I'm doing some research on users connected to Internet via cable operators and I've seen some unusual route traces.

A bit about setup:

Basically user has a home router connected to a DOCSIS modem. The router seems to receive a public IP address from the modem for its WAN interface using DHCP, but when traceroute is done, there is a number of routers in the path which have private IP addresses.

Example traceroute output:

Tracing route to google.com []
over a maximum of 30 hops:

  1     3 ms     1 ms     1 ms router has public WAN address
  2    10 ms    12 ms     8 ms
  3    11 ms    19 ms     9 ms  bg-he-m-1-pc2.sbb.rs []
  4    12 ms    11 ms    11 ms  bg-he-m-1-pc2.sbb.rs []
  5    12 ms    11 ms    12 ms  cache.google.com []

Trace complete.

I've seen some examples on other providers as well which use the addresses. The private addresses are always right after the user's CPE.

Some of my thoughts on this:

As far as I understand it, it shouldn't be possible to route private addresses over public Internet and all routers on the Internet should drop packets with private IP address destinations. I'm aware that the routers with private addresses here are in ISP's network, but I still don't understand how this all works.

I expect that there is some sort of tunneling involved, but I believe that in that case only tunnel endpoints would be visible. The other option would be carrier grade NAT. I'm not sure how that would look like. From what I've heard (but I could be wrong), in CGN systems, users usually can't directly access the Internet and can't be directly accessed from the Internet. In setup I've seen, I can connect to user from the Internet using its IP address normally.

After this rather long exposition, I've come to point where I should present the actual question:

I'd like to know which technology is used here to enable such a setup? A little bit about architecture and some keywords for Google would be helpful as well.

2 Answers 2


There is no tunneling. This is completely normal. With IP routing you are concerned about the source and destination address which doesn't change unless something is NAT'd. If a router in the middle has an RFC1918 address that's fine as it can still forward transit traffic as again the source and destination is unchanged.

Each router will simply reply to the source with their own directly connected interface address. The above is not being routed over the internet, it's simply the device that is attached to the CPE on the WAN side.

There is no magic technology being used. It's pure standard routing.


The distinction you need to draw here is that your ISP is not the internet. Any traffic leaving your ISP to the internet at large or any traffic being received by your ISP from other entities on the internet cannot use these IP addresses.

However, your ISP is well within their rights to use these IP addresses however they see fit internally on their own network. It is only at the edge of their network that they really need to worry about anything like NAT (if necessary).

Many large, older ISPs will use public IP addresses on all their router interfaces, however many smaller/newer ISPs have found it much more difficult to get IP space and find they don't have enough available to use in this way.

You may very well have a public IP address assigned to your CPE, which is then routed over devices using private IP addresses until it exits to the internet at large.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.