MAC Address Lookup

What Is a MAC Address?

A Media Access Control (MAC) address is a unique 48-bit (6-octet) identifier assigned to a network interface controller (NIC) by the manufacturer. It serves as the hardware address for devices on a local network segment and is fundamental to the operation of Ethernet, Wi-Fi, and other IEEE 802 networking technologies. MAC addresses are typically written as 12 hexadecimal digits grouped in pairs, separated by colons (e.g., 00:1A:2B:3C:4D:5E), hyphens, or a dot notation for Cisco-style (001A.2B3C.4D5E).

OUI and Vendor Identification

The first three octets (24 bits) of a MAC address form the Organizationally Unique Identifier (OUI), which is assigned by the IEEE to hardware manufacturers. By extracting the OUI, this tool can identify the vendor that manufactured the network interface. For example, a MAC starting with 00:00:0C belongs to Cisco Systems, while 00:80:41 belongs to Intel. The remaining three octets are a unique device identifier assigned by the manufacturer.

MAC Address Types

MAC addresses fall into three categories based on the first octet’s least significant bit (the I/G bit). A unicast MAC address (I/G = 0) identifies a single network interface and is the most common type. A multicast MAC address (I/G = 1) targets a group of devices, used in protocols like ARP (where 01:00:5E:* is used for IPv4 multicast) and IPv6 neighbor discovery. The broadcast address FF:FF:FF:FF:FF:FFtargets every device on the local network segment.

U/L Bit and Locally Administered Addresses

The second-least significant bit of the first octet is the U/L bit (Universal/Local). When set to 0, the address is universally administered — burned into the hardware by the manufacturer. When set to 1, the address is locally administered, meaning a network administrator or operating system has overridden the burned-in address. This is common in virtual machines, software-defined networking, and when masking device identity for privacy reasons (e.g., macOS and iOS private Wi-Fi MAC addresses).

EUI-64 and IPv6 SLAAC

A MAC address can be converted to an EUI-64 (64-bit Extended Unique Identifier) by inserting the bytes FF:FE between the first three and the last three octets, then flipping the U/L bit. The resulting 64-bit identifier is commonly used to generate IPv6 interface identifiers in Stateless Address Autoconfiguration (SLAAC). For example, the MAC 00:11:22:AA:BB:CC becomes the EUI-64 02:11:22:FF:FE:AA:BB:CC, which would form the last 64 bits of an IPv6 address like 2001:db8::211:22ff:feaa:bbcc.

Understanding MAC addressing is essential for network troubleshooting, switch configuration, ARP table analysis, and IPv6 planning. Use this tool alongside the IPv6 Subnet Calculator to explore how MAC-derived EUI-64 identifiers are used in modern networks.