Manual Pages for Commands Used in Chapter 11
There are manual pages in Appendix A for the following commands used in this chapter:
Chapter 12. Networking Background
This chapter covers the background of UNIX networking and the Chapter 13 covers networking configuration and using networking commands. If you don't have any UNIX background then you'll want to review this chapter before moving on to the
The following is a list of topics covered in this chapter:
An Overview of IEEE802.3, TCP/IP
In order to understand how the networking on your UNIX system works, you first need to understand the
Figure 12-1. ISO/OSI Network Layer Functions
I start reviewing Figure 12-1 at the bottom with layer 1 and then describe each of the four bottom layers. This is the International Standards Organization Open Systems Interconnection (ISO/OSI) model. It is helpful to visualize the way in which networking layers interact.
The beginning is the physical interconnection between the systems on your network. Without the
you can't communicate between systems, and all the great functionality that you would like to implement is not possible. The physical layer converts the data you would like to transmit to the analog signals that travel along the wire (I'll assume for now that whatever physical layer you have in place uses wires). The information traveling into a network interface is taken off the wire and prepared for use by the
In order to connect to other systems local to your system, you use the link layer that is able to establish a connection to all the other systems on your local segment. This is the layer where you have either IEEE 802.3 or Ethernet. Your UNIX system supports both of these "encapsulation"
Figure 12-2. Ethernet Encapsulation
Keep in mind that Ethernet and IEEE 802.3 will run on the same physical connection, but there are indeed differences between the two encapsulation methods. With your UNIX systems, you don't have to spend much, if any, time setting up your network interface for encapsulation.
Unfortunately, the information that IP uses does not conveniently fit inside an Ethernet frame, so you end up with
IP handles routing in a simple fashion. If data is sent to a destination connected directly to your system, then the data is sent directly to that system. If, on the other hand, the destination is not connected directly to your system, the data is sent to the default router. The default router then has the responsibility of getting the data to its destination. This routing can be a little tricky to understand, so I'll cover it in detail shortly.
The trasport level is the next level up from the network layer. It communicates with ports . TCP is the most common protocol found at this level, and it forms packets that are sent from port to port. The port used by a program is usually defined in /etc/services , along with the protocol (such as TCP). These ports are used by network programs such as telnet, rlogin, ftp , and so on. You can see that these programs, associated with ports, are the highest level we have covered while analyzing the layer diagram.