number of subnets

Question

In class C , if subnet mask is 255.255.255.224 then calculates number of subnet? (A) 6 (B) 8 (C) 4 (D) None of the Above

Comments

https://learningnetwork.cisco.com/thread/21527

Answer 1

The default mask for the class C address of 192.x.x.x is 255.255.255.0 The mask in the example is 255.255.255.224 (using 3 extra bits for subnetting, above and beyond the default). So 2^3 is 8 combinations:

000 001 010 011 100 101 110 111

In the old days, we couldn't use the all 000 or all 111 combination for subnetting, so the formula was 2^extra bits used for custom subnetting, or 2^3 in our example, -2 (for the 2 that used to not be able to be used), which would leave 6.

Today, current IOS&Cisco  has the ability to use the 000 and 111 option with an option called subnet zero and  so on current IOS there would be 8 possible subnets, not just 6.

Answer 2

since it belongs to class c so we can answer on the basis of the value of only 4 byte because first 3 byte will be used to represent different network.

the fourth byte is 224 = 11100000 (binary equivallent)

since the 1st three bit is fixeed in the host address so the address space is divided  into 2^3 = 8 parts .