private ip addresses from ISP

Ivan Groenewald ivang at
Wed May 17 14:14:18 UTC 2006

What do you mean by "reaching"?

Two quick observations from a mis-configuration point of view:
If you mean you are seeing BGP routes for those networks: Sometimes ISPs
null route private addresses with static routes in their networks and they
accidentally leak (redistribute) to customers/peers. There are obviously
other reasons too, but you can filter stuff like that yourself. Just don't
accept routes for private IP space from you upstream.

If you mean you are getting traffic destined for RFC1918 space, then make
sure you aren't announcing those networks to your upstreams by accident.
Poor upstream configs/filters could allow stuff like that to escape to peers
of the upstream. (stranger things have happened)

It's not normal or necessary to see those routes or traffic. Just contact
your upstream and point it out they should fix it.

Ivan Groenewald <ivang at>
Tel: 0845 345 0919
Xtraordinary Hosting, 6 The Clocktower, South Gyle, Edinburgh, EH12 9LB

-----Original Message-----
From: owner-nanog at [mailto:owner-nanog at] On Behalf Of
adrian kok
Sent: Wednesday, May 17, 2006 2:48 PM
To: nanog at
Subject: private ip addresses from ISP

Hi all

Have you had this experience?

Our router is running BGP and connecting to our
upstream provider with /30 network.   Our log reveals
that there are private IP addresses reaching our
router's interface that is facing our upstream ISP. 
How could this be possible?  Should upstream ISP be
blocking private IP address according to standard
configuration?  Could the packet be stripped and IP be
converted somehow during the transition? It happens in
many Tier-1 ISP though !

Thank you for your information

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