Okay so Classfull addressing is a technique which was used earlier to solve the distribution of IP address.
How this works!!
we all know that a ip address is of 32 bits and for the sake of simplicity we have divide it into 4 octets each of 8 bits
so we have 4 like this where we can fill either 0 or 1 _ _ _ _ _ _ _ _
now understand this concept assume you have 2 bits _ _
you can have 4 diff. permutations with 0 and 1 i.e. 00,01,11,10
so if you fix a bit at a position to extreme right , we can divide it into 2 possible configuration i.e if we fix first bit as 1 then we can have 10 or 11..
now if we have 3 bits _ _ _, we have 8 possible permutations, then if we fix the same right bit again as 1, then
we can have 100,101,110,111 as our posiiblities
did you see the pattren
our total space for 2 bits was 22
and after fixing one bit we are remained with 22/21(because we have fixed one bit) = 2
and in case of 3, after fixing one bit , we are remained with 23/21(again same reason) = 4
i.e when we divide 32 bits by fixing one bit, we are remained with 232/21 = 231
till now i hope you understood
so what we do in classfull addressing is we divide 32 bits first in 2 parts by fixing one bit as 0 ( 0 _ _ _ _ _ _ _ _)
and this gives us 2 parts (range 0-127 fo 1st part where 0 is fixed and 128-255 part where 1 is fixed as total sample space was 256 for 8 bits in first octat) {
we call first one (range 0 to 127) as class A {{ 0 _ _ _ _ _ _ _ 00000000 to 01111111) }}
and then we further divide by fixing second most right as 0(which gives us the range(127-191) and (192-255)
this (127-191) is called as class B {{ 10 _ _ _ _ _ _ 10000000 to 10111111}}
and we further divide(192-255)t into 2 parts by fixing 3rd bit as 0
in 192-223 adn 224-255
an we call 192-223 as class C {{ 110 _ _ _ _ _ 11000000 to 11011111}}
and then for class D we further divide 223-255 range in 2 parts by fixing 4th right bit
224-239 and 240-255
and we call 224-239 as class D {{ 1110_ _ _ _ 11100000 to 11101111}}
whereas 240-255 is class E {{ 1111 _ _ _ _ 11110000 to 11111111}}
note : i am talking about the first octet here. and octet refers to grp of 8 bits. feel free to ask ques.