Point 2 point IPs between ASes

William Herrin bill at herrin.us
Wed Jun 28 22:44:08 CST 2017

On Wed, Jun 28, 2017 at 5:09 PM, Thomas Bellman <bellman at nsc.liu.se> wrote:

> On 2017-06-28 17:03, William Herrin wrote:
> > The common recommendations for IPv6 point to point interface numbering
> are:
> > /64
> > /124
> > /126
> > /127
> I thought the only allowed subnet prefix lengths for IPv6 were /64 and
> /127.  RFC 4291 states:
>    For all unicast addresses, except those that start with the binary
>    value 000, Interface IDs are required to be 64 bits long and to be
>    constructed in Modified EUI-64 format.
> (and addresses starting with 000 are only used for special things,
> like the localhost address ::1).  And then RFC 6164 adds /127 to
> the allowed prefix lengths.
> I know that many devices allow you to configure any subnet size,
> but is there any RFC allowing you to use e.g. /124 or /126?

Hi Thomas,

AFAICT, the IETF has not caught up with operations practice... and
operations practice itself is still in flux. I do see some discussion of
longer-than-/64 prefixes in RFC 7421.

The difference between theory and practice? In theory, there is no

IPv6 overall is designed to support CIDR addressing at any netmask. Correct
implementations may not assume that any given interface will host a /64.
Some specific protocols (like SLAAC) intentionally do not work if the
interface ID is not exactly 64 bits. Others become more difficult than
necessary if the prefix is not on a nibble boundary (the /CIDR number is
not evenly divisible by 4).

In the mean time, the options that have come out of OPERATIONS activity for
point to point connections have converged on the above 4.

Bill Herrin

William Herrin ................ herrin at dirtside.com  bill at herrin.us
Dirtside Systems ......... Web: <http://www.dirtside.com/>

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