On 15/10/2010 20:26, Zaid Ali wrote: > SO I have been turning up v6 with multiple providers now and notice that > some choose /64 for numbering interfaces but one I came across use a /126. A > /126 is awfully large (for interface numbering) and I am curious if there is > some rationale behind using a /126 instead of a /64.
There are 4 general choices of netmask for ipv6 point to point interface numbering schemes: /64: the default ipv4 subnet. many people feel that this is a waste of addressing space. others feel that there is so much ipv6 address space out there that it's simpler to keep all interfaces on /64. /112: /112 is 16-bit aligned, which means that it's easy to read because 16 bits implies that the last 4 octets are link-specific. Also, your entire PoP point-to-point addressing scheme can be held within a single /64, which means good address conservation /126: this is the same as we use in ipv4: it's less easy to read, as the link-specific digits are not octet-aligned. Your entire PoP point-to-point addressing scheme can be held within a single /64, which means good address conservation /127: this is used on POS links where there is no link-layer address resolution protocol available (like ARP/ND) and consequently you can end up with unknown traffic looping between each side if you're not careful. None of these is the right or the wrong approach, unless you're using POS/STM. Otherwise all of them have their merits and demerits. Nick
