MAC

A Media Access Control (MAC) address is a hardware address that uniquely identifies each node of a network. A MAC is a 48-bit Address usually represented in hexadecimal format with colons between every 8-bits. For example:
08:00:20:95:fb:83

Each company making an ethernet card is assigned a block of MAC addresses for their cards. A list of companies and their MAC assignments is available here.

To find out your MAC address, choose your operating system:

Linux
FreeBSD/OpenBSD/NetBSD
Solaris/NeXTStep

Windows 9x
Windows 2000/NT/XP
Macintosh

Linux
From a terminal on your linux box, as any user, type the following:

/sbin/ifconfig -a |grep HWaddr

You should see something like the following:

eth0      Link encap:Ethernet  HWaddr 00:80:5F:CB:52:E7

FreeBSD/OpenBSD/NetBSD
From a terminal on your BSD box, as any user, type the following:

/sbin/ifconfig -a |grep ether

You should see something like the following:

        ether 00:d0:b7:bb:6f:f5

Solaris/NeXTStep
From a terminal on your Solaris/NeXTStep box, as root, type the following:

/sbin/ifconfig -a |grep ether

You should see something like the following:

        ether 00:d0:b7:bb:6f:f5

Windows 9x/Me
Click the Start Menu, then Run....
When the Run Window opens, type winipcfg and click Ok.
A window with the title IP Configuration should open.
$winipcfg$
Just below the ethernet adapter information heading, there should be a drop down list of adapters. It may say ppp. Click the arrow next to the adapter name to show the list.
If you have more than one adapter if your machine, select the one you will be using to connect to the IAS from the drop down list at the top of the window.
Do NOT Select the PPP Adapter.
After making your selection, the information below will change.
The MAC Address is called the Adapter Address

Windows 2000/NT/XP
Start a Command Prompt
On Windows NT 4.0 this is found on the Start Menu under Programs
On Windows 2000/XP this is found on the Start Menu under Programs subsection Accessories

In the Command Prompt, type the following:

ipconfig /all

You should see something like the following:

Windows NT IP Configuration

        Host Name . . . . . . . . . : example.math.ias.edu
        DNS Servers . . . . . . . . : 192.108.106.49
                                      192.16.204.20
        Node Type . . . . . . . . . : Hybrid
        NetBIOS Scope ID. . . . . . :
        IP Routing Enabled. . . . . : No
        WINS Proxy Enabled. . . . . : No
        NetBIOS Resolution Uses DNS : Yes

Ethernet adapter N1001:

        Description . . . . . . . . : Adapter Name Here
        Physical Address. . . . . . : 00-08-C7-82-4A-7E
        DHCP Enabled. . . . . . . . : No
        IP Address. . . . . . . . . : 192.168.192.253
        Subnet Mask . . . . . . . . : 255.255.255.0
        Default Gateway . . . . . . : 192.168.192.254
        Primary WINS Server . . . . : 192.168.192.252

The MAC Address is called the Physical Address. Make sure you use the Physical Address from the correct adapter, check the line that say's Adapter Name Here to see if it looks reasonable. Do NOT use the PPP Adapter

Macintosh

(OS 8.x and higher)

This assumes you are running OpenTransport on your Macintosh, which is the default networking software for versions 7.1 and above.

Click the Apple Menu, then Control Panels, then TCP/IP
From the Connect via: drop down list, select the adapter that contains the word Ethernet, this is usually "Ethernet Built-in" or some equivalent.
Under the File menu, click Get Info
The Information window should open. The MAC Address is called the Hardware Address:

(OS 10.1 and higher) Systems

Wired Connection

From the dock, select System Preferences.
Select the Network applet.
Next to Show, make sure Built-in Ethernet is selected. Click "Advanced" at the bottom of the page.
With the Ethernet tab selected, the Ethernet ID will look something like the following: 00:16:cb:a8:bc:46