Using IPv6 with prefixes shorter than a /64 on a LAN

George Bonser gbonser at
Sun Jan 30 13:25:26 CST 2011

> In any case, the fact you can stick a terabyte of RAM into a 4U Dell
> rack mount that sucks a whole lot of power doesn't mean we're anywhere
> near being able to do it for consumer-class hardware.  Remember, much
> of the growth is going to be in the embedded and special purpose
> systems - the smart phone/PDA/handheld game system arena, etc.  How
> many fully loaded R910's will Dell sell, and how many iPhones will
> Apple sell? How long before a Blackberry or an iPhone has a terabyte
> RAM? (For that matter, when will they get to a terabyte of SSD
> capacity?)

There are other reasons to use a /64.  Some vendors have the option of
storing either the /64 prefix or the entire /128 in CAM.  The option to
select one mode or the other is often not specific to a route but is a
global option.  If you want hardware switching of nets smaller than /64,
you need to enable the mode that places the entire address in CAM.
Doing that results in an increase in resources required to hold routes
resulting in fewer routes in hardware at any given time.

So hardware that, by default, places the entire address in CAM is fine,
but if you have the option of using only the /64 prefix then an IPv6
route becomes less expensive (only 2x the "cost" of a v4 route in
resources rather than 4x the cost).

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