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The MAC Address
An Introduction to MAC Addressing
An Article by your Guide Bradley Mitchell

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... (see below)
More of this Feature
Part 2: Finding MAC Addresses
Part 3: Changing MAC Addresses

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"I am just confused about why the router will respond with its own MAC address and not the MAC address of the destination host.

"If the router keeps an ARP table of IP-to-MAC addresses, then why doesn't it respond to the source host with the MAC of the destination host, regardless of whether they're on the same subnet?"
-TJRUGG

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IP Tutorials

Elsewhere on the Web
Vendor/Ethernet MAC Address Lookup

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 (see sidebar). 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, 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.

Next page > Finding and Changing MAC Addresses > Page 1, 2, 3

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