The following tables (Barros, 2009) are representations to further understand how these classes are organized and their address ranges within each class as well as how the network and host ID’s are broken down.
From the tables above, the major difference between each class are the number of networks and hosts per network. First off, the IP address contains two (2) parts: the network address (ID) and the host address (ID). “Each class had a specific purpose and a defined range of allowable addresses. The goal was to provide for three common scenarios in networking:
Small number of very large networks (large number of nodes per network)
Moderate number of medium-sized networks
Large number of very small networks (small number of nodes per network)
This class-based system worked well for quite some time. However, in the 1990s, when the Internet boom period (the massive growth of the Internet) began, it became clear that the addressing scheme would not support the many hundreds of thousands of networks that were popping up (and getting connected to the global network) around the world. A new classless system was devised.” A Class A has 126 networks which can accommodate up to 16,777,214 hosts per network. This class was specifically designed for large, enterprise businesses. “Class A addresses always have the high-order bit (or left-most bit) set to zero. The first octet (the left-most eight bits) is used to define the network ID. The host addresses use the second, third, and fourth octets.” (Shimonski & Alpern, 2009)
The Class B can have 16,384 networks and have 65,534 hosts per network. “Class B addresses are used for medium-sized networks that have a moderate number of hosts connected to them. Class B addresses always have the first two high-order bits (left-most) set to 10. The Class B network ID uses the first two octets for the network ID. This allows for more network IDs and fewer hosts than a Class A network. Because it uses an additional octet for the network, there is one fewer octet available for host IDs, reducing the number of hosts that can be addressed on this network by approximately a factor of two. Class B networks use the first two octets for the network ID. However, we cannot set the second bit to 1 (Class B left-most two bits must be 10).” (Shimonski & Alpern, 2009) With a Class C, there can be 2,097,152 networks. But can accommodate only 254 hosts per network. Class C addresses are for small networks with few hosts. These addresses have the first three high-order bits set to 110. Class C addresses use the first three octets for the network ID and the last octet for the host ID.
One of the last classes defined by TCP/IP protocol is the multicast or Class D. The Class D address “is reserved for IP multicast addresses. The first four high-order bits are set to 1110. The remaining 28 bits are used for individual IP multicast addresses. Multicast Backbone on the Internet (MBONE) is an extension to the Internet that supports IP multicasts and uses Class D addresses. MBONE allows a single packet to have multiple destinations and is most often used in real-time audio and video