I have a PC whit 2 eth and I’m trying to setup a router. I have 2 networks and I want to separate them with the debian router. The problem is that I don’t have access to one of the networks.
Eth1 is dynamic the eth0 is with dhcp.
I activate the forwarding with echo 1 > /proc/sys/net/ipv4/ip_forward.
When I send a ping to a PC located in the lan1 from a PC in lan2 it doesn't work but from the Debian PC ( future router) it works.
The computers above mentioned are windows pc (for the moment).
Sorry for my English and Tank’s.

debian:/etc/network# netstat -nr
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
155.208.194.192 0.0.0.0 255.255.255.192 U 0 0 0 eth0
192.168.100.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1
0.0.0.0 192.168.100.3 0.0.0.0 UG 0 0 0 eth1
0.0.0.0 155.208.194.193 0.0.0.0 UG 0 0 0 eth0

sounds like you don't have a route configured on the client machine to know where to go to reach the other network. unless the router is defined as the default gateway in general there is no wy for the clients to know how to reach a network on the other side of a router unless they are told to hit that router explicitly.