Once you figured out the block size needed for each LAN, this was actually a pretty simple question—all you need to do is look for the right clues—and, of course, know your block sizes.
Summarization, also called route aggregation, allows routing protocols to advertise many net- works as one address. The purpose of this is to reduce the size of routing tables on routers to save memory, which also shortens the amount of time for IP to parse the routing table and find the path to a remote network.
Figure 3.11 shows how a summary address would be used in an internetwork.
Summary address used in an internetwork
10.0.0.0/16 10.1.0.0/16 10.2.0.0/16… 10.255.0.0/16
Summarization is actually somewhat simple because all you really need to have down are the block sizes that we just used in learning subnetting and VLSM design. For example, if you wanted to summarize the following networks into one network advertisement, you just have to find the block size first, then you can easily find your answer:
192.168.16.0 through networks 192.168.31.0 What’s the block size? There are exactly sixteen Class C networks, so this neatly fits into a block size of 16.
Okay, now that you know the block size, you can find the network address and mask used to summarize these networks into one advertisement. The network address used to advertise the summary address is always the first network address in the block—in this example, 192.168.16.0. To figure out a summary mask, in this same example, what mask is used to get a block size of 16? Yes, 240 is correct. This 240 would be placed in the third octet—the octet where we are summa- rizing. So, the mask would be 255.255.240.0.
You’ll learn how to apply these summary addresses to a router in Chapter 6.
Here’s another example: Networks 172.16.32.0 through 172.16.50.0 This is not as clean as the previous example, because there are two possible answers, and here’s why: Since you’re starting at network 32, your options for block sizes are 4, 8, 16, 32, 64, etc. and block sizes of 16 and 32 could work as this summary address.