[nsp] using /31s

[Stephen J. Wilcox]

> > > I'm missing a nugget of clue -
> > > 
> > > edge2(config)#interface Serial6/0/4:0
> > > edge2(config-if)#ip address 216.23.183.248 255.255.255.254
> > > Bad mask /31 for address 216.23.183.248
> > > edge2(config-if)#ip address 216.23.183.249 255.255.255.254
> > > Bad mask /31 for address 216.23.183.249
> > > edge2(config-if)#ip address 216.23.183.250 255.255.255.254
> > > Bad mask /31 for address 216.23.183.250
> > > edge2(config-if)#ip address 216.23.183.251 255.255.255.254
> > > Bad mask /31 for address 216.23.183.251
> > > 
> > > 216.23.183.0/24 is VSLM'd but has mostly /30s.  I have essentially two /30s
> > > left (216.23.183.248/30, 216.23.183.252/30) and figured I'd double that with
> > > /31s but I'm having issues with IOS accepting it.
> > 
> > I am not sure if you are being serious here, but a /31 is an invalid
> > netmask.
> > 
> > It will only give you the network and broadcast, no actually usable IP
> > Addresses.
> 
> Jeff is right.  The only time I ever use /31's is to reference ip pairs 
> in filtering/acl applications.  This is really the only realm in
> which they are valid.  Try 'ip unnumbered' if you are getting
> desperate for space.

Erm, /31s are acceptable on p2p links, ip unnumbered is often not desirable. 

Your issue may be IOS, this wasnt always supported (in fact its relatively 
recent). You should check the cisco tac pages and find out if you are using a 
supported IOS..

Steve