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This is my complicated and long question. First let's talk about the context.

Network topology: (RT stands for router) enter image description here

PC A --- RT A --- RT C --- RT B --- PC B

(RT C has a WAN NIC connected to "the cloud")

With this situation :

  • PC A must send a packet to PC B
  • Default routes direct packets to the cloud
  • We haven't access to RT C's configuration
  • RT C only knows how to join network A, not network B
  • RT A knows about network B
  • RT B knows about network A

RT C's routing table:

Destination NIC     Gateway     WAN     Cloud
Network A   LAN A   RT A's WAN

RT A's routing table:

Destination NIC     Gateway     WAN     LAN A
Network B   WAN     LAN A

RT B's routing table:

Destination NIC     Gateway     WAN     LAN B
Network A   WAN     LAN B

I would like to permit PC A and PC B to communicate, but I don't have access to RT C. Networks B and BC are new.

Can PC A send a packet to RT B's WAN NIC (which is possible) and "ask RT B to direct the packet to PC B" ?

I believe replacing RT B with a VPN server should do the trick, but I would like to know if it is possible to make it without establishing a new connection.

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You say it is possible for PC A to send a packet to RT B’s WAN NIC. How is this possible if RT A and RT B are not on the same network, and RT C doesn’t know that Network B exists? – Scott Oct 12 '12 at 22:01
Because of the default route of RT A if I remember well. – Philipili Oct 13 '12 at 9:23
I don’t understand that. You say RT A’s default route is out the WAN NIC to LAN A, which is one of RT C’s addresses. And RT C doesn’t know that Network B exists — or even Network BC. – Scott Oct 15 '12 at 16:13
Well, it knows network BC, because it is a network directly connected to it. – Philipili Oct 16 '12 at 6:25

The gateway provided has to be on a directly-connected network segment, so this isn't generally possible without either some sort of VPN configuration or configuring RT C to be aware of all subnets it needs to route.

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