31-Bit IPv4 Prefixes

Sten Daniel Soersdal netslists at gmail.com
Sat Jul 28 16:34:01 UTC 2007

Lee Johnston wrote:
> Hi all,
> We've just been allocated a /31 for a point-to-point over ethernet. Any 
> idea if FreeBSD supports 31-bit prefixes as per RFC 3021? All I'm 
> getting are:
> arp: 00:02:4a:9c:b8:81 attempts to modify permanent entry for 
> xx.xx.xx.xx on em2.3211

It is likely that the /31 is not routed through your pppoe tunnel but 
instead assigned to your pppoe tunnel. Probably they expect you to use 
those two addresses in combination with NAT.
Routed operation is usually only possible (without trickery) on /30 or 
larger. But perhaps your /31 falls neatly within a /30 and they did not 
route the network/broadcast address?
Why not just ask your provider how you were thought to use these 
addresses? You are the customer, you cannot be expected to be the 
routing wiz on *their* network. I always ask anyway, the explanations 
are sometimes colorful but often i hear "oh, wait a minute, this doesn't 
make sense! hang on...".

Sten Daniel Soersdal

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