Cogent IPv6

Chris Adams cmadams at
Thu Jun 9 03:33:29 UTC 2011

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

Chris Adams <cmadams at>
Systems and Network Administrator - HiWAAY Internet Services
I don't speak for anybody but myself - that's enough trouble.

More information about the NANOG mailing list