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| In the [[Internet]] addressing architecture, a '''private network''' is a [[Computer network|network]] that uses private [[IP address]] space, following the standards set by RFC 1918 for IPv4 and RFC 4193 for IPv6. These addresses are commonly used for home, office, and enterprise [[local area network]]s (LANs), when globally routable addresses are not mandatory, or are not available for the intended network applications. Under [[Internet Protocol]] [[IPv4]], private IP address spaces were originally defined in an effort to delay [[IPv4 address exhaustion]], but they are also a feature of the next generation Internet Protocol, [[IPv6]].
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| These addresses are characterized as private because they are not globally delegated, meaning they are not allocated to any specific organization, and IP packets addressed by them cannot be transmitted onto the public Internet. Anyone may use these addresses without approval from a [[regional Internet registry]] (RIR). If such a private network needs to connect to the Internet, it must use either a [[network address translator]] (NAT) gateway, or a [[proxy server]].
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| ==Private IPv4 address spaces==
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| The [[Internet Engineering Task Force]] (IETF) has directed the [[Internet Assigned Numbers Authority]] (IANA) to reserve the following IPv4 address ranges for private networks, as published in RFC 1918:<ref>{{cite web
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| | title = RFC 1918: Address Allocation for Private Internets | url = http://tools.ietf.org/html/rfc1918#page-4
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| | page = 4 | publisher = IETF | date = February 1996}}</ref>
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| {| class="wikitable"
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| ! RFC1918 name !! IP address range !! number of addresses
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| ! ''[[classful network|classful]]'' description !! largest [[Classless Inter-Domain Routing|CIDR]] block (subnet mask) || host id size || mask bits
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| |-
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| | 24-bit block || 10.0.0.0 - 10.255.255.255 || 16,777,216 || single [[class A network]]|| 10.0.0.0/8 (255.0.0.0) || 24 bits || 8 bits
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| | 20-bit block || 172.16.0.0 - 172.31.255.255 || 1,048,576 || 16 contiguous class B networks || 172.16.0.0/12 (255.240.0.0) || 20 bits || 12 bits
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| | 16-bit block || 192.168.0.0 - 192.168.255.255 || 65,536 || 256 contiguous class C networks || 192.168.0.0/16 (255.255.0.0) || 16 bits || 16 bits
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| |}
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| [[Classful addressing]] is obsolete and has not been used in the Internet since the implementation of [[Classless Inter-Domain Routing]] (CIDR), starting in 1993. For example, while 10.0.0.0/8 was a single class A network, it is common for organizations to divide it into smaller /16 or /24 networks. Contrary to a common misconception, a /16 subnet of a class A network is not referred to as a class B network. Likewise, a /24 subnet of a class A or B network is not referred to as a class C network. The class is determined by the first three bits of the prefix.<ref>{{cite book|last=Forouzan|first=Behrouz|title=Data Communications and Networking|year=2013|publisher=McGraw Hill|location= New York|isbn= 978-0-07-337622-6|pages= 530–31}}</ref>
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| ===Dedicated space for Carrier Grade NAT deployments===
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| In April 2012, IANA allocated 100.64.0.0/10 for use in [[carrier grade NAT]] scenarios in RFC 6598.<ref name=rfc6598>{{cite web | title = RFC 6598: Reserved IPv4 Prefix for Shared Address Space | url = http://tools.ietf.org/html/rfc6598#page-8 | page = 8 | publisher = IETF | date = April 2011}}</ref> This address block should not be used either on private networks or on the public Internet: it is intended only for use within the internal operations of carrier networks. The size of the address block (<math>2^{22}</math>, approximately 4 million, addresses) was selected to be large enough to uniquely number all customer access devices for all of a single operator's [[points of presence]] in a large metropolitan area such as the [[Tokyo]] metropolitan area.<ref name=rfc6598/>
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| ==Private IPv6 addresses==
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| {{Main|Unique local address}}
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| The concept of private networks and special address reservation for such networks has been carried over to the next generation of the [[Internet Protocol]], [[IPv6]].
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| The address block <tt>fc00::/7</tt> has been reserved by IANA as described in RFC 4193. These addresses are called [[Unique Local Address]]es (ULA). They are defined as being [[unicast]] in character and contain a 40-bit random number in the routing prefix to prevent collisions when two private networks are interconnected. Despite being inherently ''local'' in usage, the [[IPv6 address#IPv6 address scopes|IPv6 address scope]] of unique local addresses is global.
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| A former standard proposed the use of so-called "site-local" addresses in the <tt>fec0::/10</tt> range, but due to major concerns about scalability and the poor definition of what constitutes a ''site'', its use has been deprecated since September 2004 by RFC 3879.
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| ==Link-local addresses==
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| {{main|Zero configuration networking}}
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| {{see also|Link-local address}}
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| Another type of private networking uses the ''link-local address range''. The validity of link-local addresses is limited to a single ''link''; e.g. to all computers connected to a [[Network switch|switch]], or to one [[wireless network]]. Hosts on different sides of a [[Network bridge|bridge]] are also on the same link, whereas hosts on different sides of a [[Network router|router]] are on different links.
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| ===IPv4===
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| In IPv4, link-local addresses are codified in RFC 5735 and RFC 3927. Their utility is in self-autoconfiguration by network devices when [[Dynamic Host Configuration Protocol]] (DHCP) services are not available and manual configuration by a network administrator is not desirable.
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| The block 169.254.0.0/16 is reserved for this purpose, with the exception of the first and the last /24 subnets in the range. If a host on an IEEE 802 ([[ethernet]]) network cannot obtain a network address via DHCP, an address from 169.254.1.0 to 169.254.254.255 may be assigned [[pseudorandom]]ly. The standard prescribes that address collisions must be handled gracefully.
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| ===IPv6===
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| In IPv6, link-local addresses are codified in RFC 4862. Their use is mandatory, and an integral part of the IPv6 standard.
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| The IPv6 addressing architecture (RFC 4291) sets aside the block <tt>fe80::/10</tt> for IP address autoconfiguration.
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| ==Common uses==
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| The most common use of private addresses is in residential networks, since most [[Internet service provider]]s (ISPs) only allocate a single publicly [[routing|routable]] IP address to each residential customer, but many homes have more than one [[computer]] or other Internet connected device, such as televisions. In this situation, a [[network address translation|network address translator]] (NAT/PAT) gateway is usually used to provide Internet connectivity to multiple hosts.
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| Private addresses are also commonly used in [[Campus network|corporate network]]s, which for security reasons, are not connected directly to the Internet. Often a proxy, [[SOCKS]] gateway, or similar devices are used to provide restricted Internet access to network-internal users.
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| In both cases, private addresses are often seen as enhancing [[network security]] for the internal network, since it is difficult for an Internet host to connect directly to an internal system.
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| ==Misrouting==
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| It is common for packets originating in private address spaces to be misrouted onto the Internet. Private networks often do not properly configure DNS services for addresses used internally and attempt [[reverse DNS lookup]]s for these addresses, causing extra traffic to the Internet [[root nameservers]]. The [[AS112]] project attempted to mitigate this load by providing special ''blackhole'' [[anycast]] nameservers for private address ranges which only return negative result codes (''not found'') for these queries.
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| Organizational edge routers are usually configured to drop ingress IP traffic for these networks, which can occur either by misconfiguration, or from malicious traffic using a spoofed source address. Less commonly, ISP edge routers drop such egress traffic from customers, which reduces the impact to the Internet of such misconfigured or malicious hosts on the customer's network.
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| ==Merging private networks==
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| Since the private IPv4 address space is relatively small, many private IPv4 networks use the same address space. This creates a common problem when merging such networks, namely the duplication of addresses on multiple devices. In this case, networks or hosts must be renumbered, often a time-consuming task, or a [[network address translation|network address translator]] must be placed between the networks to masquerade the duplicated addresses.
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| To mitigate this problem for IPv6, RFC 4193 specifies a large (40-bit) unique Global ID to be pseudo-randomly generated by each organization using Unique Global Addresses. It is very unlikely that two network addresses generated in this way will be the same.
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| ==Private use of other reserved addresses==
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| Historically address blocks other than the private address ranges have been reserved for potential future uses. Some organizations have used them for private networking applications despite official warnings of possible future address collisions. Typically these addresses are not referred to as "reserved." IPv4 addresses 240.0.0.0 to 254.255.255.254 (all addresses in 240.0.0.0/4 except 255.0.0.0/8) are designated for future use and research and development.
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| ==See also==
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| * [[Heartbeat network]]
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| * [[Intranet]], a private internet
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| * [[Reserved IP addresses]]
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| ==RFC References==
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| * RFC 1918 – ''"Address Allocation for Private Internets"''
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| * RFC 2036 – ''"Observations on the use of Components of the Class A Address Space within the Internet"''
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| * RFC 2050 – ''"Internet Registry IP Allocation Guidelines"''
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| * RFC 2101 – ''"IPv4 Address Behaviour Today"''
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| * RFC 2663 – ''"IP Network Address Translator (NAT) Terminology and Considerations"''
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| * RFC 3022 – ''"Traditional IP Network Address Translator (Traditional NAT)"''
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| * RFC 3330 – ''"Special-Use IPv4 Addresses"'' (superseded)
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| * RFC 5735 – ''"Special-Use IPv4 Addresses"''
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| * RFC 3879 – ''"Deprecating Site Local Addresses"''
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| * RFC 3927 – ''"Dynamic Configuration of IPv4 Link-Local Addresses"''
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| * RFC 4193 – ''"Unique Local IPv6 Unicast Addresses"''
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| * RFC 6598 – ''"Reserved IPv4 Prefix for Shared Address Space"''
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| ==References==
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| {{reflist}}
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| {{DEFAULTSORT:Private Network}}
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| [[Category:Internet architecture]]
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| [[Category:Network addressing]]
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