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Class of IP Addresses
Class A addresses always have the first bit of their IP addresses set to "0". Since Class A networks have an 8-bit network mask, the use of a leading zero leaves only 7 bits for the network portion of the address, allowing for a maximum of 128 possible network numbers, ranging from 0.0.0.0 – 127.0.0.0. Number 127.x.x.x is reserved for loopback, used for internal testing on the local machine.
Class B addresses always have the first bit set to "1" and their second bit set to "0". Since Class B addresses have a 16-bit network mask, the use of a leading "10" bit-pattern leaves 14 bits for the network portion of the address, allowing for a maximum of 16,384 networks, ranging from 220.127.116.11 – 18.104.22.168.
Class C addresses have their first two bits set to "1" and their third bit set to "0". Since Class C addresses have a 24-bit network mask, this leaves 21 bits for the network portion of the address, allowing for a maximum of 2,097,152 network addresses, ranging from 192.0.0.0 – 22.214.171.124.
Class D their first three bits set to "1" and their fourth bit set to "0". Class D addresses are 32D addresses are used for multicasting applications. Class D addresses have -bit network addresses, meaning that all the values within the range of 126.96.36.199 – 188.8.131.52 are used to uniquely identify multicast groups. There are no host addresses within the Class D address space, since all the hosts within a group share the group's IP address for receiver purposes.
Class Eaddresses are defined as experimental and are reserved for future testing purposes. They have never been documented or utilized in a standard way.