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232 Chapter 7: Advanced Routing Protocol Topics

Sample “Best” Summary on Yosemite

The four subnets on Yosemite cannot be summarized quite as efficiently as those on Seville. On Seville, the summary route itself covers the same set of IP addresses as the four subnets. As you will see, the best summary route at Yosemite includes twice as many addresses in the summary as there are in the original four subnets.

Yosemite has subnets 10.2.1.0, 10.2.2.0, 10.2.3.0, and 10.2.4.0, all with mask 255.255.255.0. The process starts with writing down all the subnet numbers in binary:

0000 1010 0000 0010 0000 0 001 0000 0000 - 10.2.1.0 0000 1010 0000 0010 0000 0 010 0000 0000 - 10.2.2.0 0000 1010 0000 0010 0000 0 011 0000 0000 - 10.2.3.0 0000 1010 0000 0010 0000 0 100 0000 0000 - 10.2.4.0

In this example, only the first 5 bits of the third octet are identical. So, the first 21 bits of the four subnet numbers are “in common.”

Step 2 says to create a subnet number for the summary by taking the same value for the “in common” part and binary 0s for the rest. In this case:

0000 1010 0000 0011 0000 0 000 0000 0000 - 10.2.0.0

Step 3 creates the mask by using binary 1s for the “in common” part and binary 0s for the rest. The “in common” part in this example is the first 21 bits:

1111 1111 1111 1111 1111 1 000 0000 0000 - 255.255.248.0

So, the best summary is 10.2.0.0, mask 255.255.248.0.

Step 4 suggests a method to check your work. The summary route should define a superset of the IP addresses in the summarized routes. In this case, the range of addresses starts with 10.2.0.0. The first valid IP address is 10.2.0.1, the final valid IP address is 10.2.7.254, and the broadcast address is 10.2.7.255. In this case, the summary route summarizes a larger set of addresses than just the four subnets.

Classless Routing Protocols and Classless Routing

Cisco documents occasionally mention the terms classless and classful when referring to topics related to routing and addressing. The root word, class, refers to the Class A, B, and C network classes defined by IP addressing. The terms classless and classful can be applied to several topics related to routing and addressing. This section covers the basic meaning of the terms and their application.