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
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!