This section provides a simplified overview of TCP/IP for the purposes of later discussion on Internet-related security problems. [Com91a], [Com91b], [Ford94], [Hunt92], and [Bel89] provide more complete descriptions; readers who wish to learn more should consult these references.
Part of the popularity of the TCP/IP protocol suite is due to its ability to be implemented on top of a variety of communications channels and lower-level protocols such as T1 and X.25, Ethernet, and RS-232-controlled serial lines. Most sites use Ethernet connections at local area networks to connect hosts and client systems, and then connect that network via a T1 line to a regional network (i.e., a regional TCP/IP backbone) that connects to other organizational networks and backbones. Sites customarily have one connection to the Internet, but large sites often have two or more connections. Modem speeds are increasing as new communications standards are being approved, thus versions of TCP/IP that operate over the switched telephone network are becoming more popular. Many sites and individuals use PPP (Point-to-Point Protocol) and SLIP (Serial Line IP), to connect networks and workstations to other networks using the switched telephone network.
TCP/IP is more correctly a suite of protocols including
TCP and IP, UDP (User Datagram Protocol), ICMP (Internet Control Message
Protocol), and several others.
The TCP/IP protocol suite does not conform exactly to the Open Systems
Interconnection's seven layer model, but rather could be pictured as shown
in figure 
.
Figure: Conceptual View of Services and Layers in TCP/IP.