routing invalid IP addresses

We had an attack here last night and the attack traffic was coming from an IP address of x.x.255.x which isn't a valid IP address yet the traffic was being routed over the internet (as far as I can tell anyway). When I attempted to track down the source I found our cisco routers wouldn't accept the address as valid so it was not possible to null route or trace the traffic. Has anyone else ever seen this before? Clue me in? Geo.