In the figure, notice that ISP1 has four customers, and that each customer has a variable amount of IP address space. The address space of the four customers can be summarized into one advertisement to ISP2. The 192.168.0.0/20 summarized or aggregated route includes all the networks belonging to Customers A, B, C, and D. This type of route is known as a supernet route. A supernet summarizes multiple network addresses with a mask that is smaller than the classful mask.
Determining the summary route and subnet mask for a group of networks can be done in the following three steps:
Step 1. List the networks in binary format.
Step 2. Count the number of far left matching bits. This identifies the prefix length or subnet mask for the summarized route.
Step 3. Copy the matching bits and then add zero bits to the rest of the address to determine the summarized network address.
The summarized network address and subnet mask can now be used as the summary route for this group of networks.
Summary routes can be configured by both static routes and classless routing protocols.