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What is classful addressing?

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Classful addressing is an IPv4 addressing architecture that divides addresses into five groups.
Classes A-C: unicast addresses
Class D: multicast addresses
Class E: reserved for future use
Class A :
In a class A address, the first bit of the first octet is always ‘0’. Thus, class A addresses range from 0.0.0.0 to
127.255.255.255(as 01111111 in binary converts to 127 in decimal). The first 8 bits or the first octet denote the
network portion and the rest 24 bits or the 3 octets belong to the host portion. Its Subnet mask is 255.0.0.0.
Example: 10.1.1.1
Class B :
In a class B address, the first octet would always start with ’10’. Thus, class B addresses range from 128.0.0.0 to
191.255.255.255. The first 16 bits or the first two octets denote the network portion and the remaining 16 bits or
two octets belong to the host portion. Its Subnet mask is 255.255.0.0.
Example: 172.16.1.1
Class C :
In a class C address, the first octet would always start with ‘110’. Thus, class C addresses range from 192.0.0.0
to 223.255.255.255. The first 24 bits or the first three octets denote the network portion and the rest 8 bits or the
remaining one octet belong to the host portion. Its Subnet mask is 255.255.255.0.
Example: 192.168.1.1
Class D : Class D is used for multicast addressing and in a class D address the first octet would always start with
‘1110’. Thus, class D addresses range from 224.0.0.0 to 239.255.255.255. Its Subnet mask is not defined.
Example: 239.2.2.2
Class E :
Class E addresses are reserved for research purposes and future use. The first octet in a class E address starts
with ‘1111’. Thus, class E addresses range from 240.0.0.0 to 255.255.255.255. Its Subnet mask is not defined

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