North American Network Operators Group

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Re: Strange public traceroutes return private RFC1918 addresses

  • From: Bob Snyder
  • Date: Mon Feb 02 20:44:24 2004

Matthew Crocker wrote:

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.
Which (as discussed previously) breaks things like Path MTU Discovery, traceroute, and other things that depend on the router sending back ICMP packets to the sender if any ISP along the return path (properly) filters RFC1918 address space as being bogus. You can use RFC1918 space on any device that really has no need to communicate with the outside world, but generally, un-NAT'ed routers don't qualify for this, at least on their transit interfaces.

I believe Comcast (and I'm going only on my experience as a customer) is or has moved from RFC1918 space to routable IP space for their routers, at least on interfaces I've been doing traceroutes through.