# Subnetting Part 3:

DETERMINING THE VALID NUMBER OF SUBNETS

1.  How many valid subnets exist on the 192.168.1.100/27 network?

OR,

2.  How many valid subnets exist on the 192.168.1.100 255.255.255.224 network?

The /27 in question one is called prefix notation and the 255.255.255.224 designation is the dotted decimal mask.  Both questions are the same, just written differently.

The /27 is an indicator as to how many ones (1s) are at the beginning of this network’s mask.  255.255.255.224, or /27, converted to decimal is 11111111 111111111 11111111 11100000.  One nice little tid-bit of information is that the number of network bits never changes.  Subnetting always borrows bits from the host bits, ALWAYS!

 1st Octet 2nd Octet 3rd Octet 4th Octet Default Classs C Network 11111111 11111111 11111111 00000000 This IP’s Subnet Mask 11111111 11111111 11111111 11100000

So, the question remains, how many valid subnets exist on the 192.168.1.100/27 network?

By comparison we can determine that a class C network has 24 network bits and therefore possess only 8 host bits.  On this network, we borrowed (remember) 3 bits from the host bits for our subnet.  [The number of valid subnets = 2x; where x is the number of set subnet bits (1’s)]  Therefore, 23 = 2 x 2 x 2 = 8, which is the number of valid subnets.