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, 126.96.36.199, 192.168.1.100, 188.8.131.52, & 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.
Here are some IPv4 Subnet Cheat Sheets that you may find helpful: http://packetlife.net/library/cheat-sheets/, http://www.subnetonline.com/pages/references/ipv4-cheat-sheet.php, http://www.quest4.org/ccna/subnet_cheat_sheet.htm, http://www.subnetting-secrets.com/subnetting-cheat-sheet.html, http://search.yahoo.com/search?p=subnet+cheat+sheets&ei=UTF-8&fr=moz35, http://search.yahoo.com/r/_ylt=A0oGdVfl9CxRNUgA3GBXNyoA;_ylu=X3oDMTE1ZWJyN2IxBHNlYwNzcgRwb3MDMTcEY29sbwNzazEEdnRpZANRSTAyNl8xMzQ-/SIG=143bk83dn/EXP=1361929573/**https%3a//learningnetwork.cisco.com/servlet/JiveServlet/download/102742-14596/TCPIP-Subnetting%2520cheatsheet.PDF, http://subnetmask.info/
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.:
- How many subnets? 2x = number of subnets (x is the subnetted masked bits or 1’s; 11000000 2 ones = 22 = 4 subnets)
- 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.
- What are the valid subnets? 256 – subnet mask = block size or increment; 256 – 192 = block size of 64.
- 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.
- 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.
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.