Cidr Tricks

cidr tricksCIDR/VLSM Tricks

Trick #1: When you need to know how many hosts (nodes). To find the number of useable hosts on each subnet, find the decimal equivalent to the highest order ZERO in the subnet mask and call it N, then plug N into the formula 2N-2. If the question does not specify useable hosts, then the formula is simply 2N. Example question: How many useable hosts are on each subnet with the subnet mask of 255.255.224.0? Solution: Convert the subnet mask to binary and you get 11111111.11111111.11100000.00000000 There are 13 0’s, so we’ll need 13 numbers. Starting with the lowest (far right) 0, count 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096. That last number (4096) is now N. Plug it into the formula 2N-2, and you get: 2N-2 [(2)(4096)]-2 8192-2 8190 useable hosts per subnet

Trick #2: When you need to know how many subnets (networks). To find the number of useable subnets for a subnet mask, find the decimal equivalent of the highest order ONE in the subnetted octet in the subnet mask and call it N, then plug N into the formula 2N-2. If the question does not specify useable subnets, then the formula is simply 2N. Example question: How many useable subnets are there for the subnet mask 255.255.224.0? Solution: Convert the subnet mask to binary and you get: 11111111.11111111.11100000.00000000 There are 3 high-order 1’s in the subnetted octet, so we’ll need 3 numbers. Starting with the underlined 1 above, count to the left 1, 2, 4, stopping at the beginning of that octet. That last number (4) is now N. Plug it into the formula 2N-2, and you get: 2N-2 [(2)(4)]-2 8-2 6 useable subnets Remember…

 If the question asks “how many hosts…” you’ll be counting 0’s.  If the question asks “how many subnets…” you’ll be counting 1’s.

Networks are on the left; Hosts are on the right!

Trick #2: When you need to know how many addresses. To find the number of addresses available in each subnet when given the number of subnets, take the number of...