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1.4 Network Addressing

Network addressing identifies either individual devices or groups of devices on a LAN. A pair of network devices that transmit frames between each other use a source and destination address field to identify each other. These addresses are called unicast addresses, or individual addresses, because they identify an individual network interface card (NIC).

1.4.1 MAC Addresses

The IEEE defines the format and assignment of network addresses by requiring manufacturers to encode globally unique unicast Media Access Control (MAC) addresses on all NICs. The first half of the MAC address identifies the manufacturer of the card and is called the organizationally unique identifier (OUI).

Ethernet or Token Ring router interfaces and all network interface cards (NICs) are identified with a unique burned-in address (BIA), called the MAC address or the physical address. The MAC address is implemented at the Data-Link Layer (Layer 2) of the OSI reference model to identify the station. The MAC address is 48 bits in length (6 octets) and is represented in hexadecimal format.

The first three bytes of a MAC address form the Organizational Unique Identifier (OUI), which identifies the manufacturer. The last three octets are administered by the manufacturer and assigned in sequence.

1.4.1.1 Noncanonical and Canonical Transmission

When converting hexadecimal MAC addresses to binary, each hexadecimal number is represented by its binary equivalent. Thus, dc-c0-df-fa-0f-3c is 11011100 11000000 11011111 11111010 00001111 00111100 when converted to binary.

In Ethernet networks, each octet of the MAC address is transmitted from left to right, but from LSB to MSB. The difference is that for each octet, the LSB is transmitted first and the MSB is transmitted last. Thus, the first octet of MAC address dc-c0-df-fa-0f-3c, i.e., dc is transmitted from left to right as 00111011; the second octet, c0, is transmitted as 00000011; the third octet, df, is transmitted as 1111111011; etc. This is called canonical transmission and is also known as LSB first.