On Wed, Jun 08, 2011 at 10:33:29PM -0500, Chris Adams wrote:
Once upon a time, William Herrin <bill@herrin.us> 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 http://tools.ietf.org/html/rfc3627 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: http://tools.ietf.org/html/rfc5375 IPv6 Address Assignment to End Sites: http://tools.ietf.org/html/rfc6177 Emerging Service Provider Scenarios for IPv6 Deployment: http://tools.ietf.org/html/rfc6036 IPv6 Optimal Address Plan and Allocation Tool: http://www.ipv6book.ca/allocation.html ARIN Wiki: http://www.getipv6.info/index.php/IPv6_Addressing_Plans (but some of the ARIN-related concepts here are obsolete, such as references to the HD Ratio and non-nibble-boundary allocations)