In a message written on Tue, Oct 13, 2009 at 06:26:20PM -0500, Chris Adams wrote:
The author feels that if /64 cannot be used, /112, reserving the last 16 bits for node identifiers, has probably the least amount of drawbacks (also see section 3).
I guess I'm missing something; what in section 3 is this referring to? I can understand /64 or /126 (or maybe /124 if you were going to delegate reverse DNS?), but why /112 and "16 bits for node identifiers" on a point-to-point link?
We use /112's, and do so for two (and a half) reasons: 1) If you think of all possible "network to network" interconnects they include the simple case like a single router on both ends, but they also include cases like two routers on one or both ends, and optionally with VRRP/HSRP. Maximally it appears 6 IP's may be required (two routers both ends, plus vrrp on each, statics at the VRRP). So it makes sense to have a 8 or 16 block of IP's per link so you never have to renumber the link if you switch these configurations. 2) Colon's separate 16 bit chunks in IPv6. /112's allow XXXX::1, XXXX::2 to be your IP's. The half a reason, if you have a /64 dedicate to point to point links, and use /112's, you have 2^(112-64) possible links. That's 281 trillion point to point links. Given 1, and 2, and the numbers /127's, /126's, /125's don't make any sense when you can standardize on one size fits all, and never run out. -- Leo Bicknell - bicknell@ufp.org - CCIE 3440 PGP keys at http://www.ufp.org/~bicknell/