Classes Defined

Classes Defined

We have already discussed the reason behind the provision of separate classes of IP addresses. Now we will discuss in more detail, the definition for each class of IP address. Before continuing, the following table and figure will help clarify the differences between host and network IDs. Table 3.5 illustrates the publicly available IP address classes (A to C) and their corresponding network and host ID components.

Again, it is important to understand that the IP address consists of two parts: a network ID and a host ID. As shown in Table 3.4, the most significant bits (MSBs) are used to determine how many bits are used for the network ID and the host ID. Figure 3.1 diagrams the placement of the MSBs within each of the five classes of IP addresses.

Class A addresses are assigned to networks with extremely large numbers of hosts (networked devices). The MSB is set to 0, and is combined with the remaining seven bits of the first octet to complete the network ID. This leaves the last 3 octets, or 24 bits to be assigned to subnet masking and to hosts. As we saw in Table 3.3, this allows for 126 (27-2) networks with up to 16,777,214 (221-2) hosts per network. An example of a Class A address is 10.1.2.34 where 10.0.0.0 is the network and 0.1.2.34 is the host.
Class B addresses are assigned to networks with no more than 65,534 (216-2) hosts (networked devices). The MSBs are set to 10, and are combined with the remaining 14 bits of the first two octets to complete the network ID. This leaves the last 2 octets, or 16 bits to be assigned to subnet masking and to hosts and allows for 16,384 (214) networks. Each of these networks can have as many as 65+ thousand hosts. An example of a Class B address is 120.224.21.253 here the network is 120.224.0.0 and the host is 21.253.
Class C addresses are assigned to small networks with a more limited number of hosts. The MSBs are set to 110, and are combined with the remaining 21 bits of the first three octets to complete the network ID. This leaves the last octet available to be assigned to subnet masking and to hosts, allowing for 2,097,152 (221) networks with up to 254 (28-2) hosts per network. An example of a Class B address is 207.21.32.12 which is a network of 207.21.32.0 with a host ID of 0.0.0.12.
Class D addresses are reserved for multicast groups. Multicast addresses are assigned to groups of hosts that are cooperating, or are related in some manner. Each host in a multicast group has to be configured to accept multicast packets. The MSBs of a class D address are set to 1110. The remaining bits are uniquely assigned to each group of hosts. Microsoft NT supports class D addresses for applications such as Microsoft NetShow.
Class E addresses are an experimental class of IP addresses reserved for use in the future. The MSBs for class E addresses are 1111.

You may be wondering why there are only 126 Class A networks, rather than 128 (28). As will be discussed in the next section: a network ID of all 0s is not allowed, and the Class A network ID of 127 is reserved. Read on to find out why!