18&nbsp;million addresses) and [[multicast address]]ing (≈270&nbsp;million addresses).

IPv4 addresses are usually represented in [[dot-decimal notation]], consisting of four decimal numbers, each ranging from 0 to 255, separated by dots, e.g., {{IPaddr|192.0.2.1}}.  Each part represents a group of 8 bits (an [[Octet (computing)|octet]]) of the address.<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=IBM |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> In some cases of technical writing,{{specify|date=May 2018}} IPv4 addresses may be presented in various [[hexadecimal]], [[octal]], or [[binary numeral system|binary]] representations.

===Subnetting history===
In the early stages of development of the Internet Protocol, the network number was always the highest order octet (most significant eight bits). Because this method allowed for only 256 networks, it soon proved inadequate as additional networks developed that were independent of the existing networks already designated by a network number. In 1981, the addressing specification was revised with the introduction of [[classful network]] architecture.<ref name=rfc791/>

Classful network design allowed for a larger number of individual network assignments and fine-grained subnetwork design. The first three bits of the most significant octet of an IP address were defined as the ''class'' of the address. Three classes (''A'', ''B'', and ''C'') were defined for universal [[unicast]] addressing. Depending on the class derived, the network identification was based on octet boundary segments of the entire address. Each class used successively additional octets in the network identifier, thus reducing the possible number of hosts in the higher order classes (''B'' and ''C''). The following table gives an overview of this now-obsolete system.
<!-- PLEASE DO NOT MODIFY THIS TABLE BECAUSE YOU THINK IT IS WRONG. It has been checked by knowledgeable editors for correctness within the intended purpose in this article. If you still think it is wrong, please discuss first in the TALK PAGE to avoid needless reversion of your edits. In particular, please remember that the network 127 is still a class A network no matter its status of use as loopback network. -->
{| class="wikitable"
|-
|-
|+ Historical classful network architecture
|-
! Class
! Leading <br />bits
! Size of ''network <br />number'' bit field
! Size of ''rest''<br />bit field
! Number<br />of networks
! Number of addresses<br />per network
! Start address
! End address
|-
! A
| 0
| 8
| 24
| 128 (2<sup>7</sup>)
| {{gaps|16|777|216}} (2<sup>24</sup>)
| 0.0.0.0
| 127.255.255.255<!--DO NOT CHANGE THIS, 0/8 and 127/8 ARE CLASS A NETWORKS AS WELL-->
|-
! B
| 10
| 16
| 16
| {{gaps|16|384}} (2<sup>14</sup>)
| {{gaps|65|536}} (2<sup>16</sup>)
| 128.0.0.0
| 191.255.255.255
|-
! C
| 110
| 24
| 8
| {{gaps|2|097|152}} (2<sup>21</sup>)
| 256 (2<sup>8</sup>)
| 192.0.0.0
| 223.255.255.255
|}

Classful network design served its purpose in the startup stage of the Internet, but it lacked [[scalability]] in the face of the rapid expansion of networking in the 1990s. The class system of the address space was replaced with [[Classless Inter-Domain Routing]] (CIDR) in 1993. CIDR is based on variable-length subnet masking (VLSM) to allow allocation and routing based on arbitrary-length prefixes. Today, remnants of classful network concepts function only in a limited scope as the default configuration parameters of some network software and hardware components (e.g. netmask), and in the technical jargon used in network administrators' discussions.

===Private addresses===
Early network design, when global end-to-end connectivity was envisioned for communications with all Internet hosts, intended that IP addresses be globally unique. However, it was found that this was not always necessary as private networks developed and public address space needed to be conserved.

Computers not connected to the Internet, such as factory machines that communicate only with each other via [[TCP/IP]], need not have globally unique IP addresses. Today, such private networks are widely used and typically connect to the Internet with [[network address translation]] (NAT), when needed.

Three non-overlapping ranges of IPv4 addresses for private networks are reserved.<ref name=rfc1918/> These addresses are not routed on the Internet and thus their use need not be coordinated with an IP address registry. Any user may use any of the reserved blocks. Typically, a network administrator will divide a block into subnets; for example, many [[residential gateway|home routers]] automatically use a default address range of {{IPaddr|192.168.0.0}} through {{IPaddr|192.168.0.255}} ({{IPaddr|192.168.0.0|24}}).
{{trim|{{#section:IPv4|IPv4-private-networks}}}}