Linking Ethernet interfaces

You can use a bridge interface. You can use brctl from bridge-utils to create a bridge interface. For example,

$ brctl addbr br0
$ brctl addif br0 eth0 eth1
$ brctl show
bridge name     bridge id               STP enabled     interfaces
br0             8000.00004c9f0bd2       no              eth0
                                                        eth1

So after adding interfaces eth0 & eth1 into the bridge device br0 you're left with the following setup. You can use ifconfig to see it:

$ ifconfig eth0
eth0      Link encap:Ethernet  HWaddr BC:AE:AA:34:22:11  
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
...

$ ifconfig eth1
eth1      Link encap:Ethernet  HWaddr BC:AE:AA:34:11:22  
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
...

And the bridge device with the IP address:

$ ifconfig br0
br0       Link encap:Ethernet  HWaddr BC:AE:C5:11:22:33  
          inet addr:192.168.1.20  Bcast:192.168.1.255  Mask:255.255.255.0
...