5. Subnets for other purposes
- Router loopback addresses - /128s
- Central servers (data centre networks) - /64s
- Location independent device networks (e.g. wireless) - /64s
- Transfer Networks (aka linknets or transfernets) - /64s
There are varying opinions on the correct size of a Transfer Network (what is typically a /30 router-to-router link under IPv4). For technical reasons the "obvious" choice of /127 is actually not usable, nor in fact is anything smaller than a /120 (see RFC 3627).
The three common choices are /126, /112 and /64. JANET-UK are using /126 transfernets to connect to sites, as are other ISPs such as clara.net. RFC 3627 gives reasons why /126 and /112 transfernets may be at best fiddly to manage, or worse, break certain applications.
Although the /64 is seen as wasteful, it's extremely unlikely that we will encounter a router in use on a department/college network which will not support that subnet size, as it's the RFC defined default. Masks other than /64 are not common, so may not be supported by naive hardware vendors. However, /112 is now becoming more popular, and so it's possible to assign over one /64 to contain 2^32 transfernets of size /112. Only bits 81 through 112 can be used, 65 through 80 will need to remain at zero (for reasons covered in RFC 3627).
One recent finding is that access control lists on some switches, implemented in hardware, often are limited to matching addresses of either /64 or /128 in size, or some fixed point in between such as /80. The technical reason is that these devices allocate TCAM memory in chunks suitable for IPv4 addresses and so supporting IPv6 addresses cannot be done with full bitmask flexibility. Implementing infrastructure access lists (as per BCP 38) might be awkward with /112 masks on such platforms.