You can view all network devices that are currently running by typing ifconfig -a, resulting in output that looks something like this:

eth0      Link encap:Ethernet  HWaddr 00:01:02:CB:57:3C  
          inet addr:192.168.1.1  Bcast:192.168.1.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100 
          RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)
          Interrupt:5 Base address:0xc400 

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:1065 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1065 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:262542 (256.3 Kb)  TX bytes:262542 (256.3 Kb)

Above, you can see a configured eth0 interface, as well as a configured lo (localhost) interface. The lo interface is a special virtual interface that's configured so that you can run TCP/IP applications locally, even without a network.