ITS 2330 - Linux III

Chapter 6: Navigating Network Services

Objectives:

This lesson takes place in week 7. Objectives important to this lesson:

  1. Basic network configuration
  2. Advanced network configuration
  3. Network issues
Concepts:

Chapter six begins with a discussion of network models, starting with a pretty light exposure to the ISO-OSI model. In fact, it only covers four of the seven ISO layers, but it is meant to be more like the Department of Defense (DoD) model or the TCP/IP model. In both of those models function of the bottom two and top three ISO layers are gathered into two composite layers, represented by the colors purple and green in the chart below.

DoD, TCP/IP, and OSI Models
Functional Description DoD Layers TCP/IP Layers OSI Layers What it's For
Upper Layer Processes Process/Application Application 7 - Application services and programs
6 - Presentation translation across networks
5 - Session set up and end connections
Reliable Connections Host-to-host Transport 4 - Transport guarantee delivery
Internetwork Connections Internet Internet 3 - Network find route to other networks
Hardware/Network
Connections
Network Access Network Interface 2 - Data-Link access local media
Physical 1- Physical wiring, bit transmission, send and receive signals

It's only fair to mention that a model is only that, a model, a simplified way of looking at reality. First, a bit of history. When the Internet was created, it was not popular, it was not graphic, it was not attractive, but it established a need that eventually would be felt by all computer users. The need to connect to devices on other networks led to the complications of the yellow, blue, and green layers in the chart shown above. There are lots of things that happen in networks that did not happen in the beginning, mostly related to connecting them together. If we only cared about devices in one location that never connected to anything outside their own network, we might not need any services or protocols above layer 2 in the ISO model. We care about a lot more now, and our models are a lot more complex than the early ones were. If you need a refresher on models and the way networks work, play this video from Network Chuck.


Moving back to the chapter, the author talks about basic concepts for a few pages, like wired vs. wireless, and IP addressing It is also a bit confusing that she calls a subnet mask a netmask address. I guess it gets hairier with IPv6, but her example is with IPv4.

The author reminds us that every MAC address on a network must be unique (and in the world, for that matter), and that every IP address on a network must be unique. The leads to a useful observation that if you can get to a command prompt, knowing two commands will show you the IP address and MAC address of most computers:

  • ipconfig - good for Windows operating systems (typically used by entering ipconfig /all)
  • ifconfig - good for UNIX, Linux, and Mac OS X operating systems

These commands will give you more useful information as well. They will typically show you the subnet mask (netmask address) and the default gateway (default router) for the device you are examining. The author also discusses domain names, and explains that the leftmost label (yes, the things separated by dots are labels, and the dots in a domain name are real, unlike those in an IPv4 address) is typically a hostname, standing for a particular device or service.

The author's discussion of networking hits only some of the highlights. You should make sure you understand her concepts are starting points.

The next section of the chapter is about network configuration, where some of the features already discussed are managed. Where is important in this section, because there is no agreement on a standard place to store settings for hostname, IP address, default DNS and gateway servers, and so on. We are given three locations for these files, for Red Hat, Ubuntu, and Suse distros. In the video below, Eli the Computer Guy talks about Ubuntu, and about the lack of continuity between distros. He also demonstrates using the ip command instead of the ifconfig command.


On page 288, the author starts a section on troubleshooting which continues to the end of the chapter. Practice using the well known tools to have an idea of the behaviors you should expect on a network you own or protect.

  • ping
  • traceroute
  • host
  • dig
  • netstat
  • nmap