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.An compliant subnet always uses IPv6 addresses with 64 bits for the host portion. It therefore has a /64 routing prefix (128−64 = the 64 most significant bits). Although it is technically possible to use smaller subnets, they are impractical for local area networks based on Ethernet technology, because 64 bits are required for stateless address auto configuration. The Internet Engineering Task Force recommends the use of /127 subnets for point-to-point links, which consist of only two hosts.IPv6 does not implement special address formats for broadcast traffic or network numbers, and thus all addresses in a subnet are valid host addresses. The all-zeroes address is reserved as the Subnet-Router anycast address.The recommended allocation for an IPv6 customer site was an address space with an 48-bit (/48) prefix. However, this recommendation was revised to encourage smaller blocks, for example using 56-bit prefixes. Another common allocation is a /64 prefix for a residential customer network.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.
This IPv6 subnetting reference lists the sizes for IPv6 computer networks. Different types of network links may require different subnet sizes.[1] The CIDR netmask separates the bits of the network identifier prefix from the bits of the interface identifier. Selecting a smaller prefix size results in fewer number of networks covered, but with more addresses within those networks.
CIDR Prefixes.
2001:0db8:0123:4567:89ab:cdef:1234:5678
|||| |||| |||| |||| |||| |||| |||| ||||
|||| |||| |||| |||| |||| |||| |||| |||| Single end-points and loopback
|||| |||| |||| |||| |||| |||| |||| |||128 Point-to-point links (inter-router)
|||| |||| |||| |||| |||| |||| |||| ||124
|||| |||| |||| |||| |||| |||| |||| |120
|||| |||| |||| |||| |||| |||| |||| 116
|||| |||| |||| |||| |||| |||| |||112
|||| |||| |||| |||| |||| |||| ||108
|||| |||| |||| |||| |||| |||| |104
|||| |||| |||| |||| |||| |||| 100
|||| |||| |||| |||| |||| |||96
|||| |||| |||| |||| |||| ||92
|||| |||| |||| |||| |||| |88
|||| |||| |||| |||| |||| 84
|||| |||| |||| |||| |||80
|||| |||| |||| |||| ||76
|||| |||| |||| |||| |72
|||| |||| |||| |||| 68
|||| |||| |||| |||64 Single End-user LAN (default prefix size for SLAAC)
|||| |||| |||| ||60 Some (very limited) 6rd deployments (/60 = 16 /64)
|||| |||| |||| |56 Minimal end sites assignment (e.g. Home network) (/56 = 256 /64)
|||| |||| |||| 52 (/52 = 4096 /64)
|||| |||| |||48 Typical assignment for larger sites (/48 = 65536 /64)
|||| |||| ||44
|||| |||| |40
|||| |||| 36 possible future Local Internet registry extra-small allocations
|||| |||32 Local Internet registry minimum allocations
|||| ||28 Local Internet registry medium allocations
|||| |24 Local Internet registry large allocations
|||| 20 Local Internet registry extra large allocations
|||16
||12 Regional Internet Registry allocations from IANA
|8
4