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Todd Lammle's CCNA has a chapter titled Easy Subnetting. I read that chapter countless times, then it just clicked. The first thing I did was learn to subnet class C addresses just to learn the concept Then I moved onto class B and A. Rule number 1, the numbers NEVER change and you must learn the powers of 2. You also need to know what octet you are subnetting in. Always remember that 256 is your absolute limit. You must ask yourself several questions. Example: The IP address for this example is below: 192.168.10.62/25 What is the subnet mask for a /25? 255.255.255.128 What is 256 - 128? This will determine your block size. 128 How many times can 128 go into 256? This will determine the amount of networks possible using a 255.255.255.128 mask. 2 What are the network addresses for the original IP address.? A /25 is subnetted in the 4th octet. You always want to count your block size starting with 0. Network Addresses and Broadcast Addresses for 192.168.10.62/25: 192.168.10.0 and 192.168.10.128 - Networks 192.168.10.127 and 192.168.10.255 - Broadcast(The broadcast is always the ip address prior to the next subnet.) What is the valid host address range? You cannot use the network address or the broadcast address. The network addresses are above and cannot be assigned to host. Your valid host ranges would be: 192.168.10.1 - 192.168.10.126 192.168.10.129 - 192.168.10.254 So 192.168.10.62/25 is in the 192.168.10.0 subnet. Try 192.168.10.62/26. What is the subnet mask for /26? 255.255.255.192 Then the rest of the questions.... Todd Lammle explains why these number work out the way this do. It has to do with on and off bits in the ip address, 1's and 0's. I'll let you read that part.