# Subnetting

Part 2

As mentioned prior to this, one of the key elements to subnetting is Binary, Octal, Decimal, and Hexadecimal conversion.  While I do like the Sybex Study Guide for CCNA by Todd Lammle, I do believe with respect to the subnetting aspect there are other better resources.  For example the Bryant Advantage system, the ICND 1 & 2 Study Guides for CCENT & CCNA by Wendell Odom (especially the Appendicies), and Test King (http://www.testking.com/).  I am certain that there are others, but I have come to an appreciation and an understanding after these; of course, the CCNA Bootcamp was an extreme help as well!

One trick that has stuck with me is this:  Key on the first several digits to the first segment of the IP Address (i.e., 10.0.0.1, 172.10.120.1, 192.168.1.100, 224.10.10.1, & 240.0.0.100)

10   = 00001010 = A first four digits 0000-0111

172 = 10101100 = B first two digits 1000-1011

192 = 11000000 = C first two digits 1100-1101

224 = 11100000 = D first four digits 1110

240 = 11110000 = E first four digits 1111

This is a quick assessment trick which can cut off some time while determining to what subnet class the address is a part of.  Of course, it is just as easy to memorize the entire spectrum.

Look through some of these cheat sheets, you may pick something up from them; on the other hand, you may already know it – to which I say GREAT!

Some questions you need for determining subnets, hosts, etc.:

1. How many subnets?  2= number of subnets (x is the subnetted masked bits or 1’s; 11000000 2 ones = 22 = 4 subnets)
2. How many hosts per subnet?  2y-2 where y is the number of unmasked bits or 0’s; 11000000 = 26-2 = 64-2 = 62 hosts.
3. What are the valid subnets?  256 – subnet mask = block size or increment; 256 – 192 = block size of 64.
4. What is the broadcast address for each subnet?  Our broadcast address is the last address prior to the next subnet; i.e., our block size is 64 then our starting addresses are 0, 64, 128, 192…therefore, the broadcast address would be the on prior to the last which is 63, 127, 191, & 255.
5. What are the valid hosts? The valid hosts are all of the addresses in between the subnet and the broadcast addresses; i.e., with the block size of 64 then our valid hosts are: 1-62, 65-126, 129-190, & 193-254.