This totally sounds like a bug to me, because my subnet mask and IP address should be valid by all accounts.

My ISP has assigned me a fixed IP address of 65.110.7.20, with a subnet mask of 255.255.255.252. Which sounds about right, because we're trying to set up a router with a /31 network - just 4 ip addresses.

But when I try to set this IP and netmask up in Windows XP, I get the following error:

"The combination of IP address and subnet mask are invalid. All of the bits in the host address portion of the IP address are set to 0."

But they're not all set to 0. That would be a subnet mask of 255.255.255.255. Even then, that too is a valid subnet mask, consisting of a network of 1 IP address.

Is there a way around this? A registry hack maybe?