Cannot communicate between 2 machines on different subnets & netmask

Posted on 2002-05-14
Last Modified: 2010-03-18

I have Linux 7.1 using IPChaines and IP_FORWARDING=on.

I have an external network with the following setup:
Cisco router =
Windows 2000 =
(NetMask =

The above machines are connected to a switch.
I then connected a network cable from the switch into my linux box via (eth3) I configured eth2 as follows:
eth2 IP=
Netmask =

I am able to ping the Windows 2000 machine from the linux box.

However on my Internal network I cannot ping the Windows 2000 machine but of course I can ping eth2.
My internal network is:

My IP address on that network (, this is the machine I am using to try and ping the Windows 2000 machine.

I also have a direct internet connection and the router for that internet connection is listed as my default gateway.
Default gateway =

Please ask if you need any clarification on the above network.

I know it may make my life easier if I use IP Filtering however I do not have the expertise to get that up and going quickly because I am looking for a solution by today.
Question by:MCI_Consultants
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LVL 40

Expert Comment

ID: 7008428
You've got a routing problem. Right now the machines in the network believe that any destinations other than that network should go to the Cisco, which is the default gateway for that network. There are two possible solutions for that. One is to include a static route on the Cisco telling it that the network is reachable by the gateway at (Linux's eth2 interface). The other solution is to add a static route to the w2k machine and tell it the same thing. With either of those routes in place you should be able to w2k machine as it'll know (either from it's static route or by a redirect from the Cisco) how to get packets back to the Linux box and thus to a client on the inside of the Linux box.

A possible complication is that you've also got a direct Internet connection via the Linux (presumably via eth1). I can't tell how you've set that up, so I don't know if additional routing is needed on Linux. Fix the routing on the 192.168.160 network and see if you can connect to w2k.
LVL 51

Expert Comment

ID: 7010389
echo 1 >/proc/sys/net/ipv4/ip_forward
LVL 16

Expert Comment

ID: 7017984
Please post output of:

ifconfig -a
netstat -nr
cat /proc/sys/net/ipv4/ip_forward


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Expert Comment

ID: 9078488
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Expert Comment

ID: 10004248
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