Using /31 for router links

Nathan Ward nanog at
Sat Jan 23 04:11:12 UTC 2010

On 23/01/2010, at 1:31 PM, Jay Nugent wrote:

> Greetings,
> On Fri, 22 Jan 2010, Seth Mattinen wrote:
>> In the past I've always used /30's for PTP connection subnets out of old 
>> habit (i.e. Ethernet that won't take unnumbered) but now I'm considering 
>> switching to /31's in order to stretch my IPv4 space further. Has anyone 
>> else does this? Good? Bad? Based on the bit of testing I've done this 
>> shouldn't be a problem since it's only between routers.
>   Yes, this *IS* done *ALL* the time.  P-t-P means that there are ONLY
> two devices on the wire - hence point to point.  It ONLY uses two IP
> addresses (one on each end) and there is no reason or need to ARP on this
> wire.  So no need for a broadcast or network addresses - it is just the
> two end points.

ARP is still required on ethernet links, so that the MAC address can be discovered for use in the ethernet frame header. /31 does not change the behavior of ARP at all.

Nathan Ward

More information about the NANOG mailing list