Doubt:mrgreen:

sKGP

i was going through the subnetting part and could not understand this thing.
It says the
Number of networks in class A=2^7 - 2 , B=2^14 , C =2^21

Where as i understand the number of hosts per network part clearly

thanks in advance

apr911

A bit of an odd way to describe it but to explain it

Class A networks are /8 networks with a range from 0.x.x.x to 127.x.x.x however, 0 is not a usable address and 127 is used as loopback and thus also not usable. Therefore, there are 128 - 2 available networks available in class A.

2 ^ 7 = 128

Class B networks are /16 networks with a range from 128.0.x.x to 191.255.x.x. All of these networks are usable and therefore there are 16,384 networks available in class B.

2 ^ 14 = 16,384

There is 63 major networks in the first octet
191 - 128 = 63 in binary is 00111111 thus 2^6

and 255 additional networks in the second octet of each primary octet
255 - 0 = 255 in binary is 11111111 thus 2 ^ 8

2 ^ 6 * 2 ^ 8 = 2 ^ 14

Class C networks are /24 networks with a range from 192.0.0.x to 223.255.255.x. Again all of these networks are usable and therefor there are 2,097,152 networks in class C.

2 ^ 21 = 2,097,152

There are 31 major networks in the first octet
223 - 192 = 31 in binary is 11111 thus 2 ^ 5

There are 255 additional networks in the second of each primary octet
255 - 0 = 255 in binary is 11111111 thus 2 ^ 8

There is a further 255 additional networks in the third octet of each higher order (1st and 2nd) octet
255 - 0 = 255 in binary is 11111111 thus 2 ^ 8

2^5 * 2 ^8 * 2^8 = 2 ^ 21

Hope this helps.

For the CCNA the important thing to remember is the leading bits.

Class A has 0 so the first octet of a binary translated class A address will be between 00000000 and 011111111

Class B is 10 so the first octet of a binary translated class B address will be between 10000000 and 10111111

Class C is 110 so the first octet of a binary translated class C address will be between 11000000 and 11011111

Class D (multicast) is 1110 so the first octet of a binary translated class D address will be between 11100000 and 11101111

Class E (experimental) is 1111 so the first octet of a binary translated class E address will be between 11110000 and 11111111