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I wondered if someone could explain this to me.

At work we have a firewall that connects to the Internet and has an external IP configured on it via DHCP. The firewall's WAN port connects to an Alcatel NTE which is Virgin Media's Kit. This has a private IP address of 10.2.2.67 (it has a sticker and when you do a trace route it includes this as a hop).

traceroute to google.com, 64 hops max, 52 byte packets
 1  10.9.1.254 (10.9.1.254)  6.657 ms  2.436 ms  2.234 ms
 2  10.2.2.67 (10.2.2.67)  18.802 ms *  66.547 ms

I don't understand how the firewall can have an external ip configured on it and then something with a private IP address as the next hop, please could someone explain how the routing would work?

Topology:

Virgin Media
|
|
Alcatel NTE (Virgin Media 10.2.2.67)
|
|
Firewall (89.x.x.x)
|
|
Internal Network
5

IPv4 doesn't know anything about public or private IP addresses. To IPv4, they are all IP addresses that can be treated the same. ISPs have agreed not to route packets with addresses in the private IPv4 ranges (and other ranges, too) between each other. Your packets destined to the public Internet will have public addressing on the source of the packets, and the destination addresses on the packets will also be public addresses.

What an ISP does in its internal network is of no concern to you because it doesn't affect your packets. Your packets could pass through various different public or private networks, and it simply doesn't matter because the source and destination addresses on the packets are public addresses.

  • Thanks Ron, how can each end of one link have two different ip addresses? The Wan port is External and then the other end of that cable the interface has a private ip. Surely that wouldn’t work? – Nick Carlton Apr 5 at 18:59
  • Point-to-point links do not need to be in the same network. If this is a DSL connection, then it is using PPPoA or PPPoE. – Ron Maupin Apr 5 at 19:01
  • It’s a leased line, does that make a difference? – Nick Carlton Apr 5 at 19:05
  • It could still be using PPP. With PPP, there are only two endpoints, so any traffic sent will be destined for the other end. That is why PPP doesn't have any layer-2 addressing. Your device does not need to resolve the layer-3 address to the layer-2 address, so the layer-3 address doesn't matter. Traffic is delivered by layer-2 on the link, but there is no layer-2 addressing with PPP. – Ron Maupin Apr 5 at 19:07

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