Class-Based Routing
Since the network portion of a packet’s destination IP address determines the destination network, routing a packet requires examining the IP address, determining the network portion of that address, and then sending the traffic to that specific network. However, IP packets do not carry subnet mask data around with them, and only contain the 32-bit IP address of the destination system. Without this information, devices are unable to determine what portion of the IP address belongs to the network, and which portion belongs to the host, meaning they cannot determine the destination network number and thus cannot route the packets.
In order to resolve this problem, IP systems had to use some other form of logic to determine which portion of the IP address applied to the destination network, and which portion applied to the destination system. This was achieved through the use of the address classes defined above, and the way in which network addresses were assigned.
Class A network addresses always have the first bit of their IP address set to 0. Since Class A networks only use the first eight bits for the network number, this leaves seven bits for the network portion of the address, allowing for 128 possible network numbers. However, the “all-on” and “all-off” networks (numbered and 127) were also reserved, so there were really only 126 possible network numbers (1 through 126). Therefore, any IP packet with a destination network of 1.x.x.x through 126.x.x.x is for ...