Network administrators and security professionals may need to look-up the manufacturer of an Ethernet MAC address seen in equipment logs and active traffic listings. The burned-in MAC address of each system is unique and registered with the IEEE. These registrations are allocated in 24-bit blocks called MA-L (MAC Address – Large) blocks containing 16,777,216 addresses. Each MA-L assignment is registered as a 24-bit OUI (Organizationally Unique Identifier). Ethernet devices are identified by a 48-bit MAC address that starts with the registered OUI and ends with a manufacturer-assigned 24-bit number (usually allocated sequentially). The OUI is normally written as a six-digit hexadecimal number, each eight bits (two hex digits) are usually separated by a “-” or “:” character for readability. Technically these apply to more than just Ethernet – IEEE says they are for “Universal LAN MAC addresses” (wired ethernet, wifi, token ring, etc).
You can lookup registered OUI codes on the IEEE website at: http://standards.ieee.org/develop/regauth/oui/public.html