A MAC address is 48 bits long and is represented as a hexadecimal number. Represented in hex, it is 12 characters in length, where each character is 4 bits. Three numbering spaces, managed by the Institute of Electrical and Electronics Engineers (IEEE), are in common use for formulating a MAC address: 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. To make it easier to read, the MAC address is represented in a dotted hexadecimal format, like this: FFFF. FFFF.FFFF. Some formats use a colon (:) instead; and in some cases, the colon separator is spaced after every two hexadecimal digits, like this: FF:FF:FF:FF:FF:FF. As mention in Chapter 2, the first six digits of a MAC address are associated with the vendor, or maker, of the NIC. Each vendor has one or more unique sets of six digits. These first six digits are commonly called the organizationally unique identifier. (OUI). The last six digits are used to represent the NIC uniquely within the OUI value. In theory, each NIC has a unique MAC address. In reality, however, this is probably not true. What is important for your purposes is that each of your NICs has a unique MAC address within the same physical or logical segment. A logical segment is a virtual LAN (VLAN) and is referred to as a broadcast domain. Some devices, such as Cisco routers, might allow you to change the MAC address for a NIC, while others won’t.
In TCP/IP networks, the MAC address of a subnet interface can be queried with the IP address using the Address Resolution Protocol (ARP) for Internet Protocol Version 4 (IPv4) or the Neighbor Discovery Protocol (NDP) for IPv6. On broadcast networks, such as Ethernet, the MAC address uniquely identifies each node and allows frames to be marked for specific hosts. It thus forms the basis of most of the Link layer (OSI Layer 2) networking upon which upper layer protocols rely to produce complex, functioning networks.