Network interface

A network interface is the interface to a network node or a certain Network Interface Card (NIC) perhaps an ethernet card. An interface can be virtual or physical. To see all network interfaces on a system use the ifconfig command.

$ ifconfig -a 
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 33192
        groups: lo 
        inet 127.0.0.1 netmask 0xff000000 
        inet6 ::1 prefixlen 128
        inet6 fe80::1%lo0 prefixlen 64 scopeid 0x9
wi0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        lladdr 00:02:2d:09:4b:44
        media: IEEE802.11 autoselect (DS2)
        status: active
        ieee80211: nwid ATLAS -12dBm (auto) 
        inet 10.0.0.2 netmask 0xff000000 broadcast 10.255.255.255
        inet6 fe80::202:2dff:fe09:4b44%wi0 prefixlen 64 scopeid 0x1
xl0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        lladdr 00:60:08:5a:86:82
        media: Ethernet 100baseTX full-duplex
        status: no carrier
        inet 172.16.2.2 netmask 0xfffffe00 broadcast 172.16.3.255
        inet6 fe80::260:8ff:fe5a:8682%xl0 prefixlen 64 scopeid 0x2

When a computer has multiple network interfaces and passes IP traffic from one interface to another interface it is called a router. Similarely if a computer passes ethernet frames from one NIC to another it is called a bridge.

If you want to be sure that traffic is being routed or bridged you can attach a bpf program like tcpdump to a network interface and watch a packet entering interface a and exiting on interface b.