The internet community originally defined five IP address classes as a way of accommodating various sized networks. Microsoft TCP/IP supports Class A, B and C addresses that have been assigned to hosts. The specific address class is used to define the bits that are utilized for the network ID, as well as the bits which are utilized for the host ID. The address class is also used to define the possible number of networks available and the possible number of hosts within each network.
IP Address Classes
Class A addresses are typically assigned to networks that comprise of a very large number of hosts. Class A addresses will always have the high order bit or the first bit of their IP address set to 0. The next seven bits that complete the first octet similarly complete the network ID. The remaining 24 bits or the last three octets are used to represent the host ID. This allows for the availability of 16,777,214 hosts in every network.
Because Class A networks possess an 8-bit network mask, using a leading zero will leave only seven bits for the part of the address representing the network. This therefore allows for a maximum of 128 possible network numbers that range from 0.0.0.0 - 127.0.0.0. The number 127.x.x.x is typically reserved for loopback and used for the internal testing performed on the local machine.
Class B IP addresses are typically assigned to networks of a medium to large size. Class B addresses will always have the first bit of their IP addresses set to 1, and the second bit set to 0. What this means is that the two high-order bits in every Class B address will always be set to the binary 1 0. Because Class B IP addresses contain a 16-bit network mask, using a leading 10 bit pattern will leave the next 14 bits that complete the first two octets for the network ID part of the address. The remaining 16 bits or the last two octets will therefore represent the host ID. This therefore permits a maximum of 16, 384 networks that range from 220.127.116.11 - 18.104.22.168; as well as 65,534 hosts within each network.
Class C IP addresses are typically assigned to small networks. Class C addresses will always have their first two bits set to 1 and their third bit set to 0. What this means is that the 3 high-order bits in every Class C address will always be set to the binary 1 1 0. Because Class C addresses contain a 24-bit network mask, this leaves the next 21 bits that complete the first 3 octets for the portion of the address that represents the network ID. As a result, this allows for a maximum of 2,097,152 network addresses that range between 192.0.0.0 - 22.214.171.124; along with 254 hosts per network.
Class D IP addresses are typically reserved for IP multicast addresses and are as such utilized in multicasting applications. The first 3 bits of Class D addresses are set to 1, while the fourth bit is set to 0. Because Class D addresses are 32 bit network addresses, all the values within the range of 126.96.36.199 - 188.8.131.52 serve in the unique identification of multicast groups. The Class D address space does not contain any host addresses because all hosts within a group share the IP address of the group for receiving purposes.
Class E addresses are described as experimental as they are reserved for purposes of testing in the future. It is for this reason that Class E addresses have never been utilized or documented in a standard manner.
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