DETERMINING THE VALID NUMBER OF SUBNETS
1. How many valid subnets exist on the 192.168.1.100/27 network?
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.
Lammle, T. (2007). CCNA Cisco Certified Network Associate Study Guide. Indianapolis: Wiley Publishing, Inc.
Odom, W. (2012). Official Cert Guide ICND1 640-822. Indianapolis, IN: Cisco Press.
Odom, W. (2011). Official Cert Guide ICND2 640-816. Indianapolis, IN: Cisco Press.