Ifconfig

ifconfig is short for Interface Configuration, the UNIX command for configuring a network interface. Usually given in the form of ifconfig interface name -options ifconfig eth0 192.168.0.1 netmask 255.255.255.0

Just typing ifconfig will show information of all the currently configured network interfaces.

Discovering your MAC address
You can generally find your MAC address with ifconfig eth0    Link encap:Ethernet  HWaddr 00:90:F5:03:4B:69 inet addr:192.168.0.2 Bcast:192.168.0.255  Mask:255.255.255.0 inet6 addr: fe80::290:f5ff:fe03:4b69/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500  Metric:1 RX packets:911573 errors:0 dropped:0 overruns:0 frame:0 TX packets:656393 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:604721438 (576.7 Mb) TX bytes:58964684 (56.2 Mb) Interrupt:10 Base address:0xf000 The entry "HWaddr" on the first line tells the hardware address for your network card.
 * 1) /sbin/ifconfig eth0

Set up two IPs on one interface
To set up an additional ip address on one NIC, issue ifconfig eth0:0 192.168.0.1 This will set up the additional ip address 192.168.0.1 on your network device eth0.

Changing the address
In Linux the MAC address can be changed with most network card drivers. This can be useful in cases where there is security based on MAC addresses and you get a new network card. Example to change it:
 * 1) ifconfig eth0 hw ether 00:00:00:00:00:00

This just changes it in the software drivers, not the actual MAC address on the hardware (so it has to be done after every boot). Some on-board network cards allow the address to be changed in the BIOS.