ISPs Blocking Private Addresses?

Mon Feb 9 03:19:44 UTC 1998

Peter,

> Do most ISPs explicitly block private IP addresses (e.g. 10.X.X.X) at their
> borders?
> 
> Do the "default-less" ISPs filter private addresses or do they let
> routing/forwarding do the work?

More clueful ISPs do in fact block private address space exchange via BGP.
Many also filter. Smaller ISPs tend to miss these steps, and there is
frequently someone advertising private space at the NAPs from time to
time. And some believe it, surprisingly. 

Many cable networks, like Rogers and @Home use net 10 for numbering
internal devices for SNMP survellience, like modems and switches. While
INTERNIC may not mandate it, they are sensitive to 2 IP addresses per
household drawn from the public space. As such, when @Home and Rogers
merged networks, there was some private address space reconcilication
required. Thus, you cannot assume that private address space is not routed
"privately" within a given ISP's backbone. Needless to say, they are
usually very vigilant about route and packet filtering.

Regards,

Eric Carroll			eric.carroll at acm.org
Tekton Internet Associates