Editing Subnet (section)

==Internet Protocol version 6==
<!--[[IPv6 subnetting]] redirects here-->
{{see also|IPv6 subnetting reference}}
The design of the [[IPv6]] address space differs significantly from IPv4. The primary reason for subnetting in IPv4 is to improve efficiency in the utilization of the relatively small address space available, particularly to enterprises. No such limitations exist in IPv6, as the large address space available, even to end-users, is not a limiting factor.

As in IPv4, subnetting in IPv6 is based on the concepts of variable-length subnet masking (VLSM) and the [[Classless Inter-Domain Routing]] methodology. It is used to route traffic between the global allocation spaces and within customer networks between subnets and the Internet at large.

A compliant IPv6 subnet always uses addresses with 64 bits in the host identifier.<ref name="rfc4291-2-5-1"/> Given the address size of 128 bits, it therefore has a /64 routing prefix. Although it is technically possible to use smaller subnets,<ref name="rfc4862"/> they are impractical for local area networks based on Ethernet technology, because 64 bits are required for [[stateless address autoconfiguration]].<ref name="rfc2464"/> The [[Internet Engineering Task Force]] recommends the use of {{IPaddr||127}} subnets for point-to-point links, which have only two hosts.<ref name="rfc6164"/><ref name="rfc6547"/>

IPv6 does not implement special address formats for broadcast traffic or network numbers,<ref name="rfc4291-2"/> and thus all addresses in a subnet are acceptable for host addressing. The all-zeroes address is reserved as the subnet-router anycast address.<ref name="rfc4291-2-6-1"/> The subnet router anycast address is the lowest address in the subnet, so it looks like the “network address”. If a router has multiple subnets on the same link, then it has multiple subnet router anycast addresses on that link.<ref>{{cite web | url=https://into6.com.au/2014/03/30/subnet-router-anycast-addresses-what-are-they-how-do-they-work/ | title=Subnet Router Anycast Addresses – what are they, how do they work? – Into6 | date=30 March 2014 | access-date=2022-06-09 | archive-date=2022-07-02 | archive-url=https://web.archive.org/web/20220702054220/https://into6.com.au/2014/03/30/subnet-router-anycast-addresses-what-are-they-how-do-they-work/ | url-status=live }}</ref> The first and last address in any network or subnet is not allowed to be assigned to any individual host.

In the past, the recommended allocation for an IPv6 customer site was an address space with a 48-bit ({{IPaddr||48}}) prefix.<ref name="ARIN"/> However, this recommendation was revised to encourage smaller blocks, for example using 56-bit prefixes.<ref name="RFC 6177"/> Another common allocation size for residential customer networks has a 64-bit prefix.