Each node has a physical address for the specific hardware device that connects it to a network. For instance, a physical address on an Ethernet network is a 6-byte numeric value, such as 08-00-14-57-69-69. It is assigned by the manufacturer of the Ethernet interface hardware. X.25 networks, which conform to the specification of the ITU-T (International Telecommunications Union, Telecommunications sector), previously CCITT, use the X.121 standard for physical addresses, which consist of 14-digit numbers.

NOTE: Physical addresses are also called media access control (MAC) addresses. Throughout the rest of this chapter, all references to MAC or physical addresses assume physical addresses on Ethernet, token ring, or FDDI networks.

The IP address for a node is a logical address and is independent of any particular hardware or network topology. It has the same form, regardless of the media type. The IP address is a 4-byte (32-bit) numeric value that identifies both a network and a local host or node (computer or other device) on that network. The 4-byte IP address is usually represented in dotted decimal notation. Each byte is represented by a decimal number, and periods separate the bytes, for example, 129.47.6.17.

A conflict arises because IP uses a 32-bit address and Ethernet uses a 48-bit Ethernet address. To associate the IP address to a physical address on an Ethernet network, a mapping must occur between the two types. The address resolution protocol (ARP) provides a mapping between the two different forms of addresses. As a result of the mapping performed by ARP, the IP address is mapped to a physical address. ARP mapping is limited to networks that support hardware broadcast.