Editing IPv4 (section)

=={{anchor|IPv4_Addressing}}Addressing==
<!--redirect from [[IPv4 address]]-->
{{broader|IP address}}
[[File:IPv4 address structure and writing systems-en.svg|right|300px|thumb|Decomposition of the quad-dotted IPv4 address representation to its [[Binary numeral system|binary]] value]]
IPv4 uses 32-bit addresses which limits the [[address space]] to {{gaps|4|294|967|296}} (2<sup>32</sup>) addresses.

IPv4 reserves special address blocks for [[private network]]s (2<sup>24</sup>&nbsp;+&nbsp;2<sup>20</sup>&nbsp;+&nbsp;2<sup>16</sup>&nbsp;≈&nbsp;18 million addresses) and [[multicast]] addresses (2<sup>28</sup>&nbsp;≈&nbsp;268 million addresses).

===Address representations===
IPv4 addresses may be represented in any notation expressing a 32-bit integer value. They are most often written in [[dot-decimal notation]], which consists of four [[Octet (computing)|octet]]s of the address expressed individually in [[decimal]] numbers and separated by [[Full stop|periods]].

For example, the quad-dotted IP address in the illustration ({{IPaddr|172.16.254.1}}) represents the 32-bit [[decimal]] number 2886794753, which in [[hexadecimal]] format is 0xAC10FE01.

[[CIDR notation]] combines the address with its routing prefix in a compact format, in which the address is followed by a slash character (/) and the count of leading consecutive ''1'' bits in the routing prefix (subnet mask).

Other address representations were in common use when [[classful network]]ing was practiced. For example, the [[loopback]] address {{IPaddr|127.0.0.1}} was commonly written as {{IPaddr|127.1}}, given that it belongs to a class-A network with eight bits for the network mask and 24 bits for the host number. When fewer than four numbers were specified in the address in dotted notation, the last value was treated as an integer of as many bytes as are required to fill out the address to four octets. Thus, the address {{IPaddr|127.65530}} is equivalent to {{IPaddr|127.0.255.250}}.

==={{anchor|Allocation}}Allocation===
In the original design of IPv4, an IP address was divided into two parts: the network identifier was the most significant octet of the address, and the host identifier was the rest of the address. The latter was also called the ''rest field''. This structure permitted a maximum of 256 network identifiers, which was quickly found to be inadequate.

To overcome this limit, the most-significant address octet was redefined in 1981 to create ''network classes'', in a system which later became known as ''classful'' networking. The revised system defined five classes. Classes A, B, and C had different bit lengths for network identification. The rest of the address was used as previously to identify a host within a network. Because of the different sizes of fields in different classes, each network class had a different capacity for addressing hosts. In addition to the three classes for addressing hosts, Class D was defined for [[multicast]] addressing and Class E was reserved for future applications.

Dividing existing classful networks into subnets began in 1985 with the publication of {{IETF RFC|950}}. This division was made more flexible with the introduction of variable-length subnet masks (VLSM) in {{IETF RFC|1109}} in 1987. In 1993, based on this work, {{IETF RFC|1517}} introduced [[Classless Inter-Domain Routing]] (CIDR),<ref>{{cite web|url=http://www.3com.com/other/pdfs/infra/corpinfo/en_US/501302.pdf|archive-url=https://web.archive.org/web/20010616012053/http://www.3com.com/other/pdfs/infra/corpinfo/en_US/501302.pdf|archive-date=June 16, 2001|title=Understanding IP Addressing: Everything You Ever Wanted To Know|publisher=3Com|url-status=dead}}</ref> which expressed the number of bits (from the [[Most significant bit|most significant]]) as, for instance, {{IPaddr|/24}}, and the class-based scheme was dubbed ''classful'', by contrast. CIDR was designed to permit repartitioning of any address space so that smaller or larger blocks of addresses could be allocated to users. The hierarchical structure created by CIDR is managed by the [[Internet Assigned Numbers Authority]] (IANA) and the [[regional Internet registries]] (RIRs). Each RIR maintains a publicly searchable [[WHOIS]] database that provides information about IP address assignments.

===Special-use addresses===
The [[Internet Engineering Task Force]] (IETF) and IANA have restricted from general use various [[reserved IP addresses]] for special purposes.{{Ref RFC|6890}} Notably these addresses are used for [[multicast]] traffic and to provide addressing space for unrestricted uses on private networks.
<section begin=IPv4-special-address-blocks/><!--This table is transcluded to other pages, such as [[Reserved IP addresses]]. (see: [[Wikipedia:transclusion]])-->
:{|class="wikitable sortable"
|+Special address blocks
!Address block
!Address range
!Number of addresses
!Scope
!Description
|-
|0.0.0.0/8
|0.0.0.0–0.255.255.255
|align=right|{{val|16,777,216}}
|Software
|Current (local, "this") network{{Ref RFC|6890}}
|-
|10.0.0.0/8
|10.0.0.0–10.255.255.255
|align=right|{{val|16,777,216}}
|Private&nbsp;network
|Used for local communications within a private network{{Ref RFC|1918}}
|-
<!--Note: 14.0.0.0/8, 24.0.0.0/8 and 39.0.0.0/8 are outdated and irrelevant. Do not add them.-->
|100.64.0.0/10
|100.64.0.0–100.127.255.255
|align=right|{{val|4,194,304}}
|Private&nbsp;network
|[[IPv4 shared address space|Shared address space]]{{Ref RFC|6598}} for communications between a service provider and its subscribers when using a [[carrier-grade NAT]]
|-
|127.0.0.0/8
|127.0.0.0–127.255.255.255
|align=right|{{val|16,777,216}}
|Host
|Used for [[loopback address]]es to the local host<ref name=rfc6890/>
|-
<!--Note: 128.0.0.0/16 is "no longer needed" and "subject to future allocation" so it is not relevant here.-->
<!--"Note that 128.66.0.0/24 has been used for some examples in the past. However, this block did not appear in the list of special prefixes in RFC 3330 or its successors, and the block is therefore not reserved for any special purpose. The block can be used for regular address assignments with caution." excerpt from RFC 5737.-->
|169.254.0.0/16
|169.254.0.0–169.254.255.255
|align=right|{{val|65,536}}
|Subnet
|Used for [[link-local address]]es{{Ref RFC|3927}} between two hosts on a single link when no IP address is otherwise specified, such as would have normally been retrieved from a [[DHCP]] server
|-
|172.16.0.0/12
|172.16.0.0–172.31.255.255
|align=right|{{val|1,048,576}}
|Private&nbsp;network
|Used for local communications within a private network<ref name=rfc1918/>
|-
<!--Note: 191.255.0.0/16 and 192.0.0.0/24 is "no longer needed" and "subject to future allocation" so it is not relevant here.-->
|-
|192.0.0.0/24
|192.0.0.0–192.0.0.255
|align=right|{{val|256}}
|Private&nbsp;network
|IETF Protocol Assignments, [[DS-Lite]] (/29)<ref name=rfc6890/>
|-
|192.0.2.0/24
|192.0.2.0–192.0.2.255
|align=right|{{val|256}}
|Documentation
|Assigned as TEST-NET-1, documentation and examples{{Ref RFC|5737}}
|-
|192.88.99.0/24
|192.88.99.0–192.88.99.255
|align=right|{{val|256}}
|Internet
|Reserved.{{Ref RFC|7526}} Formerly used for [[6to4|IPv6 to IPv4]] relay{{Ref RFC|3068}} (included [[IPv6]] address block [[IPv6 address#Special addresses|2002::/16]]).
|-
|192.168.0.0/16
|192.168.0.0–192.168.255.255
|align=right|{{val|65,536}}
|Private network
|Used for local communications within a private network<ref name=rfc1918/>
|-
|198.18.0.0/15
|198.18.0.0–198.19.255.255
|align=right|{{val|131,072}}
|Private&nbsp;network
|Used for benchmark testing of inter-network communications between two separate subnets{{Ref RFC|2544}}
|-
|198.51.100.0/24
|198.51.100.0–198.51.100.255
|align=right|{{val|256}}
|Documentation
|Assigned as TEST-NET-2, documentation and examples<ref name=rfc5737/>
|-
|203.0.113.0/24
|203.0.113.0–203.0.113.255
|align=right|{{val|256}}
|Documentation
|Assigned as TEST-NET-3, documentation and examples<ref name=rfc5737/>
|-
<!--223.255.255.0/24 presently unused, but subject to future allocation-->
|224.0.0.0/4
|224.0.0.0–239.255.255.255
|align=right|{{val|268,435,456}}
|Internet
|In use for [[IP multicast|multicast]]{{Ref RFC|5771}} (former Class D network)
|-
|233.252.0.0/24
|233.252.0.0–233.252.0.255
|align=right|{{val|256}}
|Documentation
|Assigned as MCAST-TEST-NET, documentation and examples (This is part of the above multicast space.)<ref name="rfc5771"/>{{Ref RFC|6676}}
|-
|240.0.0.0/4
|240.0.0.0–255.255.255.254
|align=right|{{val|268,435,455}}
|Internet
|Reserved for future use{{Ref RFC|3232}} (former Class E network)
|-
|255.255.255.255/32
|255.255.255.255
|align=right|{{val|1}}
|Subnet
|Reserved for the "limited [[Broadcast address|broadcast]]" destination address{{Ref RFC|6890}}
|}<section end=IPv4-special-address-blocks/>

====Private networks====
Of the approximately four billion addresses defined in IPv4, about 18 million addresses in three ranges are reserved for use in private networks. Packets addresses in these ranges are not routable in the public Internet; they are ignored by all public routers. Therefore, private hosts cannot directly communicate with public networks, but require [[network address translation]] at a routing gateway for this purpose.
<section begin=IPv4-private-networks/><!--This table is transcluded to other pages, such as [[Blackhole server]]. (see: [[Wikipedia:transclusion]])-->
:{|class=wikitable
|+Reserved private IPv4 network ranges{{Ref RFC|1918}}
|-
!Name!![[CIDR]] block!!Address range!!Number of<br>addresses!!''[[Classful]]'' description
|-
|24-bit block||10.0.0.0/8||10.0.0.0 – 10.255.255.255||align=right|{{val|16777216}}||Single Class A
|-
|20-bit block||172.16.0.0/12||172.16.0.0 – 172.31.255.255||align=right|{{val|1048576}}||Contiguous range of 16 Class B blocks
|-
|16-bit block||192.168.0.0/16||192.168.0.0 – 192.168.255.255||align=right|{{val|65536}}||Contiguous range of 256 Class C blocks
|}<section end=IPv4-private-networks/>

Since two private networks, e.g., two branch offices, cannot directly interoperate via the public Internet, the two networks must be bridged across the Internet via a [[virtual private network]] (VPN) or an [[IP tunnel]], which [[Encapsulation (networking)|encapsulates]] packets, including their headers containing the private addresses, in a protocol layer during transmission across the public network. Additionally, encapsulated packets may be encrypted for transmission across public networks to secure the data.

===Link-local addressing===
RFC 3927 defines the special address block 169.254.0.0/16 for link-local addressing. These addresses are only valid on the link (such as a local network segment or point-to-point connection) directly connected to a host that uses them. These addresses are not routable. Like private addresses, these addresses cannot be the source or destination of packets traversing the internet. These addresses are primarily used for address autoconfiguration ([[Zeroconf]]) when a host cannot obtain an IP address from a DHCP server or other internal configuration methods.

When the address block was reserved, no standards existed for address autoconfiguration. [[Microsoft]] created an implementation called [[Automatic Private IP Addressing]] (APIPA), which was deployed on millions of machines and became a [[de facto standard]]. Many years later, in May 2005, the [[IETF]] defined a formal standard in RFC 3927, entitled ''Dynamic Configuration of IPv4 Link-Local Addresses''.

===Loopback===
{{Main|Localhost}}
The class A network {{IPaddr|127.0.0.0}} (classless network {{IPaddr|127.0.0.0|8}}) is reserved for [[loopback]]. IP packets whose source addresses belong to this network should never appear outside a host. Packets received on a non-loopback interface with a loopback source or destination address must be dropped.

===First and last subnet addresses===
{{See also|IPv4 subnetting reference}}
The first address in a subnet is used to identify the subnet itself. In this address all host bits are ''0''. To avoid ambiguity in representation, this address is reserved.{{Ref RFC|923|quote=Special Addresses: In certain contexts, it is useful to have fixed addresses with functional significance rather than as identifiers of specific hosts. When such usage is called for, the address zero is to be interpreted as meaning "this", as in "this network".}} The last address has all host bits set to ''1''. It is used as a local [[broadcast address]] for sending messages to all devices on the subnet simultaneously. For networks of size {{IPaddr||24}} or larger, the broadcast address always ends in 255.

For example, in the subnet {{IPaddr|192.168.5.0|24}} (subnet mask {{IPaddr||24|netmask=dotted}}) the identifier {{IPaddr|192.168.5.0}} is used to refer to the entire subnet. The broadcast address of the network is {{IPaddr|192.168.5.255}}.

{|class=wikitable style=margin:left
!Type!!Binary form!!Dot-decimal notation
|-
|Network space
|<code>11000000.10101000.00000101.<span style=color:red>00000000</span></code>
|192.168.5.0 
|-
|Broadcast address
|<code>11000000.10101000.00000101.<span style=color:red>11111111</span></code>
|192.168.5.255
|-
|colspan=3|<small>In red, is shown the host part of the IP address; the other part is the network prefix. The host gets inverted (logical NOT), but the network prefix remains intact.</small>
|}

However, this does not mean that every address ending in 0 or 255 cannot be used as a host address. For example, in the {{IPaddr||16}} subnet {{IPaddr|192.168.0.0|16|netmask=dotted}}, which is equivalent to the address range {{IPaddr|192.168.0.0}}–{{IPaddr|192.168.255.255}}, the broadcast address is {{IPaddr|192.168.255.255}}. One can use the following addresses for hosts, even though they end with 255: {{IPaddr|192.168.1.255}}, {{IPaddr|192.168.2.255}}, etc. Also, {{IPaddr|192.168.0.0}} is the network identifier and must not be assigned to an interface.{{Ref RFC|1122|rp=31}} The addresses {{IPaddr|192.168.1.0}}, {{IPaddr|192.168.2.0}}, etc., may be assigned, despite ending with 0.

In the past, conflict between network addresses and broadcast addresses arose because some software used non-standard broadcast addresses with zeros instead of ones.{{Ref RFC|1122|rp=66}}

In networks smaller than {{IPaddr||24}}, broadcast addresses do not necessarily end with 255. For example, a CIDR subnet {{IPaddr|203.0.113.16|28}} has the broadcast address {{IPaddr|203.0.113.31}}.

{|class=wikitable style=margin:left
!Type!!Binary form!!Dot-decimal notation
|-
|Network space
|<code>11001011.00000000.01110001.0001<span style=color:red>0000</span></code>
|203.0.113.16
|-
|Broadcast address
|<code>11001011.00000000.01110001.0001<span style=color:red>1111</span></code>
|203.0.113.31
|-
|colspan=3|<small>In red, is shown the host part of the IP address; the other part is the network prefix. The host gets inverted (logical NOT), but the network prefix remains intact.</small>
|}

As a special case, a {{IPaddr||31}} network has capacity for just two hosts. These networks are typically used for point-to-point connections. There is no network identifier or broadcast address for these networks.{{Ref RFC|3021}}

===Address resolution===
{{Main|Domain Name System}}
Hosts on the [[Internet]] are usually known by names, e.g., www.example.com, not primarily by their IP address, which is used for routing and network interface identification. The use of domain names requires translating, called ''resolving'', them to addresses and vice versa. This is analogous to looking up a phone number in a phone book using the recipient's name.

The translation between addresses and domain names is performed by the [[Domain Name System]] (DNS), a hierarchical, distributed naming system that allows for the subdelegation of [[namespace]]s to other DNS servers.

===Unnumbered interface===
An unnumbered [[Point-to-point (telecommunications)|point-to-point]] (PtP) link, also called a transit link, is a link that does not have an IP network or subnet number associated with it, but still has an IP address. First introduced in 1993,<ref>{{cite journal|journal=Internet Engineering Task Force|last1=Almquist|first1=Philip|last2=Kastenholz|first2=Frank|date=December 1993|title=Towards Requirements for IP Routers|url=https://datatracker.ietf.org/doc/draft-ietf-rreq-iprouters-require/00}}</ref>{{Ref RFC|1716}}{{Ref RFC|1812}}<ref>{{Cite web|title=Understanding and Configuring the ip unnumbered Command|url=https://www.cisco.com/c/en/us/support/docs/ip/hot-standby-router-protocol-hsrp/13786-20.html|access-date=2021-11-25|website=Cisco|language=en}}</ref> Phil Karn from Qualcomm is credited as the original designer.

The purpose of a transit link is to [[Router (computing)|route]] [[datagram]]s. They are used to free IP addresses from a scarce IP address space or to reduce the management of assigning IP and configuration of interfaces. Previously, every link needed to dedicate a {{IPaddr||31}} or {{IPaddr||30}} subnet using 2 or 4 IP addresses per point-to-point link. When a link is unnumbered, a ''router-id'' is used, a single IP address borrowed from a defined (normally a [[loopback]]) interface. The same ''router-id'' can be used on multiple interfaces.

One of the disadvantages of unnumbered interfaces is that it is harder to do remote testing and management.