MAC Addresses

MAC addresses, or physical addresses, are 48 bits long and are expressed in hexadecimal, twelve hex digits in total. The first six hex digits identify the manufacturer of the NIC card, this unique code is called the Organisational Unique Identifier (OUI). The other six hex digits comprise the interfaces serial number, which is set by the vendor of the NIC card during the manufacturing process and is also a unique code for that vendor. No two MAC addresses are the same. There are 1612 possible MAC addresses in total, so unlike IP addresses, we are not close to running out just yet.

MAC addresses are usually stored in ROM on the NIC card and copied to memory when the host boots up. They are added to frames at the Data Link layer in the form of headers and distinguish hosts on a network, and are the first parts of a frame a host will look at to determine if the data was intended for them.

MAC addresses are considered a flat addressing scheme since they have no structural hierarchy. This is where IP addressing comes into it’s own.

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