How to Do Route Summarization

Step 1 - List all routes that need to be summarized in dotted decimal format in order of lowest to highest. Also list their matching subnet broadcast addresses.

Example Networks 10.3.4.0/24, 10.3.5.0/24, 10.3.6.0/24,10.3.7.0/14

Subnet ID    Broadcast ID
10.3.4.0        10.3.4.255
10.3.5.0        10.3.5.255
10.3.6.0        10.3.6.255
10.3.7.0        10.3.7.255

Step 2 - Write down the the network ID of the smallest Network and write down the broadcast of the largest network.

10.3.4.0 & 10.3.7.255

Step 3 - Determine the starting subnet mask

Take the existing subnet mask and subtract 1 as the starting point



24 -1 equals 23

Step 4 - Take the lowest subordinate subnet ID and find the network ID and broadcast with the defined mask from Step 3

network 10.3.4.0 /23 - 10.3.5.255 broadcast

( /23 does not work because it does not include all Subnet ID defined in Step 1 )

Step 3 & 4 Repeat - Subtract one more from the Subnet Mask

/23 -1 = /22

Recalculate the network and broadcast

network 10.3.4.0 - 10.3.7.255 broadcast

WINNER - this subnet mask successfully summarized all subnets listed in Step one and therefor is the answer!