UNDERSTANDING NETWORK CLASSES

All the addresses must have two parts, the network part and the nodes, or host, part. In this section you look at Ipv4 addresses. Ipv4 addresses used in TCP/IP networks are 4 bytes long, called IP addresses, and are written in standard dot notation, which, means that a decimal number separated by dots (for example, 192.168.1.2). The decimal numbers must be within the numeric range of 0 to 255 to conform to the 1-byte requirement, IP addresses are divided into classes with the most significant being classes A,B and C, depending on the value of the first byte of the address. Table shows valid numbers for these classes.

RESERVED NETWORK CLASSES AND THEIR IP NUMBER RANGES.

CLASS	FIRST BYTE	NET MASK	RESERVED ADDRESSES	Hosts / Network
Class A	0-126	255.0.0.0	10.0.0.0-10.255.255.255.255	Roughly 1.6 Million /1
Class B	128-191	255.255.0.0	172.16.0.0- 172.31.255.255	65,024 / 16,320
Class C	192-223	255.255.255.0	192.168.0.0-192.168.255.255	254 / Nearly 2 Million

Classes D, E, and F

Addresses falling into the range of 224.0.0.0 through 254.0.0.0 are either experimental or are reserved for special purpose use and don't specify any network. IP Multicast, which is a service that allows material to be transmitted to many points on an internet at one time, has been assigned addresses from within this range.

A number of network addresses are reserved for special purposes. 0.0.0.0 and 127.0.0.0 are two such addresses. The first is called the default route, and the latter is the loopback address. The default route has to do with the way the IP routes datagrams.

Network 127.0.0.0 is reserved for IP traffic local to your host. Usually, address 127.0.0.1 will be assigned to a special interface on your host, the loopback interface, which acts like a closed circuit. Any IP packet handed to this interface from TCP or UDP will be returned to them as if it had just arrived from some network. This allows you to develop and test networking software without ever using a “real” network. The loopback network also allows you to use networking software on a standalone host. This may not be as uncommon as it sounds; for instance, many UUCP sites don't have IP connectivity at all, but still want to run the INN news system. For proper operation on Linux, INN requires the loopback interface.

The reason for the class division is to enable efficient use of the address numbers. If the division were the first 2 bytes to the network part, and the last 2 bytes to the host part, then no network could have more than 2¹⁶ hosts. This would be impractical for large networks and wasteful for small networks.

There are a few ways to assign IP addresses to the devices, depending on the purpose of the network. If the network is internal, an intranet, not connected to an outside network, any class A,B or C, network number can be used. The only requirement is choosing a class that allows for the appropriate number of hosts to be connected. Although this is possible, in the real world this approach would not allow for connecting to the internet.

Understand TCP/IP

This is a acronym for Transmission Control Protocol/Internet Protocol, and refers to a family of protocols used for computer communications. TCP and IP are just two of the separate protocols contained in the group of protocols developed by the separate protocols contained in the group of protocols developed by the Department of Defense, sometimes called the DoD Suite but more commonly known as TCP/IP. In addition to Transmission Control Protocol and Internet Protocol, this family also includes Address Resolution Protocol (ARP); User Datagram Protocol (UDP); Internet Control Message Protocol (ICMP); Domain Name System (DNS); Routing Information Protocol (RIP); Simple Mail Transfer Protocol (SMTP); Tenet, and many others. These protocols provide the necessary services for basic network functionality, and you will take a closer look at them for a better understanding of how the network works.

To be able to send and receive information on the network, each device connected to it must have an address. The address of any device on the network must be unique and have a standard, defined format by which it is known to any other device on the network. This device address consists of two parts:

1. The address of the network to which the device is connected
2. The address of the device itself-its node or host address


Devices that are physically connected to each other (not separated by routers) would have the same network number but different node, or host, numbers. This would be typical of an internal network at a company or university. These types of networks are now often referred to as intranets.

The two unique addresses I’ve been talking about are typically called the network layer addresses and the Media Access Control (MAC) addresses. Network Layer addresses are IP addresses that have been assigned to the device. The MAC address is built into the card by the manufacturer and refers to only the lowest-level address by which all data is transferred between devices.

Now that u knows a little about addressing u need to learn how the addresses and also the data is transmitted across the network. This transform is a accomplished the braking the information into small pieces of data called packets or Datagram. Why is it necessary to use packets instead of just sending the entire message as one long stream of data? There ate tow reasons for this - sharing resources and error correction.

Lets look at the first, the line is busy. If these computers were sharing a large amount of data, other devices on the network would be unable to transfer their data. When long streams of data ate broken into small packets, each packet is sent individually, and the other devices can send their packets between the packets of the long stream. Since each packet is uniquely addressed and has instructions on how to reassemble it, it does not matter the order that it arrives in or that it arrives in small pieces.

The second reason to breaking the data into packets is error correction. Because the data is transmitted across media that is subject to interference, the data can become corrupt. One way to deal with the corruption is to send a checksum along with the data. A checksum is a running count of the bytes sent in the message. The receiving device compares its total to the total transmitted. If these numbers are the same, the data is good; but if they are different, either the checksum or the data itself is corrupt. The receiving device then asks the sender to resend the data. By breaking the data into small packets, each with its own checksum, it is easier to ensure that a good message arrives, and if not, only a small portion needs to be resent instead of the entire message.

In the description of packets, I mentioned unique addressing and reassembly instructions. Because packets also contain data, each is made up of two parts, the header, which contains the address and reassembly instructions. Because peackets also contain data, each is made up of two parts, the header, which contains the address and reassembly instructions, and the body, which contains the data, Keeping all this information in order is the protocol. The protocol is a set of rules that specifies the format of the package and how it is used.