I regularly see questions about IPv6 subnetting (despite the fact that the answers are out there with a simple Google search). ‘How do you subnet in IPv6?’ Well, there are some general guidelines that can help you a long way.

An IPv6 address is 128 bits, so prefix size can vary between /0 and /128. I’m going to list prefix sizes and their meanings below here, but remember, these are guidelines, and in no way mandatory. If you want to make a /103 subnet, you can do that.

  • A /64 is considered a basic subnet in IPv6. Reason for this is Stateless Address Autoconfiguration, or SLAAC (covered in RFC 4291). This is a method in which an IPv6 host forms his own address using the MAC address and the /64 prefix advertised by a router in the broadcast domain. It only works on /64, hence why you will usually see a lot of /64 subnets in an IPv6 network. Visually they are easy too: the first four sedets (there’s no definitive naming convention how to call a group of 16 bits in an IPv6 address) are the network part, the last four sedets the host part.
  • A /127 is a point-to-point link, as IPv6 has no concept of network and broadcast addresses. While many use a /64 on point-to-point links as well so the routing table looks simple, a /127 can more secure to work with.
  • A standard network with subnets is a /56, which gives 256 subnets (eight bits), each a /64. This is usually enough for a small to even medium network and should be considered a minimum assigned address space by an ISP for one connection (RFC 6177 refers to this best-practice).
  • A more common assignment by a provider, and a former best-practice, is a /48. It makes the visual part of addressing easy again: the first three sedets are the globally routed prefix, the fourth sedet is used for internal subnetting, and the remaining four sedets are again the host part for the /64 subnets. A /48 prefix is the smallest prefix allowed by most carriers in a global BGP routing table. It can also be seen in large companies that efficiently use route summarization.
  • Anything bigger, /44, /40, /32, are globally assigned IPv6 address spaces to companies with their own Autonomous System number, just like IPv4 global address space. You’re not likely to see this outside a BGP routing table.
  • A /16 is only used so far for 2002::/16 (6to4 addresses), just like a FE80::/10 (link-local),  FF00::/8 (multicast) and FC00::/7 (unique-local) addresses. None of these should ever appear in a routing table.
  • Finally a /0 refers to ::/0, which is a default route, just like IPv4’s 0.0.0.0/0 route.

As you may have noticed by now, subnetting in IPv6 is done for efficient route summarization, not conserving address space. Why would you? This is why a /103 subnet would not be necessary, although together with DHCPv6, you can perfectly make such a subnet work.

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