I have a problem I've seen more than once and on different Linux systems. I normally see it when I want my laptop to temporarily connect to a different network; for example use the internal ethernet adapter to connect to a 192.168.0.x network; rather than my normal wireless connection to a 192.168.1.x network.

I take the wireless adapter down (or remove it completely) and configure the internal adapter to use DHCP. It picks up a DHCP address OK and the routing table looks fine. I can ping the local interface but when I try to ping other interfaces on the same network (which can ping each other and even ping my laptop) I get a "host destination unreachable" or similar error.

I have a few years experience with Unix networking and I've checked all the obvious stuff like phyical connectivity; but I'm a bit stumped. Does anyone have any ideas?

Do you have a firewall up?

Do you have a firewall up?

Yes, but only a simple Shorewall configuration with a few open ports; nothing IP address specific. Might that cause a problem?

*could* be a routing issue? perhaps traffic defaults to your Wireless... May be this will work

ping -I eth0 192.168.0.x

Have a look at your routing table while on the temporary network, route -n
The line with destination set to 0.0.0.0 is your gateway setting, is this pointing to the right gateway on the temporary network?
If not then this setting might need to be configured on the DHCP server of the temporary network, or else you will have to set it manually every time.

If the routing table looks fine then I would bet on some setting in the firewall. Even a "simple" firewall setting in a firewall script usually set up the firewall rules quite strictly and specific for the present network. If you set a simple firewall it usually narrow the rules down so that it only considers your current ip range as LAN, all other addresses are considered WAN or foreign traffic.