MAC Address

A MAC (Media Access Control) address is a 48-bit identifier, written like 3c:22:fb:1a:90:7e, that identifies a network interface at the data link layer. The first three bytes are the manufacturer's OUI; the rest is assigned by the vendor.

Why it matters

Switches use MAC addresses to forward frames, and ARP maps IP addresses to MACs on a LAN. MAC addresses are only meaningful on the local segment; routers replace them at every hop, so a remote website never sees yours. Networks use them for DHCP reservations and access filtering, though they can be changed in software.

Example

$ ip link show eth0
link/ether 3c:22:fb:1a:90:7e brd ff:ff:ff:ff:ff:ff

The address ff:ff:ff:ff:ff:ff is the broadcast address, delivered to every device on the segment.

Many phones now use randomized MAC addresses per Wi-Fi network for privacy.

Common mistakes

• Relying on MAC filtering as real security. Addresses are visible on the air and easy to spoof.
• Assuming a MAC address is globally unique and permanent. Virtual machines and privacy features change it.
• Expecting to find a remote server's MAC address. It is not carried across routers.

Related

• ARP
• Network switch
• Ethernet
• DHCP

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