For those who are wondering WTH is a /31 subnet (255.255.255.254), here's some very interesting information.

Traditionally, PtP links were assigned a /30 subnet (255.255.255.252), yeilding a network address, 2 host addresses and a broadcast address. Objectively speaking, this is a 100% waste of network addresses - for every 2 host I need, I created 2 unused addresses... unless you use IP-directed broadcast, that's a different story.

The 31-bit netmask is proposed by RFC 3021. This reduces the number of wasted addresses and makes the 2 addresses in the network completely available for use by hosts and removes the network and broadcast addresses.

So what happens to broadcast? Directed broadcasts are not supported, but 255.255.255.255 broadcasts can still be accepted. Also, the network now becomes the first host addresses, e.g. 192.168.0.0/31 is the network where 192.168.0.0 and 192.168.0.1 are both useable IPs.

For the Cisco gurus, it is currently supported by Cisco devices running 12.2(T) and above. Here's how:

ip subnet-zero
interface Vlan1
 ip address 192.168.0.0 255.255.255.254

My copy of CentOS 4 running kernel 2.6.9 supports this as well.

eth0      Link encap:Ethernet  HWaddr 00:0C:29:CD:7C:00
          inet addr:192.168.0.1  Bcast:192.168.0.1  Mask:255.255.255.254

Here's the two boxes communicating...

PING 192.168.0.0 (192.168.0.0) 56(84) bytes of data.
64 bytes from 192.168.0.0: icmp_seq=0 ttl=255 time=7.02 ms
64 bytes from 192.168.0.0: icmp_seq=1 ttl=255 time=2.32 ms
64 bytes from 192.168.0.0: icmp_seq=2 ttl=255 time=3.02 ms

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.0.0     0.0.0.0         255.255.255.254 U     0      0        0 eth0

Vincent has also verified that this is completely routable in his lab router setup (via OSPF). I'm sure it would work for other routing protocols... it doesn't really matter.

However, Windows XP gives me an error indicating that there's only 1 host bit so that means it's not supported. I'm not sure if Vista or Windows 2008 supports this, maybe somebody can verify?

ISPs, why are you still wasting your limited IP address space!?