Re: Choice of network space when numbering interfaces with IPv6

[Nick Hilliard <nick () foobar org>]

On 15/10/2010 20:26, Zaid Ali wrote:
> SO I have been turning up v6 with multiple providers now and notice that
> some choose /64 for numbering interfaces but one I came across use a /126. A
> /126 is awfully large (for interface numbering) and I am curious if there is
> some rationale behind using a /126 instead of a /64.

There are 4 general choices of netmask for ipv6 point to point interface
numbering schemes:

/64: the default ipv4 subnet.  many people feel that this is a waste of
addressing space.  others feel that there is so much ipv6 address space out
there that it's simpler to keep all interfaces on /64.

/112: /112 is 16-bit aligned, which means that it's easy to read because 16
bits implies that the last 4 octets are link-specific.  Also, your entire
PoP point-to-point addressing scheme can be held within a single /64, which
means good address conservation

/126: this is the same as we use in ipv4: it's less easy to read, as the
link-specific digits are not octet-aligned.  Your entire PoP point-to-point
addressing scheme can be held within a single /64, which means good address
conservation

/127: this is used on POS links where there is no link-layer address
resolution protocol available (like ARP/ND) and consequently you can end up
with unknown traffic looping between each side if you're not careful.

None of these is the right or the wrong approach, unless you're using
POS/STM.  Otherwise all of them have their merits and demerits.

Nick