11.1 IEEE 802 Family, LAN Standards, and IP

The 802 family of standards from the Institute of Electrical and Electronics Engineers (IEEE) defines various types of LANs. Of these, 802.3 (which is commonly known as the Ethernet) is the most popular LAN technology and is deployed widely throughout the world. Other LAN standards from IEEE include 802.4 (token bus), 802.5 (token ring), and 802.6 (FDDI) (Figure 11-1).

Figure 11-1. IP and the IEEE 802 family.

The IEEE 802.1 standard provides an introduction to the set of 802 series of standards and defines interface primitives. The IEEE 802 series of standards operate in the layer 1 and layer 2 (i.e., the physical and data link layers ), of the seven-layer OSI or the TCP/IP protocol models.

The logical link control (LLC) is the upper part of the data link layer and helps establish peer-to-peer logical links between two 802 LAN protocol entities. The LLC layer is common to all 802 LAN protocols, creating a layer of transparency between data link and network layer protocols. This allows several 802 LAN protocols to be defined beneath the LLC layer that differ in their physical layer and media access control (MAC) layer characteristics.

The MAC layer is the lower part of the data link layer. The MAC layer defines different addressing formats and specific protocols used to arbitrate access to the 802 LAN system, for the various 802 LAN standards. The MAC address is 48 bits in length and is a layer 2 address, used only in the data link layer. Of the 48 bits, the first two bits are for control and administrative purposes. The next field is the vendor ID, which is generally assigned by the IEEE to any organization that needs it for use on devices such as personal computers, servers, and printers.

The second part of the MAC address is "locally assigned" by the organization that owns the vendor ID. When the IEEE assigns these addresses, they are truly global addresses and ensure that no two stations in the world have the same MAC address.

The physical layer defines signaling mechanisms and transmission characteristics for the different LAN standards. For example, in the 802.3 protocol, the physical layer defines data over media such as coaxial cable, copper , and fiber- optic cable.

11.1.1 IP over LANs

IP is a network layer protocol, and the power of internetworking is that the IP layer does not depend on any specific layer 2 protocol. The existence of the IEEE 802.2 LLC layer beneath the IP layer was designed to allow e-mail, business applications, or network management software to work on different types of 802 LANs, creating transparency at the LLC layer. The LLC layer also allows other protocols, such as AppleTalk, to run over 802 LANs as well.

The most common LAN in the 802 family is the 802.3 Ethernet LAN. Mechanisms are clearly defined to map IP addresses to 48-bit MAC layer addresses. For example, this may be provided as part of the driver software when installing a LAN card in a computer. This driver software creates a mapping of the IP address to a specific MAC address of the computer.

Other mechanisms to help associate IP addresses to MAC addresses include the Address Resolution Protocol (ARP). In addition, the IEEE 802 standards define mechanisms to map unicast, multicast, and broadcast IP addresses to MAC layer addresses, allowing various types of applications to run over Ethernet LANs.