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TCP/IP Features

TCP/IP includes many important features that you'll learn about in this book. In particular, pay close attention to the way the TCP/IP protocol suite addresses the following problems:

  • Logical addressing

  • Routing

  • Name service

  • Error control and flow control

  • Application support

These issues are at the heart of TCP/IP. The following sections introduce these important features. You'll learn more about these features later in this book.

Logical Addressing

A network adapter has a unique and permanent physical address. The physical address is a number that was given to the card at the factory. On a local area network, low-lying hardware-conscious protocols deliver data across the physical network using the adapter's physical address. There are many network types, and each has a different way of delivering data. On a basic ethernet network, for example, a computer sends messages directly onto the transmission medium. The network adapter of each computer listens to every transmission on the local network to determine whether a message is addressed to its own physical address.

By the Way

As you'll learn in Hour 9, "Network Hardware," today's ethernet networks are a bit more complicated than the idealized scenario of a computer sending messages directly onto the transmission line. Ethernet networks sometimes contain hardware devices such as switches and hubs to manage the signal.

On large networks, of course, every network adapter can't listen to every message. (Imagine your computer listening to every piece of data sent over the Internet.) As the transmission medium becomes more populated with computers, a physical addressing scheme cannot function efficiently. Network administrators often segment networks using devices such as routers to reduce network traffic. On routed networks, administrators need a way to subdivide the network into smaller subnetworks (called subnets) and impose a hierarchical design so that a message can travel efficiently to its destination. TCP/IP provides this subnetting capability through logical addressing. A logical address is an address configured through the network software. In TCP/IP, a computer's logical address is called an IP address. As you'll learn in Hour 4, "The Internet Layer," and Hour 5, "Subnetting," an IP address can include

  • A network ID number identifying a network

  • A subnet ID number identifying a subnet on the network

  • A host ID number identifying the computer on the subnet

The IP addressing system also lets the network administrator impose a sensible numbering scheme on the network so that the progression of addresses reflects the internal organization of the network.

By the Way

If your network is isolated from the Internet, you are free to use any IP addresses you want (as long as your network follows the basic rules for IP addressing). If your network will be part of the Internet, however, Internet Corporation for Assigned Names and Numbers (ICANN), which was formed in 1998, will assign a network ID to your network, and that network ID will form the first part of the IP address. (See Hours 4 and 5.)

In TCP/IP, a logical address is resolved to and from the corresponding hardware-specific physical address using the ARP and RARP protocols, which are discussed in Hour 4.


A router is a special device that can read logical addressing information and direct data across the network to its destination. At the simplest level, a router divides a local subnet from the larger network (see Figure 1.3).

Figure 1.3. A router connecting a LAN to a large network.


Data addressed to another computer or device on the local subnet does not cross the router and therefore doesn't clutter up the transmission lines of the greater network. If data is addressed to a computer outside the subnet, the router forwards the data accordingly. As has already been mentioned this hour, very large networks such as the Internet include many routers and provide multiple paths from the source to the destination (see Figure 1.4).

Figure 1.4. A routed network.


TCP/IP includes protocols that define how the routers will find a path through the network. You'll learn more about TCP/IP routing and routing protocols in Hour 10, "Routing."

By the Way

As you'll also learn in Hour 9, network devices such as bridges, switches, and smart hubs also can filter traffic and reduce network traffic. Because these devices work with physical addresses rather than logical addresses, they cannot perform the complex routing functions shown in Figure 1.4.

Name Resolution

Although the numeric IP address is probably more user friendly than the network adapter's prefabricated physical address, the IP address is still designed for the convenience of the computer rather than the convenience of the user. People might have trouble remembering whether a computer's address is or TCP/IP, therefore, provides for a parallel structure of user-oriented alphanumeric names, called domain names or DNS names. This mapping of domain names to an IP address is called name resolution. Special computers called name servers store tables showing how to translate these domain names to and from IP addresses.

The computer addresses commonly associated with email or the World Wide Web are expressed as DNS names (for example,,, and TCP/IP's name service system provides for a hierarchy of name servers that supply domain name/IP address mappings for DNS-registered computers on the network. This means that the everyday user rarely has to enter or decipher an actual IP address.

DNS is the name resolution system for the Internet and is the most common name resolution method. However, some TCP/IP networks also support other methods for resolving alphanumeric names to IP addresses. Another common name resolution scheme is the Windows Internet Name Services (WINS) for resolving Microsoft Windows NetBIOS names to IP addresses.

You'll learn more about TCP/IP name resolution in Hour 11, "Name Resolution."

Error Control and Flow Control

The TCP/IP protocol suite provides features that ensure the reliable delivery of data across the network. These features include checking data for transmission errors (to ensure that the data that arrives is exactly what was sent) and acknowledging successful receipt of a network message. TCP/IP's Transport layer (see Hour 6, "The Transport Layer") defines many of these error-checking, flow-control, and acknowledgment functions through the TCP protocol. Lower-level protocols at TCP/IP's Network Access layer (see Hour 3) also play a part in the overall system of error control.

Application Support

Several network applications might be running on the same computer. The protocol software must provide some means for determining which incoming packet belongs with each application. In TCP/IP, this interface from the network to the applications is accomplished through a system of logical channels called ports. Each port has a number that is used to identify the port. You can think of these ports as logical pipelines within the computer through which data can flow from the application to (and from) the protocol software (see Figure 1.5).

Figure 1.5. Applications access the network through port addresses.


Hour 6 describes TCP and UDP ports at TCP/IP's Transport layer. You'll learn more about application support and TCP/IP's Application layer in Hour 7, "The Application Layer."

The TCP/IP suite also includes a number of ready-made applications designed to assist with various network tasks. Some typical TCP/IP utilities are shown in Table 1.1. You'll learn more about these TCP/IP utilities in Part IV, "TCP/IP Utilities."

Example light iptables for ubuntu.

Table 1.1. Typical TCP/IP Utilities




File Transfer








Remote Terminal Access



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By the Way

TCP/IP is actually entering into a new phase at the time of this writing. New technologies such as wireless networks, virtual private networks, and network address translation are adding new complexities that the creators of TCP/IP wouldn't have imagined. You'll learn more about these technologies in later chapters.

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