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Wikipedia states:

Unicast and anycast addresses are typically composed of two logical parts: a 64-bit network prefix used for routing, and a 64-bit interface identifier used to identify a host's network interface.

Can you use more than those 64 bits to identify a network - or is this not in compliance with any RFC? Also: Can you use more than 64 bits to identify a host?

I know that those questions are probably not of any practical use, since 2 ^ 64 addresses will be enough for most networks. I am just curious.

yas4891
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    It says **typically** - of course other prefix lengths would be as valid as well. See [RFC 4291 section 2.3](http://tools.ietf.org/html/rfc4291#section-2.3) for reference - it does not impose any limits on prefixes. – the-wabbit Dec 18 '11 at 14:34
  • @syneticon-dj thanks for your fast comment. If you turn this into an answer, I'll gladly accept it – yas4891 Dec 18 '11 at 14:42

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Some IPv6 implementations allowed netmasks to be smaller than /64, but the last I saw those died in the 1990's. The modern ones only allow /64 because of RFC and how auto-addressing works. The network part is always 64-bits. When Comcast and company get around to passing out IPv6 to customers, they'll likely be passing out a /64 when they do. When I go to my upstream and ask for an allocation I'll probably get a /48, which I'll then chop into /64's for each subnet I need.

How the addresses end up written in the network documentation is another matter though.

sysadmin1138
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  • so what you're saying is: The convention is 64 bits for interface ID and everyone expects it to be, but there is no formal document requiring this. Did I get that right? – yas4891 Dec 18 '11 at 14:45
  • /64 is what you get asking for a public assignment. However, this is a regulatory decision of the [RIRs](http://en.wikipedia.org/wiki/Regional_Internet_registry), not a technical limitation of IPv6. The "formal document" for this is the address allocation policy - like in [this example from RIPE](http://www.ripe.net/ripe/docs/ripe-538#assignment_size) – the-wabbit Dec 18 '11 at 14:50
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    @yas4891 Correct. In fact many small networks use /112 as that leaves the last hex group for the host ID (which gives somewhat familiar looking IPs for people used to /24 IPv4 subnets). – Chris S Dec 18 '11 at 15:14
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It says typically - of course other prefix lengths would be as valid as well. See RFC 4291 section 2.3 for reference - it does not impose any limits on prefixes.

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the-wabbit
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It is possible, but only recommended in special situations. One example is using a /127 on point-to-point links. I have also seen /48 prefixes on a datacenter colocation LAN, where each customer is allowed to use a /64. Weird, but it happens...

Sander Steffann
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