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This is the basic setup. The goal is to get Host 2 to use Host 1's second NIC to connect to the internet without using any special routing software or anything on Host 1.

network diagram

What I can do: Configure static routing tables for both Cisco-routers and Host 1/2.
What I can't do: Configure the router at and use dynamic routing on the Cisco routers. What I was able to achieve: Ping (Host 1s 2nd NIC) from (Host 2's NIC) directly.
Were I got stuck: Ping from

What I did:

  • Told Cisco 2 about via Ethernet, via Serial and via (Cisco 1) aswell as via Serial.
  • Told Cisco 1 about via Ethernet, via Serial, via (Cisco 2) and via aswell as specifically via (probably not neccessary)
  • And last I added routes on Host 1 for via NIC 2 aswell as via

I feel I'm so close to solving this, but I seem to bang my head against a wall, adding unneccessary routes and not really solving the issue.

Both Hosts run Windows XP.

I would be SO glad for some help here, this is exciting for me (especially since I'm more the programming type) and the fact I can ping from already got me extra credits from my teacher, but I basically promised him I was able to make Host 2 go into the internet via Host 1, and I intend to show everyone the solution.

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up vote 1 down vote accepted

Since you cannot alter the routing tables on and it apparently has no route to the network, you will have the problem that replies to packets which originated from would never be able to find their routes.

You can work around this problem by using a technique called NAT where the source address of all outgoing packets would be rewritten by Host 1 to be - where the router at would have a valid route to. Host 1 would maintain connection tables and perform the translation back to the original addresses for incoming reply packets.

NAT support is built in into Windows XP in two flavors. There is the easy to set up "Internet Connection Sharing", but it is mainly targeted at a single One internet-connected XP-PC in a single private subnet scenario and there are several restrictions to the implementation so you would not be able to use it in your setup. And there is the NAT implementation of the Remote Access Service (RAS) which can perform the task as well but is more flexible. You would have to use the routing ip nat context of the netsh command to do the configuration. See the Microsoft Technet documentation for more details on this feature or the numerous third-party walkthroughs on "NAT with Windows RAS" like this one which you'll find by querying the search engine of your choice.

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I never thought about the replying packet, I only pretended to be a packet going out when thinking about this. This makes sense, and while we never used NAT/RAS practically we went through it in theory and it makes most sense. Sadly I can't instantly test this solution since next lesson is in 2 weeks but I'm sure with these new ressources I'll be able to solve the problem. I'll accept this answer this it makes sense to me in theory. Do you want me to tell you if I successfully implement it? thanks! – xNidhogg Jan 9 '12 at 10:15
@xNidhogg You should un-accept the answer and wait two weeks then :) Just drop a comment to my answer if you have further issues or to tell if you were able to make it work - I'll get notified. And welcome to Serverfault. – the-wabbit Jan 9 '12 at 11:01

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