Determining IP Class


IP address basics:
IPv4 addresses are 32 bit with each octet being 8 bits each. To determine the class of a network in classful network, you look at the first octet:

Class A:
– First bit in the first octet is always set to 0.
– The first octet ranges from 1 – 127.
– Default subnet mask or Class A IP addresses is 255.0.0.0.
– Class A addressing can have 126 networks (27– 2) and 16777214 hosts (224– 2)
– 0nnnnnnn.hhhhhhhh.hhhhhhhh.hhhhhhhh

Class B:
– First two bits in the first octet are always set to 10.
– The first octet ranges from 128 – 191
– Default subnet mask or Class B IP addresses is 255.255.0.0.
– Class B addressing can have 16384 networks (214– 2) and 65534 hosts (216– 2)
– 10nnnnnn.nnnnnnnn.hhhhhhhh.hhhhhhhh

Class C:
– First three bits in the first octet are always set to 110.
– The first octet ranges from 192 – 223
– Default subnet mask or Class C IP addresses is 255.255.255.0.
– Class C addressing can have 2097152 networks (221– 2) and 254 hosts (28– 2)
– 110nnnnn.nnnnnnnn.nnnnnnnn.hhhhhhhh

Class D:
– First four bits in the first octet are always set to 1110.
– The first octet ranges from 224 – 239
– Class D has IP address range from 224.0.0.0 to 239.255.255.255.
– Reserved for multicasting
– Does not have a subnet mask

Class E:
– Reserved for experimental purposes; R&D or Study
– Class E has IP address range from 240.0.0.0 to 255.255.255.254
– Does not have a subnet mask


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