IPv4 Address Classes

Though Classless Inter-Domain Routing (CIDR) and Variable-Length Subnet Mask (VLSM) are the order of today's IPv4 networking, classification of IPv4 address spaces into classes A, B, C, or D remains common. The following table shows the classes that were designated during the days of classful routing:

Class	First bits	Address range	Subnet mask
Class A	0	1.x.x.x ~ 127.x.x.x	255.0.0.0 (/8)
Class B	10	128.0.x.x ~ 191.255.x.x	255.255.0.0 (/16)
Class C	110	192.0.0.x ~ 223.255.255.x	255.255.255.0 (/24)
Class D	1110	224.0.0.0 ~ 239.255.255.255	-
Class E	1111	240.0.0.0 ~ 255.255.255.254	-