A+ Certification / Beginners

The TCP/IP Protocol

Since the dramatic growth of the Internet, Transmission Control Protocol/Internet Protocol (TCP/IP) has become the preferred protocol on networks today. TCP/IP is the common protocol on all desktops, including Windows, Linux, and Macintosh systems - allowing all of these different operating systems to communicate over a common protocol. It doesn't matter what operating system you are running or what kind of network you have - as long as you are running a common protocol such as TCP/IP, you can access resources across any platform.

TCP/IP is installed by default with all the major operating systems such as Windows 2000, Windows XP, and Linux. When configuring TCP/IP on these systems there are three major settings that need to be configured to allow the computer to communicate with other computers on different networks or the Internet:

  • IP address
  • Subnet mask
  • Default gateway

In order to troubleshoot communication across TCP/IP, it is important that you understand the types of settings that need to be configured. In the sections that follow, you look at how to configure TCP/IP and at some utilities to help you troubleshoot the protocol.

IP address

The IP address is a 32-bit number that is unique to your computer. No two systems can have the same IP address. An IP address is similar to the address of your home, which is the method by which other people send mail to you. An IP address works the same way on a TCP/IP network - you will assign the number to your computer, and it is the method other computers use to send information to your computer.

An IP address is made up of four sets of numbers separated by periods. This is called the dotted decimal notation format of the IP address. An example of an IP address in the dotted decimal notation format is Each of the four sets of numbers is referred to as an octet (because each octet represents 8 bits of data).

The IP address is made up of a network ID and a host ID:

  • The network ID is a unique number used only by your network and is the same for all computers on the same network.
    For example, in the IP address, the first two octets (131.107) make up the network ID. So if Computer A with an IP address of talks to Computer B, which has the IP address of, you can assume that the two computers are on the same network because 131.107.x.y is the network ID for both computers.
  • The host ID portion uniquely identifies a computer on the network.
    For example, in the IP address, the last two octets (2.200) make up the host ID. Only one computer on the 131.107.x.y network can have the host ID of 2.200.

Is the network ID always the first two octets? The answer is no. The network ID isn't always the first two octets of the IP address. So how do you know which octets make up the network ID and which octets make up the host ID portion of the IP address? What class of IP address you have dictates which numbers correspond to which IDs.

There are three major classes of IP addresses: Class A, Class B, and Class C. The different IP address classes support a different total number of workstations on the network. For example, a Class A network (a network using Class A addresses) supports up to 16,777,214 network devices, while a Class B network supports 65,534 network devices, and a Class C network supports only 254 network devices.

Class A networks use the first octet as their network ID and the remaining three octets as the host ID. A Class B network uses the first two octets for the network ID and the last two octets as the host ID. A Class C network uses the first three octets as the network ID and the last octet for the host ID.

How do you know what class IP address you have? Look at the first octet. If it has a value between 1 and 126, it's a Class A IP address; if it has a value between 128 and 191, it's a Class B IP address; if it has a value between 192 and 223, it's a Class C IP address. Table below summarizes IP address classes.

IP Address Classes
Network ClassNumber of HostsOctet SummaryFirst Octet Value
Class A16,777,214n.h.h.h1-126
Class B65,534n.h.h.h128-191
Class C254n.h.h.h192-223

You will notice that in the number of hosts column there are actually two numbers missing. For example, a class C address can have 256 possible addresses numbered 0 through 255. But you are not allowed to use the 0 because it is reserved for what is known as the network ID. Also, you are not allowed to use the 255 because it is the broadcast address - which is how systems send data to every computer on the network. To summarize, with each of the address classes you will loose two addresses due to the network ID and the broadcast address.

Notice in Table above that the number 127 is skipped in the First Octet Value column. This is because it is illegal for a system on the network to have an IP address that starts with 127. Any address starting with 127 is illegal because this address is reserved for the loopback address. The loopback address is an address that always refers to the TCP/IP software stack that initialized on your system. Typically, the loopback address is referred to as, but you could use any address that starts with 127. For example, you could issue the following command in a command prompt to test that TCP/IP is functioning properly on your system:


No matter what your IP address is, the loopback address is a constant that you can use to verify that your system is running correctly. Here's an IP address example: Computer A has an IP address of, and it initiates communication with Computer B, whose IP address is Are the two workstations on the same network? Looking at the first octet, you can see that the number 194 is a Class C address, which means that the first three octets make up the network ID, and the last octet is the host ID. Since there is a difference in the first three octets (which is the network ID), these two computers are on different networks.

Identifying whether the computer you are trying to communicate with is on your network could be important when troubleshooting communications. If the computer that you are trying to communicate with is not on your network, then the problem could be with your computer, the remote computer, or the router. This means you have more places to look for the cause of the problem.

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