Computer Networking: Internet Protocols in Action

Chapter 5.1. MAC Addresses and the Address Resolution Protocol (ARP)

INTRODUCTION

At every layer of the network protocol stack, unique numbers are assigned to differentiate entities. IP addresses are used to identify hosts on the Internet and port numbers are used to distinguish network applications executing on a host. At the link layer, there are Media Access Control or MAC addresses.

Within a local area network, each network device must have a unique MAC address. Devices listen to the shared communication medium for packets with a destination MAC address that matches their own. If two devices had the same MAC address, they would both attempt to act upon and reply to the same messages.

MAC addresses are typically hardcoded into the network interface card itself. For example, if you install an Ethernet card or a wireless card into your computer, the card itself contains its unique MAC address.

MAC addresses are 48 bits or 6 bytes in length. They are typically written in a colon-separated hexadecimal format such as 00:06:5b:e3:4d:1a. In this format, each colon-separated number represents a byte of the address.

To ensure the uniqueness of MAC addresses, the IEEE (Institute of Electrical and Electronics) Registration Authority administers these addresses. Companies that manufacture network interface cards apply to the IEEE for a 24 bit OUI or Organizationally Unique Identifier. Typically, this involves an application fee and a yearly renewal fee. This company ID can then be used as the ...