A plain-English guide to Media Access Control addresses - what they are, where they come from, and how they're written.

A Media Access Control address (MAC address) is a unique identifier assigned to a network interface for communications on the physical network segment. MAC addresses are used by numerous network technologies and most IEEE 802 standards, including Ethernet and Wi-Fi. Logically, they live in the Media Access Control protocol sub-layer of the OSI reference model.

MAC addresses are most often assigned by the manufacturer of a network interface card (NIC) and are stored in its hardware - typically in the card's read-only memory or some other firmware mechanism. When assigned by the manufacturer, a MAC address usually encodes that vendor's registered identification number and may be referred to as the burned-in address (BIA). It may also be known as an Ethernet hardware address (EHA), hardware address, or physical address. A single device may have multiple NICs and will then have one unique MAC address per NIC.

// Numbering Spaces

MAC addresses are formed according to the rules of one of three numbering name spaces managed by the Institute of Electrical and Electronics Engineers (IEEE): MAC-48, EUI-48, and EUI-64. The IEEE claims trademarks on the names EUI-48 and EUI-64, where EUI stands for Extended Unique Identifier.

MAC-48 / EUI-48 - 48-bit (6-byte) addresses. The original MAC-48 designation has effectively been superseded by EUI-48, which uses an identical format.
EUI-64 - 64-bit (8-byte) addresses, used by technologies such as IPv6 (for stateless address autoconfiguration) and IEEE 802.15.4 (Zigbee, Thread, and similar low-power wireless networks).

For universally administered addresses, the leading portion is an IEEE assignment registered to an organization. IEEE assignments vary in size: MA-L uses 24 bits, MA-M uses 28 bits, and MA-S uses 36 bits. The legacy Individual Address Block (IAB) registry also uses 36-bit assignments. A 24-bit MA-L assignment is traditionally known as an Organizationally Unique Identifier (OUI).

The IEEE also maintains the 24-bit Company ID (CID) registry for use in locally administered addresses. These registry assignments allow MACVendors to map an address to its registered organization, which is often—but not always—the hardware manufacturer.

// Notational Conventions

The standard (IEEE 802) format for printing MAC-48 addresses in human-friendly form is six groups of two hexadecimal digits, separated by hyphens (-) or colons (:), in transmission order. For example:

01-23-45-67-89-AB
01:23:45:67:89:AB

This same form is also commonly used for EUI-64. Another convention, used predominantly by networking equipment from vendors like Cisco, uses three groups of four hexadecimal digits separated by dots - again in transmission order:

0123.4567.89AB

Tip: MAC addresses are case-insensitive. aa:bb:cc:dd:ee:ff and AA:BB:CC:DD:EE:FF refer to the same interface.

// Universal vs. Local, Unicast vs. Multicast

Two bits in the first byte of a MAC address carry special meaning:

The U/L bit (universally / locally administered) indicates whether the complete address is universally assigned or constructed for local use—for example by virtualization software, privacy-focused MAC randomization, or an administrator. IEEE CID prefixes are a notable case: their registered company identifier intentionally occupies locally administered address space, while the organization constructs the rest of the address locally.
The I/G bit (individual / group) indicates whether the address identifies a single interface (unicast) or a group of interfaces (multicast). The broadcast address FF:FF:FF:FF:FF:FF is a special multicast address that targets every device on the segment.

// Modern Privacy Considerations

Because hardware MAC addresses are globally unique and persistent, they can be used to track devices across networks. To address this, modern operating systems - including iOS, Android, Windows, and macOS - now use MAC randomization by default, generating a different locally-administered MAC address for each network the device joins.

This means the address you see advertised on a Wi-Fi network is often not the burned-in hardware MAC. If you need the hardware address for a router allow-list or asset inventory, look for an option labelled Use device MAC or Private address: Off in your device's Wi-Fi settings.

// Looking Up a Vendor

By matching the leading digits of a MAC address against IEEE MA-L, MA-M, MA-S, CID, and legacy IAB assignments, you can identify the organization to which the most specific matching block is registered. That's exactly what MACVendors does - and you can automate the lookup using our MAC Address Vendor Lookup API.

References: Wikipedia