ENTERPRISE NETWORKS II: WIDE AREA NETWORKS
Connecting one LAN to another LAN via a WAN with routers equipped with IP.
from an originating host, which runs an applications program, to another host in another
network as shown in Figure 12.2. This may be a LAN-to-WAN-to-LAN connectivity as
shown in the figure. It may also be just a LAN-to-WAN or it may be a WAN-to-WAN
connectivity. The host would enter its own network by means of a network access protocol
such as HDLC or an IEEE 802 series protocol (Chapter 11).
A LAN connects via a router (or gateway) to another network. Typically a router (or
gateway) is loaded with three protocols. Two of these protocols connect to each of the
attached networks (e.g., LAN and WAN), and the third protocol is the IP, which provides
the network-to-network interface.
Hosts typically are equipped with four protocols. To communicate with routers or
gateways, a network access protocol and Internet protocol are required. A transport layer
protocol assures reliable communication between hosts because end-to-end capability is
not provided in either the network access or Internet protocols. Hosts also must have
application protocols such as e-mail or file transfer protocols (FTPs).
TCP/IP and Data-Link Layers
TCP/IP is transparent to the type of data-link layer involved, and it is also transparent
whether it is operating in a LAN or WAN domain or among them. However, there is
document support for Ethernet, IEEE 802 series, ARCNET LANs, and frame relay for
WANS (Refs. 2, 5).
Figure 12.3 shows how upper OSI layers are encapsulated with TCP and IP header
information and then incorporated into a data-link layer frame.
For the case of IEEE 802 series LAN protocols, advantage is taken of the LLC common
to all 802 protocols. The LLC extended header contains the SNAP (sub-network access
protocol) such that we have three octets of the LLC header and five octets in the SNAP.
The LLC header has its fields fixed as follows (LLC is discussed in Chapter 11):
The five octets in the SNAP have three assigned for protocol ID or organizational code and
two octets for "EtherType." EtherType assignments are shown in Table 12.1. EtherType