Could it be your cable modem?
I got this on my log but do not know where or who is it from. Does any one know this IP address?
[Internet connected] IP address: 192.168.100.10, Tuesday, Jan 31,2012 03:12:37
It is from "inside" your network. 192.168... is a Class-C IP address, and is not routable. Check your local wireless security and encryption settings.
What is the IP address of LAN interface of your router ? Usually it is 192.168.1.1. But I guess in your case it is 192.168.100.1 and DHCP server is assigning 192.168.100.10 to you LAN machine.