what is mac address

A media access control (MAC) address is a binary number that uniquely identifies a computer's network adapter. These numbers (sometimes called "hardware addresses" or "physical addresses") are embedded into the network hardware during the manufacturing process, or stored in firmware, and are designed not to be modified.

Some people also refer to them as "Ethernet addresses," but many types of networks use MAC addressing, including Ethernet, Wi-Fi, and Bluetooth.

Format of MAC address

The traditional MAC address is a 12-bit (6 bytes or 48 bits) hexadecimal number. By convention, they are usually written in one of the following three formats:

MM：MM：MM：SS：SS：SS
MM-MM-MM-SS-SS-SS
MMM.MMM.SSS.SSS

The leftmost 6 bits (24 bits) are called the "prefix" associated with the adapter manufacturer. Each provider registers and receives a MAC prefix assigned by the IEEE. Suppliers often have many prefix numbers associated with their different products. For example, the prefixes 00:13:10, 00:25:9C and 68:7F:74 (plus many others) all belong to Linksys (Cisco Systems).

The rightmost digit of the MAC address represents the identification number of the specific device. Across all devices manufactured with the same vendor prefix, each device has its own unique 24-digit number. Note that hardware from different vendors may happen to share the same device portion of this address.

64-bit MAC address

Although the traditional MAC address length is 48 bits, a few types of networks require 64-bit addresses. For example, ZigBee wireless home automation and other similar networks based on IEEE 802.15.4 require a 64-bit MAC address to be configured on their hardware devices.

Compared with mainstream IPv4, IPv6-based TCP/IP networks also implement different methods of communicating with MAC addresses. However, instead of using a 64-bit hardware address, IPv6 automatically converts a 48-bit MAC address into a 64-bit address by inserting a fixed (hard-coded) 16-bit value FFFE between the vendor prefix and the device identifier. IPv6 calls these numbers "identifiers" to distinguish them from true 64-bit hardware addresses.

For example, the 48-bit MAC address 00:25:96:12:34:56 appears on an IPv6 network (usually written in one of these two forms):

00：25：96：FF：FE：12：34：56
0025：96FF：FE12：3456

MAC vs. IP Address Relationship

TCP/IP networks use MAC addresses and IP addresses, but for separate purposes. The MAC address remains fixed to the device's hardware, while the IP address of the same device can change based on its TCP/IP network configuration. Media Access Control operates at Layer 2 of the OSI model, while Internet Protocol operates at Layer 3. This allows MAC addressing to support other types of networks besides TCP/IP.

IP networks use the Address Resolution Protocol (ARP) to manage the translation between IP and MAC addresses. The Dynamic Host Configuration Protocol (DHCP) relies on ARP to manage the unique assignment of IP addresses to devices.

MAC Address Cloning

Some internet service providers link each of their residential customer accounts to the MAC address of the home network router (or another gateway device). The address seen by the provider does not change until the customer changes their gateway, for example by installing a new router. When a residential gateway changes, ISPs will now see a different MAC address reported and prevent that network from coming online.

A process called "cloning" solves this problem by enabling the router (gateway) to continue reporting the old MAC address to the provider, even if its own hardware address is different. Administrators can configure their router (assuming it supports this feature, as much as possible) to use the clone option and enter the MAC address of the old gateway in the configuration screen. When cloning is not available, customers must contact the service provider to register their new gateway device.

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