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.