Cogent IPv6

Chuck Anderson cra at WPI.EDU
Thu Jun 9 12:01:47 UTC 2011

On Wed, Jun 08, 2011 at 10:33:29PM -0500, Chris Adams wrote:
> Once upon a time, William Herrin <bill at> said:
> > Now, as to why they'd choose a /112 (65k addresses) for the interface
> > between customer and ISP, that's a complete mystery to me.
> I had to ask this here a while back, so I can now share. :-)
> IPv6 addresses are written as 8 16-bit chunk separated by colons
> (optionally with the longest consecutive set of :0 sections replaced
> with ::).  A /112 means the prefix is 7 of the 8 chunks, which means you
> can use ::1 and ::2 for every connection.
> Of course, just because you allocate a /112 (or shorter) in your
> database doesn't mean you have to use it.  You could also allocate a
> /112 for a point-to-point link and use a /127 (e.g. addresses ::a and
> ::b).

Please don't use /127:

Use of /127 Prefix Length Between Routers Considered Harmful

More below on use of various prefix lengths.  You need to watch out
for the EUI-64 'u' and 'g' bits, as well as subnet anycast addresses
(top 127 addresses of every subnet):

IPv6 Addressing Considerations:

IPv6 Address Assignment to End Sites:

Emerging Service Provider Scenarios for IPv6 Deployment:

IPv6 Optimal Address Plan and Allocation Tool:

ARIN Wiki:
(but some of the ARIN-related concepts here are obsolete, such as
references to the HD Ratio and non-nibble-boundary allocations)

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