Tcp/ip

A crash course in TCP/IP

At first glance, TCP/IP (Transmission Control Protocol/Internet Protocol) may seem baffling. Many other protocols, such as NetBEUI and IPX/SPX, require no configuration. TCP/IP is different. Due to the seemingly endless number of options that you can configure within TCP/IP, many people become intimidated at first. In reality, however, TCP/IP isn’t very difficult, but you have to gain some understanding of what you’re configuring. So, I’m going to give you a crash course in TCP/IP. Although I won’t be able to explore every feature in detail, I’ll cover the important points.

The IP address

The most basic element of TCP/IP is the IP address. The IP address is a number that’s unique to each computer. If you know a computer’s IP address, you can communicate with that computer from anywhere in the world. Since TCP/IP is the protocol that the Internet uses and since Internet servers are located all over the world, TCP/IP must be routable. Thus, when you try to access an IP address, your computer must be able to tell whether or not that IP address is located on your local network. If the desired address is located on your local network, you won’t have a problem reaching it. If it isn’t on your local network, TCP/IP must know which network the IP address is located on in order to reach the address.

The network number represents the network that contains a given IP address. If you look through the various tabs of the TCP/IP properties sheet, you’ll see that there’s no field that allows you to specify the network number. Instead, the network number is part of the IP address.

An IP address is composed of a network number and a computer number. Your computer can distinguish those two numbers because of something called the subnet mask. The subnet mask is located in a field directly below the IP address on the TCP/IP properties sheet. A simple subnet mask would be something like 255.255.0.0. The numbers that make up the subnet mask indicate which portion of the IP address is the network number and which portion is the computer number. The four numbers of the subnet mask correspond directly to the four numbers in the IP address. For example, if you had a computer with an IP address of 147.100.100.25 and a subnet mask of 255.255.0.0, the first two numbers in the subnet mask (both are 255) indicate that the first two numbers of the IP address are the network number. The second two numbers (both are 0) indicate that the second two numbers of the IP address are the computer number. Therefore, in the IP address 147.100.100.25, the 147.100 portion denotes which network the computer is located on, and the 100.25 portion represents a particular computer on that network. Of course, subnet masks become much more complex than this example. For instance, you can subdivide an individual network.

The default gateway

Now that you know how TCP/IP determines whether a destination address is located on a local network or on a foreign network, you may wonder how it attaches to a foreign network, especially if that network is halfway around the world. Well, it completes this job by using routing tables. Routing tables, which are stored in your routers, tell the router where to connect in certain situations.

You should note that one of the TCP/IP configuration options is for a default gateway. The default gateway is the address where a TCP/IP packet is sent if TCP/IP can’t locate the destination IP address on the local network. Usually, the default gateway points