2.13. Summary

In this chapter, we have made a quick tour of the TCP/IP protocols and their data formats. We began by discussing layering and encapsulation and their roles in networking architecture. These conceptual tools allow us to divide the network's functions into discrete sets, or layers, that can be more easily understood in isolation. Communication between layers follows a well-defined API (application programming interface) that allows changes to one layer without affecting the others.

In our examination of IP addressing, we saw that the classical division of addresses into five classes leads to the growth of routing tables and the depletion of globally routable addresses. CIDR helps solve these problems by generalizing classful addressing to allow fine-grained control over the boundary between the network and host IDs in the address. Although classful addressing is still mandated by the IETF as the standard, virtually all addressing uses CIDR.

Next we studied the IP, TCP, UDP, and ICMP protocols. These protocols make up the core of the TCP/IP suite and are the foundation on which applications are built. While discussing these protocols, we studied their operation on the wire by examining tcpdump traces. Although examining the protocols at this level may seem like overkill in an overview, it helps us to develop the skill of reading tcpdump output and dissecting network packets. We will use these skills often as we discuss tunneling and VPNs in the rest of the text.

We continued our study of addressing by considering RFC 1918 private addresses and NAT. Although NAT has several undesirable effects, it is a useful and popular tool to conserve IP global addresses, foster independence from any particular ISP, and reduce costs by allowing several hosts to share the same global IP address. The combination of NAT and CIDR is providing a short-term solution to the IP address depletion problem until the long-term solution, IPv6, can be deployed.

PPP is, of course, just another interface layer protocol like Ethernet or Token Ring, but because it is often used as a vehicle for IP datagrams in tunnels, we made a cursory examination of it. We usually think of PPP as a protocol running on serial links, but it can run over a variety of media. PPP combines the simple point-to-point serial-link protocol of SLIP with the framing and some of the functionality of HDLC.

Next, we examined IPv6. Although we will not dwell much on IPv6, it is important to understand some of its features. This is particularly true because IPsec, which we discuss in Part 3, is part of IPv6.

Finally, we briefly discussed routing. We introduced the notion of autonomous systems and noted that the interior gateway protocols used within an AS differ from the exterior gateways protocols used among autonomous systems.