AT&T normally rejects bogons such as RFC1918, urpf-detected forgeries from customers, traffic pointed at internal network routers, etc. However, AT&T's network does support MPLS, so if InsightBB is part of the Comcast cloud, it may be that this _looks_ like the public internet but is really an MPLS private network cloud that happens to use similar addresses and only reaches the Internet through gateways, in spite of being carried on much of the same hardware. Disclaimer: As a Comcast stockholder, I probably should know their architecture and whether or not InsightBB is part of their company, but all I really know about it is that cable companies have a history of doing funky things, particularly with NAT, which is one of many reasons I use DSL at home instead of cable modems. And this posting is strictly my private speculation, not my employer's. Bill Stewart