I need to set up a router.

My network has 4 segments:
10.5.0.0 at eth0
10.5.1.0 at eth1
10.5.2.0 at eth2
10.5.3.0 at eth3

Netmask is 255.255.252.0

The problem is when I assign IP addresses to corresponding NICs (network cards) all traffic goes through eth0 (10.5.0.0 incorporates all other networks)

Routing table looks like this:

Dest_______Gateway___Netmask________Iface
10.5.0.0___0.0.0.0___255.255.252.0__eth0
10.5.1.0___0.0.0.0___255.255.252.0__eth1
.....
127.0.0.1__0.0.0.0___255.0.0.0______lo


If I change order of networks (i.e. eth3 has 10.5.0.0 and eth0 has 10.5.3.0) I got the similar results.

I don't want to make 4 different subnets (with 255.255.255.0 netmask) because then AFAIK I need to use NAT in that case. Traffic between these 4 segments must flow with minimal additional efforts. NAT will be used only for accessing external networks

How can I make each network use only it's NIC?

you say "each network" but there is only one network there, not 4. i take it you don't really have much knowledge on subnetting? firstly i'd suggest that your ip structure is very wrong, and you should change your netmask to 255.255.255.0 so you DO have 4 subnets. then just enable ip forwarding (echo 1 > /proc/sys/net/ipv4/ip_forward) and you'll be able to reach each other just fine, assumign this machine is the default gateway for all networks.

if you are going to insist on doing what you are, then you're actually looking to turn your machine into a switch, not a router, which to me seems like a complete waste as you could just use a $5 switch to do the job for you in 10 seconds.

Thanks! That's what I was looking for.

Switch is not a solution in my case because this PC will be a gateway for a huge number of PC-s.

well that's the point... that *IS* a switch...