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I have a Windows XP machine acting as internet gateway. It is connected to two different networks, one, say LAN1, connects to internet and other, say LAN2, to outside LAN. My machine is sitting behind the gateway. I have set up internet connection sharing on LAN1 and can access internet on my machine but i can't access anything from LAN2. Is it possible to access internet from LAN1 and yet be able to access PC's on LAN2?

--------------   ---------
| Lan 1      |   | Lan 2 |
| (Internet) |   ---------
--------------      ^
    ^               |
    |               |
--------------------------
|    Win XP Gateway      |
--------------------------
           ^
           |
      --------------
      | My Machine |
      --------------

EDIT: Gateway is equipped with 3 lan sockets, two are connected to Lan 1 & 2, third one is connected to switch. And my machine also connects to that same switch.

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Are you using a switch or hub? or how many Lan socket your computer got? –  user44509 Aug 18 '10 at 10:19
    
I have updated my question to answer this. –  Babar Aug 18 '10 at 10:24
    
can you ping LAN2?(if Not,what s the message) and what s the light color on the port involved with LAN2 on SW? –  user44509 Aug 18 '10 at 10:37
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1 Answer

So, your machine has as Gateway the XP Gateway (let's call it XPG).

Now, all your traffic, except what goes to the same IP class your machine is in, goes through XPG. Now, XPG has a gateway of it's own from LAN1 (I presume), and whatever it talks it says over it's gateway.

You have to check out your routing table, and possibly add a route on XPG.

To see current routes type route print. To add e new route check out the exact syntax for route command on Windows (beware, on Linux and Mac the same command has different syntax). Your route command should specify that if a signal comes from "My Machine"'s network toward LAN2 it should be routed over the network interface LAN2 is connected to.

EDIT: I forgot to mention, don't try to ping a windows machine from LAN2 from "My Computer". You obviously are in different IP classes (networks) and Windows do not reply to pings from other networks than their own, unless you explicitly specify a rule in the Windows firewall on the machines from LAN2. However, you will be able to access their shared folders.

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