MACaddress

A MAC address, short for Media Access Control address, is a unique identifier assigned to a network interface for communications on the physical network segment. In Ethernet and similar technologies, MAC addresses are typically 48 bits long and are written as six hexadecimal octets, commonly separated by colons or hyphens (for example, 00:1A:2B:3C:4D:5E).

A MAC address generally consists of two parts. The first 24 bits form the Organizationally Unique Identifier (OUI), which identifies the hardware manufacturer. The remaining 24 bits identify the specific network interface device. The first octet contains flags that indicate whether the address is universally administered or locally administered and whether the frame is intended for a unicast or multicast destination. Specifically, the I/G bit indicates individual versus group (multicast) addresses, and the U/L bit indicates globally unique versus locally administered addresses.

MAC addresses may be permanently burned into hardware by the vendor (universally administered) or modified at the software level (locally administered). In virtualized environments, virtual network interfaces are assigned MAC addresses, which can sometimes be randomized for privacy or altered for network policy reasons. MAC spoofing, the practice of changing the MAC address, is possible on many systems and has various legitimate and malicious uses.

Functionally, MAC addresses operate at the data link layer to deliver frames within the local network. When a device needs to communicate with an IP address on the same broadcast domain, the Address Resolution Protocol maps IP addresses to MAC addresses. For IPv6, neighbor discovery performs similar mappings without ARP. Broadcast and multicast addresses have special forms, such as ff:ff:ff:ff:ff:ff for broadcast, and multicast prefixes like 01:00:5e or 33:33 for IPv6-related traffic.

Privacy and security considerations include potential device tracking via MAC addresses, leading to practices like address randomization on some operating systems and networks enforcing MAC-based controls.