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 - [email protected] - CCIE 3440
PGP keys at http://www.ufp.org/~bicknell/
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