CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [413]
Each packet has two ports assigned, a destination port and an ephemeral port. The destination port is a fixed, predetermined number that defines the function or session type. Common TCP/IP session types use destination port numbers in the range 0–1023. The ephemeral port is an arbitrary number generated by the sending computer; the receiving computer uses the ephemeral port as a destination address so that the sending computer knows which application to use for the returning packet. Ephemeral ports usually fall in the 1024–5000 range, but this varies slightly among the different operating systems.
Ports enable one computer to serve many different services, such as a Web server and e-mail server, at the same time. We will discuss the most common ports and the associated services in the next chapter.
TCP/IP Services
TCP/IP is a different type of protocol. Although it supports File and Printer Sharing, it adds a number of special sharing functions unique only to it, lumped together under the umbrella term TCP/IP services. The most famous TCP/IP service is called Hypertext Transfer Protocol (HTTP), the language of the World Wide Web. If you want to surf the Web, you must have TCP/IP. But TCP/IP supplies many other services beyond just HTTP. By using a service called Telnet, for example, you can access a remote system as though you were actually in front of that machine.
Another example is a handy utility called PING. PING enables one machine to check whether it can communicate with another machine. Figure 23-24 shows an example of PING running on a Windows Vista system. Isn’t it interesting that many TCP/IP services run from a command prompt? Good thing you know how to access one! I’ll show you other services in a moment.
Figure 23-24 PING in action
The goal of TCP/IP is to link any two hosts (remember, a host is just a computer in TCP/IP lingo), whether the two computers are on the same LAN or on some other network within the WAN. The LANs within the WAN are linked together with a variety of connections, ranging from basic dial-ups to dedicated high-speed (and expensive) data lines (Figure 23-25). To move traffic between networks, you use routers (Figure 23-26). Each host sends traffic to the router only when that data is destined for a remote network, cutting down on traffic across the more expensive WAN links. The host makes these decisions based on the destination IP address of each packet.
Figure 23-25 WAN concept
Figure 23-26 Typical router
TCP/IP Settings
TCP/IP has a number of unique settings that you must set up correctly to ensure proper network functioning. Unfortunately, these settings can be quite confusing, and there are quite a few of them. Not all settings are used for every type of TCP/IP network, and it’s not always obvious where you go to set them.
Windows makes this fairly easy by letting you configure both dial-up and network connections by using the Network Connections dialog box (Figure 23-27). To get there, right-click on My Network Places (Windows 2000/XP) or Network (Windows Vista/7) and select Properties. In Vista/7, you have to click the Manage network connections button, but in 2000 and XP, you simply select the connection you wish to configure and then set its TCP/IP properties.
Figure 23-27 Network Connections dialog box showing dial-up and LAN connections
The CompTIA A+ certification exams assume that someone else, such as a tech support person or some network guru, will tell you the correct TCP/IP settings for the network. Your only job is to understand roughly what they do and to know where to enter them so the system works. Following are some of the most common TCP/IP settings.
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EXAM TIP The CompTIA A+ certification