There's a common misconception reflected here that I wanted to correct. I
don't have nanog-post, so I apologize if its not appropriate to reply
directly. You may repost my comments if you'd like.
Comcast and many others seem to
blithely ignore this for convenience sake. (It's not like they need a
huge amount of space to give private addresses to these links.)
ARIN required cable operators to use RFC 1918 space for the management
agents of the bridge cable modems that have been rolled out to the millions
of residential cable modem customers. Doing so obviously requires a 1918
address on the cable router, but Cisco's implementation requires that
address to be the primary interface address. There is also a publicly
routable secondary which in fact is the gateway address to the customer, but
isn't the address returned in a traceroute. Cisco has by far the lead in
market share of the first gen Docsis cable modem router market so any trace
to a cable modem customer is going to show this.
In fact, Comcast and others _do_ need a huge amount of private IP space
because of this. We didn't "blithely ignore" the RFC, but didn't have a
choice in implementation. Perhaps Cisco will improve their implementation
for the next round of CMTS development...
Filtering of RFC 1918 space by cable ISPs is of course another topic.
-Doug-