the original ipv4 only supported 254 networks, so in 1981 the internet addressing specification was changed to a classful network architecture. classful network design allowed for a larger number of individual networks. the first three bits of an ip address determined its "class". three classes ("a", "b", and "c") were defined for normal computer communication (unicast). the size of the network id was based on the class of the ip address. each class used more octets for the network id, making the host id smaller and reducing the number of possible hosts.