The Transmission Control Protocol/Internet Protocol (TCP/IP) has become the common language for the networking world. It is the protocol suite (or stack) that serves as the foundation for the mega-network known as the Internet. Most network operating systems, such as Windows 2000 Server, Novell NetWare 5.x, and the many different flavors of Unix and Linux, now embrace TCP/IP as their default networking protocol.

TCP/IP was developed by the Defense Advanced Research Projects Agency (DARPA). It was originally developed as a wide area networking protocol suite that could be used to maintain communication links between sites even if certain sites became inoperable during a worldwide nuclear war; however, another underlying reason for the creation of the TCP/IP protocol stack was that the Department of Defense needed a protocol stack that could communicate across dissimilar networks. These different types of networks existed because the government uses a bidding system and suddenly found itself with different computer systems at various branches of the Defense Department: the army, navy, and so on. Therefore, TCP/IP is jokingly called the "protocol of low bid."

TCP/IP contains a number of "member" protocols that make up the actual TCP/IP suite. Because the TCP/IP protocol stack was developed before the completion of the OSI reference model, these protocols do not map perfectly to the various layers of the model.

The Department of Defense Model

The TCP/IP suite was actually developed using a different conceptual model, called the Department of Defense (DoD) model . The DoD model has four layers:

  • Application layer . This layer in the DoD stack is equivalent to the OSI Application, Presentation, and Session layers. This layer provides the user interface for the various protocols and applications that access the network, and it handles file transfer, remote logon to other nodes, email functionality, and network monitoring.

  • The Host-to-Host layer . Equivalent to the OSI Transport layer, this layer provides flow control and connection reliability as data moves from a sending to a receiving computer. This layer takes the data from the Application layer protocols and begins the process of readying the data for movement out over the network.

  • The Internet layer . This layer corresponds to the OSI Network layer. It is responsible for the routing of data across logical network paths and provides an addressing system to the upper layers of the DOD stack. This layer also defines the packet format used for the data as it moves onto the network.

  • The Network Access layer . This layer is equivalent to the Data Link layer of the OSI model and consists of the protocols that take the packets from the Internet layer and package them in an appropriate frame type. The various frame types are described by the IEEE specifications we discussed in Chapter 4, "Building the Network Infrastructure."

We have been concentrating on the OSI model in our discussion of network protocol stacks, which is the accepted conceptual model for networking today. Figure 5.3 shows some selected protocols from the TCP/IP stack mapped to the OSI layers.

Figure 5.3. The TCP/IP stack mapped to the OSI model.


TCP/IP Protocols

A number of different protocols make up the TCP/IP stack. There are protocols that handle network access, data transport, and the logical addressing of data packets as they are routed over a network. Table 5.3 provides a brief explanation of some of the key TCP/IP protocols in the stack (including those shown in Figure 5.3).

Table 5.3. TCP/IP Protocol Stack Members




The Hypertext Transport Protocol defines the message format and transmission used by Web servers and Web browsers.


The Telnet protocol is the terminal emulation protocol that allows you to connect a local computer with a remote computer or other remote device.


The File Transfer Protocol provides an interface and services for file transfer on a network.


The Simple Mail Transport Protocol provides email services on the Internet and IP networks.


The Transmission Control Protocol is a connection-oriented transport protocol.


The User Datagram Protocol is a connectionless transport protocol.


The Internet Protocol is the basis for all addressing on TCP/IP networks and provides a connection-oriented Network layer protocol.


The Address Resolution Protocol maps IP addresses to MAC hardware addresses.


The Routing Information Protocol is a routing protocol used by routers to determine the best path for packets traveling on an internetwork.

As you can see from Table 5.3, the TCP/IP stack contains protocols that fulfill all the needs for data movement on the network as related to the OSI conceptual model. Protocols such as HTTP and FTP actually provide functions that operate at three layers of the OSI model (Application, Presentation, and Session). Although we refer to FTP as a protocol , it is actually a large protocol that provides a user interface and functions like a mini-application. FTP client software was used a great deal to transfer files on the Internet before HTTP became available.

IP Addressing

Another important aspect of TCP/IP is the logical addressing scheme that TCP/IP uses at the Network layer of the OSI model. This logical addressing is provided by the IP protocol. Everyone with access to the Internet has probably run across an IP address. It takes the form of four decimal numbers separated by periods. This format is referred to as dotted decimal .

This dotted decimal notation actually shows us the address in a format we understand (a series of decimal numbers). Computers and other devices on the network actually see this address as a decimal stream of 32 bits. Each of the four parts of the address are referred to as an octet . That is because each octet contains eight bits of information (a bit being either a 1 or 0).

For example, the IP address would be seen by devices on the network as this:

10000010 00000001 00010000 00000001



TCP/IP is the protocol of choice for routing data between networks. These large networks are best described as "networks of networks" (many LANs connected together) and are referred to as internetworks . Another term that is often used to refer to internetworks is enterprise networks . This describes the entire network infrastructure for a large company or institution that may be made up of multiple sites across a geographical area, a country, or even the world.

IP Address Classes

Because IP-based networks can be of varying sizes (very large, large, medium, and small), the architects of the IP addressing system set up different classes of IP addresses to accommodate different sizes of networks. Three classes of addresses for use on networks exist: Class A, B, and C. Here's more information on each class type:

  • Class A . Used for very large networks and supplies more than 16 million node addresses for the network. Due to the way IP addresses are structured, a Class A network can serve a huge number of host computers (nodes); however, there can only be 127 Class A networks. ARPAnet (built in the early days of the Internet) is an example of a Class A network.

  • Class B . Used for networks that still need a lot of node addresses, such as for large companies and institutions. There are 16,384 Class B network addresses available, with each Class B network supplying more than 65,000 host addresses.

  • Class C . Used for small networks. There are more than 2 million Class C network addresses available. Class C networks only provide 254 node addresses, however.

Two other classes of IP addresses should also be mentioned: Class D and Class E. Class D network addresses are used by multicast groups receiving data from a particular application or server service. An example of a multicast use of Class D addresses is Microsoft NetShow, which can broadcast the same content to a group of users at one time. Class E addresses belong to an experimental class, which is not available for use by folks like you and me.

Finding your Class

You can quickly identify the class to which an IP address belongs by looking at the decimal value of the first octet; each class (A, B, or C) has a specific first octet range:

  • Class A (1126)

  • Class B (128191)

  • Class C (192223)

Notice that 127 is missing from the range of numbers. It is used for special testing of the IP configuration of a computer.



You can easily convert dotted decimal IP addresses to their binary equivalents using any scientific calculator, such as the Windows Calculator, that can be viewed in scientific mode. Be sure that the Dec (decimal) option button is selected and then enter the first octet's decimal number. Then select the Bin (binary) option button. Each binary number should consist of eight digits. You might have to add a leading zero or two to make up the eight positions . Repeat this procedure for each octet.

The Subnet Mask

In the Old West, a cowboy was nothing without his horse. Likewise, IP addresses really don't mean anything without an accompanying subnet mask. Devices on a network need to be able to tell what part of the IP address is providing information related to which network the computer with a particular address is on, as opposed to what part of the address actually refers to just the computer or node itself. This is determined by the subnet mask. Devices on the network use the subnet mask to "mask" out the portion of the IP address that refers to the network that the node sits on. If you look back at Figure 5.4, you'll notice that a computer must be configured with both an IP address and the appropriate subnet mask.

Figure 5.4. The IPX/SPX stack mapped to the OSI model.


Each class has a default subnet mask:

  • Class A:

  • Class B:

  • Class C:

Without going into a lengthy and complex explanation of how computers and other devices use a subnet mask to determine certain information from an IP address, the default subnet masks provide some obvious visual clues. If you remember, a Class A network provides more than 16 million node addresses. Also, there are only 126 Class A network addresses available.

If you look at the Class A subnet mask, you will notice that 255 only appears in the first octet. In binary, 255 would be 11111111, meaning that all eight bits are turned on. These "turned on" bits in the subnet mask actually mask out the first octet of any Class A IP address. This tells a computer that the first octet holds the network information.

Notice that all the other octets in the Class A subnet mask are 0this would be 00000000 in binaryand would not mask out the information in the second, third, and fourth octets. This allows these octets to be used for node addresses. Each octet contains eight bits, so in a Class A network, eight bits are used to determine network information, and the other 24 bits are used for node addresses. This is why so many different possibilities are available for node addresses (again, more than 16 million).

In the case of Class B networks, only the third and fourth octets are used for node addresses; this is because the first and second octets are masked out by the subnet mask and provide the network information. With only 16 bits available for node addresses, this means that Class B networks supply fewer node addresses (around 65,000).

Using this logic, you can see why Class C networks supply so few IP addresses (254). This is because only the fourth octet is reserved for node addressing, and the rest of the octets are used for network information. The subnet mask masks out all the octets in a Class C address except for the last octet.



IP networks can be divided into segments called subnets . We take a look at some of the basics of subnetting in Chapter 12, "TCP/IP Network Administration." We also look at configuring IP addresses and subnet masks on network computers in Chapter 12.

Absolute Beginner's Guide to Networking
Absolute Beginners Guide to Networking (4th Edition)
ISBN: 0789729113
EAN: 2147483647
Year: 2002
Pages: 188
Authors: Joe Habraken © 2008-2017.
If you may any questions please contact us: