MAC
Address 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 formats: MM:MM:MM:SS:SS:SS MMMM-MMSS-SSSS The
first half (24 BITS) of a MAC address contains the ID number of the adapter
manufacturer. These IDs are regulated by an Internet standards body (see
sidebar). The second half (24 MORE BITS) 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
(association) between the IP address of a device and its MAC address. This
mapping is known as the ARP cache or ARP table. ARP, the 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 one 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 MAC
addresses. In Windows NT and any newer versions of Windows, the ipconfig utility (using the /all option) can also be
used. Both winipcfg and ipconfig can display multiple MAC addresses. First, one MAC
address is shown for each network adapter. Then, one or more additional MAC
addresses are shown for other network adapters. Windows, for example, utilizes built-in
MAC addressing to support Windows dial-up connections. Some Windows VPN clients
likewise use their own MAC address. Because these other adapters are really
software constructs that do not involve unique hardware, these are often
referred to as virtual adapters. Find a MAC Address in Unix or Linux
The
specific command used in Unix to find a MAC address
varies depending on the "flavor" of the operating system. In Linux
and in some forms of Unix, the command ifconfig
-a
returns MAC addresses. It's also possible to find MAC addresses
in Unix and Linux by reading the boot message
sequence, either on-screen as the system boots or from the startup message
file. The log file for boot messages is usually /var/log/messages
or /var/adm/messages. Find a MAC Address on
the Macintosh
MAC
addresses on the Macintosh are generally found on the TCP/IP Control Panel.
If the system is running Open Transport, the MAC address can be found under the
Info or User Mode/Advanced screens. If the system is running MacTCP, the MAC address can be found under the Ethernet
icon. Finding a MAC Address
Summary
The table below summarizes options for
finding a computer's MAC address.
MAC addresses were designed to be fixed
numbers that cannot be changed. However, there are some valid reasons to want
to change your MAC address. Changing MAC Addresses
to Support Your ISP
Some ISPs, typically cable modem
providers, sell Internet subscriptions on a per-address basis. Occasionally the
ISP manages these subscriptions by assigning a static (fixed) IP address to the
customer. However, this approach is an inefficient use of IP addresses that are
currently in short supply. More typically, ISPs manage single-address
subscriptions by registering the MAC address of the device that connects to the
ISP. This device could be a broadband modem, for example. The customer is free
to build a home or small business network behind this modem, but the ISP
expects the MAC address to match the registered value at all times. Whenever a customer replaces their modem
or adds a broadband router, the MAC address will no longer match that
registered at the ISP, and the ISP will disable the customer's Internet
connection. Cloning MAC Addresses
One way to solve this problem is to call
the ISP and ask them to update the registered MAC address to match the new
hardware. A more efficient way to solve this problem is to configure the device
so that it advertises the original MAC address, even though it's
hardware is built to utilize a different MAC address. This process is called cloning.
Many broadband routers today support MAC
address cloning as an advanced configuration option. The exact procedure varies
depending on the type of router. MAC Addresses and Cable
Modems
Note that in addition to MAC addresses
stored at the ISP, some broadband modems also store the MAC address of the host
computer's network adapter. However, in this case, cloning is not required.
It's true that changing network adapters usually causes the cable modem
connection to fail. To remedy this problem, though, requires only that the
cable modem and computer be reset (and perhaps a waiting period for the ISP to
release the old IP address). Changing MAC Addresses
through the Operating System
Starting with Windows 2000, users can
change their MAC address through the Windows My Network Places
interface. This feature relies on software support built into the adapter
driver program and thus does not work for all adapters. Likewise, the ifconfig command available in Linux and other flavors of Unix supports changing MAC addresses with the necessary
network card and driver support. In Conclusion
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. Changing MAC addresses may also increase
privacy in some situations, though MAC addresses do not reveal any geographic
or ISP location information like IP addresses do.
|