Firewall Help - IP Spoof

[Slap]

I keep getting the following message in my firewall log. How can I stop this?

12/10/2001 13:15:19.112 - IP spoof detected - Source:223.1.1.128, 137, LAN - Destination:193.255.1.1, 137, WAN - MAC address: 00.10.A4.A2.81.E7 -