In IP addressing, there are five classes of IP addresses: A, B, C, D, and E.
Class A addresses have a default mask of 255.0.0.0 and a range of 1.0.0.0 to 127.255.255.255. The first octet is used for the network address and the remaining three octets are used for the host address.
Class B addresses have a default mask of 255.255.0.0 and a range of 128.0.0.0 to 191.255.255.255. The first two octets are used for the network address and the remaining two octets are used for the host address.
Class C addresses have a default mask of 255.255.255.0 and a range of 192.0.0.0 to 223.255.255.255. The first three octets are used for the network address and the last octet is used for the host address.
Class D addresses are reserved for multicast groups and have a range of 224.0.0.0 to 239.255.255.255
Class E addresses are reserved for experimental or future use and have a range of 240.0.0.0 to 254.255.255.255
Note that, nowadays Class A,B,C are mostly used for subnetting, class D and E are not used for subnetting