matthew@crocker.com disait :
Search the archives, Comcast and other cable/DSL providers use the 10/8 for their infrastructure. The Internet itself doesn't need to be Internet routable. Only the edges need to be routable. It is common practice to use RFC1918 address space inside the network. Companies like Sprint and Verio use 'real' IPs but don't announce them to their peers on customer edge routes.
Are you sure about Sprint ? I was told that Sprint DOES announce edge blocks to peers/custom (For URPF i guess) but blackholes this block at the edge. Thus you can still traceroute the IP up to Sprint edge, but cannot get into Sprint network. This is a hot issue for Opentransit since we are considering not announcing some infrastructure blocks. I think that Sprint way is rather smart : . It prevent/mitigate infrastructure DDOS . It keeps working with URPF enable peers. Vincent, Opentransit - France Telecom