efense communities to share research resources. It dates back to 1998 and was well into existence before ISO and CCITT took interest in layered protocols. The TCP/IP suite of protocols (Refs. 2, 3) has wide acceptance today, especially in the commercial and industrial community worldwide. These protocols are used on both LANs and WANs. They are particularly attractive for their Internet-working capabilities. The Internet protocol (IP) (Ref. 2) competes with CCITT Rec. X.75 protocol (Ref. 4), but is notably more versatile and has much wider application. The architectural model of the IP (Ref. 2) uses terminology that differs from the OSI reference model. 1 Figure 12.1 shows the relationship between TCP/IP and related DoD (Department of Defense) protocols and the OSI reference model. Tracing data traffic TCP/IP and Related Protocols Application File transfer Electronic mail Terminal emulation Presentation File transfer Simple mail transfer Telnet protocol (FTP) protocol protocol Session (SMTP) Transmission control User datagram protocol Transport protocol (TCP) (UDP) Address resolution Internet Network Internet control message protocol (ARP) protocol (IP) protocol (ICMP) Network interface cards Data link CSMA/CD (Ethernet), Token Ring, ARCNET, StarLan Transmission media Physical Wire pair, fiber optics, coaxial cable, radio Figure 12.1 How TCP/IP and associated protocols relate to OSI. 1 IP predates OSI. ENTERPRISE NETWORKS II: WIDE AREA NETWORKS LAN #1 IP router IP router LAN #2 Figure 12.2 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. 