What is my personal computer's wired ethernet (MAC) address?

Every computer's network interface has a unique identifer called a MAC address. The MAC address is a 12 digit hexadecimal number usually delimted by colons, e.g. ab:01:23:ef:45:dc. Both wired and wireless network interfaces have these addresses.

Microsoft Windows 2000/XP/Vista

Go to Start Menu->Programs->Accessories->DOS Command Prompt Type cmd. When the command window appears, type ipconfig /all. The "Physical Address" value for your ethernet card is your MAC address. Make sure you're looking at your ethernet interface and not your wireless interface.

Apple MacOS X

Open the Network Preferences, and, for any Location, choose "Built-in Ethernet" from the drop down menu that's labeled "Show:". Click on the "Ethernet" tab (or in Leopard click on the Advanced button then the "Ethernet" tab), and the MAC address is displayed as the Ethernet ID.

Linux

Type /sbin/ifconfig. The HWaddr field associated with your ethernet interface is your MAC address. If your ethernet interface is not yet configured, your MAC address may be found in /var/log/messages. For example

kernel: eth0: 3Com 3c905B Cyclone 100baseTx at 0xb800, 00:50:04:73:76:ae, IRQ 11 ^---------------^ <-- MAC address