Cogent IPv6

William Herrin bill at
Thu Jun 9 02:24:31 UTC 2011

On Wed, Jun 8, 2011 at 9:58 PM, Kelly Setzer <Kelly.Setzer at> wrote:
> IPv6 newbie alert!
> I thought the maximum prefix length for IPv6 was 64 bits,
> so the comment about a v6 /112 for peering vexed me.  I
> have Googled so much that Larry Page called me and
> asked me to stop.
> Can someone please point me to a resource that explains
>how IPv6 subnets larger than 64 bits function and how
>they would typically be used?

Hi Kelly,

IPv6 netmasks work exactly like IPv4 netmasks. You can even route
/128's if you want. Two major caveats:

1. SLAAC (stateless autoconfiguration, the more or less replacement
for DHCP) only works if the subnet on your LAN is exactly /64. So
unless you're manually configuring the IPv6 address on every machine
on your subnet, you're using a /64.

2. Reverse DNS delegates every 4 bits (in IPv4 its every 8 bits). And
when you write the address, every 4 bits is one digit. So unless you
want to make things needlessly hard, you're also going to choose 4-bit
boundaries for everything. I.e. a /56 or a /60 but never a /57.

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.

Bill Herrin

William D. Herrin ................ herrin at  bill at
3005 Crane Dr. ...................... Web: <>
Falls Church, VA 22042-3004

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