In computer networking, the Media Access Control (MAC) address is every bit as important as an IP address. Learn in this article how MAC addresses work and how to find the MAC addresses being used by a computer.
















What Is a MAC Address?
The MAC address is a unique value associated with a network adapter. MAC addresses are also known as hardware addresses or physical addresses. They uniquely identify an adapter on a LAN.

MAC addresses are 12-digit hexadecimal numbers (48 bits in length). By convention, MAC addresses are usually written in one of the following two formats:

MM:MM:MM:SS:SS:SS

MM-MM-MM-SS-SS-SS
The first half of a MAC address contains the ID number of the adapter manufacturer. These IDs are regulated by an Internet standards body. The second half of a MAC address represents the serial number assigned to the adapter by the manufacturer. In the example,
00:A0:C9:14:C8:29
The prefix
00A0C9
indicates the manufacturer is Intel Corporation.
Why MAC Addresses?
Recall that TCP/IP and other mainstream networking architectures generally adopt the OSI model. In this model, network functionality is subdivided into layers. MAC addresses function at the data link layer (layer 2 in the OSI model). They allow computers to uniquely identify themselves on a network at this relatively low level.
MAC vs. IP Addressing
Whereas MAC addressing works at the data link layer, IP addressing functions at the network layer (layer 3). It's a slight oversimplification, but one can think of IP addressing as supporting the software implementation and MAC addresses as supporting the hardware implementation of the network stack. The MAC address generally remains fixed and follows the network device, but the IP address changes as the network device moves from one network to another.

IP networks maintain a mapping between the IP address of a device and its MAC address. This mapping is known as the ARP cache or ARP table. ARP (Address Resolution Protocol), supports the logic for obtaining this mapping and keeping the cache up to date.

DHCP also usually relies on MAC addresses to manage the unique assignment of IP addresses to devices.

The method used to find a MAC address depends on the type of network device involved. All popular network operating systems contain utility programs that allow you to find (and sometimes change) MAC address settings.

Find a MAC Address in Windows

In Windows 95, Windows 98 and Windows ME, the winipcfg utility displays the computer's MAC address. Use the ipconfig utility (with the /all option) in Windows NT and any newer versions of Windows.

Both 'winipcfg' and 'ipconfig' may display multiple MAC addresses for one computer. One MAC address exists for each installed network card. Additionally, Windows maintains one or more MAC addresses that are not associated with hardware cards.

For example, Windows dial-up networking uses virtual MAC addresses to manage the phone connection as if it were a network card. Some Windows VPN clients likewise have their own MAC address. The MAC addresses of these "virtual" network adapters are the same length and format as true hardware addresses.

Find a MAC Address in Unix or Linux

The specific command used in Unix to find a MAC address varies depending on the version of the operating system. In Linux and in some forms of Unix, the command "ifconfig -a" returns MAC addresses.

You can also find MAC addresses in Unix and Linux in the boot message sequence. These operating systems display the computer's MAC address on-screen as the system reboots. Additionally, boot-up messages are retained in a log file (usually "/var/log/messages" or "/var/adm/messages").

Find a MAC Address on the Macintosh

You can find MAC addresses on the Macintosh in the TCP/IP Control Panel. If the system is running Open Transport, the MAC address appears under the "Info" or "User Mode/Advanced" screens. If the system is running MacTCP, the MAC address appears under the "Ethernet" icon.

Summary - How to Find a MAC Address

The list below summarizes options to find a computer's MAC address:
  • Windows 95 and newer: winipcfg
  • Windows NT and newer: ipconfig /all
  • Linux and some Unix: ifconfig -a
  • Macintosh with Open Transport: TCP/IP Control Panel - Info or User Mode/Advanced
  • Macintosh with MacTCP: TCP/IP Control Panel - Ethernet icon
MAC addresses were designed to be fixed numbers that cannot be changed. However, there are several valid reasons to want to change your MAC address

Changing a MAC Address To Work With Your ISP

Most Internet subscriptions allow the customer only a single IP address. The Internet Service Provider (ISP) may assign one static (fixed) IP address to each customer. However, this approach is an inefficient use of IP addresses that are currently in short supply. The ISP more commonly issues each customer dynamic IP address that may change each time the customer connects to the Internet.

ISPs ensure each customer receives only one dynamic address using several methods. Dial-up and many DSL services typically require the customer to log in with a username and password. Cable modem services, on the other hand, do this by registering and tracking the MAC address of the device that connects to the ISP.

The device whose MAC address is monitored by an ISP can be either the cable modem, a broadband router, or the PC that hosts the Internet connection. The customer is free to build a network behind this equipment, but the ISP expects the MAC address to match the registered value at all times.

Whenever a customer replaces that device, however, or changes the network adapter inside it, the MAC address of this new equipment will no longer match the one registered at the ISP. The ISP will often disable the customer's Internet connection for security (and billing) reasons.

Change a MAC Address through Cloning

Some people contact their ISP to request they update the MAC address associated with their subscription. This process works but takes time, and Internet service will be unavailable while waiting for the provider to take action.

A better way to quickly workaround this problem is to change the MAC address on the new device so that it matches the address of the original device. While an actual physical MAC address cannot be changed in hardware, the address can be emulated in software. This process is called cloning.

Many broadband routers today support MAC address cloning as an advanced configuration option. The emulated MAC address appears to the service provider identical to the original hardware address. The specific procedure of cloning varies depending on the type of router; consult product documentation for details.

MAC Addresses and Cable Modems

In addition to MAC addresses tracked by the ISP, some broadband modems also track the MAC address of the host computer's network adapter within the home network. If you swap the computer connected to the broadband modem, or change its network adapter, your cable Internet connection may not function afterward.

In this case, MAC address cloning is not required. Resetting (including recycling power) on both the cable modem and the host computer will automatically change the MAC address stored inside the modem.

Changing MAC Addresses through the Operating System

Starting with Windows 2000, users can sometimes change their MAC address through the Windows My Network Places interface. This procedure does not work for all network cards as it depends on a certain level of software support built into the adapter driver.

In Linux and versions of Unix, the "ifconfig" also supports changing MAC addresses if the necessary network card and driver support exists.

Summary - Change a MAC Address

The MAC address is an important element of computer networking. MAC addresses uniquely identify a computer on the LAN. MAC is an essential component required for network protocols like TCP/IP to function.

Computer operating systems and broadband routers support viewing and sometimes changing MAC addresses. Some ISPs track their customers by MAC address. Changing a MAC address can be necessary in some cases to keep an Internet connection working. Some broadband modems also monitor the MAC address of their host computer.

Although MAC addresses do not reveal any geographic location information like IP addresses do, changing MAC addresses may improve your Internet privacy in some situations.