The Transmission Control Protocol/Internet Protocol is by far the most popular protocol suite in use today, thanks to the growth of the Internet. TCP/IP has changed a great deal over the years, as different protocols were created or adapted to address different needs. For example, TCP was originally developed back in 1974, while TCP and IP came together in 1978. It wasn’t until January 1st, 1983 that all systems on the Internet (then known as the ARPANET) officially had to use TCP/IP. The groups who set the direction for the Internet (and decide what will become a standard) are the Internet Engineering Task Force (IETF) and the Internet Engineering Steering Group (IESG).

Before we go any further, you should be aware of what are known as Requests For Comments (RFCs). Before anything becomes an Internet standard, it has to go through the RFC process. While all Internet standards are defined in RFCs, not all RFCs become standards. If you take the time to read RFC 1149 or 1438, you’ll understand what I mean. It’s also worth noting that RFCs can be superceded by newer versions. If a standard is changed, a new RFC document is created, and a new number is associated with it. An RFC contains the definitive information for any Internet standard. If you’re just starting out in the world of networking, get in the habit of using RFCs when you need clarification. You can be sure that the information they provide outlines the facts and they are a freely available resource, too. Just make certain that you’re reading the most current version.

Tip: One of the best resources for searching RFCs is The results supplied by this page outline RFC status information, as well as whether a particular RFC has been updated or is now obsolete.

A number of key protocols make up the TCP/IP suite, some of which we looked at briefly in Chapter 1. In this chapter we’ll go into more detail on each. A solid understanding of the protocols that make up the TCP/IP protocol stack is not only imperative for the exams, but also in real life. Troubleshooting any network is relatively simple when you truly understand how communication processes take place

TCP/IP and the OSI Model

The TCP/IP protocol stack is comprised of protocols that exist at the upper three layers of the TCP/IP model (the lower layer is the Network Interface layer, and is responsible for network technologies that we’ve already looked at such as Ethernet, Token Ring, and so forth). The main protocols that can be found at each layer have different roles and responsibilities, and having an appreciation of how they work (and their purposes) is imperative. The figure below outlines the protocols that we’re going to look at in this section, and how they map to the OSI model.

Figure: TCP/IP protocols and the OSI model.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.