84 Chapter 3 IP Addressing Route Summarization In the Design Considerations with VLSM section we briefly mentioned the concept of route summarization So what is it and why do we need it On very large networks there may be hundreds or even thousands of individual networks and subnetworks being advertised All these routes can be very taxing on a router s memory and processor In many cases the router doesn t even need specific routes to each and every subnet e g 172 16 1 0 24 It would be just as happy if it knew how to get to the major network e g 172 16 0 0 16 and let another router take it from there A router s ability to take a group of subnetworks and summarize them as one network i e one advertisement is called route summarization as shown in Figure 3 5 In some of the literature you may find route summarization referred to as route aggregation FIGURE 3 5 Route summarization 172 16 1 0 24 I am the way to get to network 172 16 0 0 16 172 16 2 0 24 172 16 3 0 24 Besides reducing the number of routing entries that a router must keep track of route summarization can also help protect an external router from making multiple changes to its routing table due to instability within a particular subnet For example let s say that we were working on a router that connected to 172 16 2 0 24 As we were working on the router we rebooted it several times If we were not summarizing our routes an external router would see each time 172 16 2 0 24 went away and came back Each time it would have to modify its own routing table However if our external router Copyright 2001 SYBEX Inc Alameda CA www sybex com Extending IP Addresses 85 were receiving only a summary route i e 172 16 0 0 16 then it wouldn t have to be concerned with our work on one particular subnet We will get the most benefit from route summarization when the networks or subnetworks that we are summarizing are contiguous To illustrate this point let s look at an example Route Summarization Example 1 We have the following networks that we want to advertise as a single summary route 172 16 100 0 24 172 16 101 0 24 172 16 102 0 24 172 16 103 0 24 172 16 104 0 24 172 16 105 0 24 172 16 106 0 24 To determine what the summary route would be for these networks we can follow a simple two step process 1 Write out each of the numbers in binary as shown in Table 3 14 TABLE 3 14 Summary Example IP Network Address Binary Equivalent 172 16 100 0 10101100 0001000 01100100 0 172 16 101 0 10101100 0001000 01100101 0 172 16 102 0 10101100 0001000 01100110 0 172 16 103 0 10101100 0001000 01100111 0 172 16 104 0 10101100 0001000 01101000 0 172 16 105 0 10101100 0001000 01101001 0 172 16 106 0 10101100 0001000 01101010 0 Copyright 2001 SYBEX Inc Alameda CA www sybex com 86 Chapter 3 IP Addressing 2 Examine the table to determine the maximum number of bits starting from the left that all of the addresses have in common where they stop lining up we bolded them to make them easier for you to see The number of common bits is the subnet mask for the summarized address 20 In our example we can see from the table that all of the addresses have the first 20 bits in common The decimal equivalent of these first 20 bits is 172 16 96 0 So we can write our new summarized address as 172 16 96 0 20 If we were to later add a network 172 16 98 0 it would need to come off the router summarizing this address space If we didn t it could cause problems Okay this is confusing we know This is why we re going to give you three more examples Route Summarization Example 2 In this example we will summarize 10 1 0 0 through 10 7 0 0 First put everything into binary and then follow the bits starting on the left and stopping when the bits do not line up Notice where we stopped boldfacing the following 10 1 0 0 00001010 00000001 00000000 00000000 10 2 0 0 00001010 00000010 00000000 00000000 10 3 0 0 00001010 00000011 00000000 00000000 10 4 0 0 00001010 00000100 00000000 00000000 10 5 0 0 00001010 00000101 00000000 00000000 10 6 0 0 00001010 00000110 00000000 00000000 10 7 0 0 00001010 00000111 00000000 00000000 Now create the network number using only the boldfaced bits Do not count the bits that are not in boldface The second octet has no bits on bits in the bolded section so we get this 10 0 0 0 Copyright 2001 SYBEX Inc Alameda CA www sybex com Extending IP Addresses 87 To come up with the summary mask count all the bolded bits as ones Because eight bits are boldface in the first octet and five bits in the second we ll get this 255 248 0 0 Route Summarization Example 3 This example will show you how to summarize 172 16 16 0 through 172 16 31 0 First let s put the network addresses into binary and then line up the bits 172 16 16 0 10101100 0001000 00010000 00000000 172 16 17 0 10101100 0001000 00010001 00000000 172 16 18 0 10101100 0001000 00010010 00000000 172 16 19 0 10101100 0001000 00010011 00000000 172 16 20 0 10101100 0001000 00010100 00000000 172 16 21 0 10101100 0001000 00010101 00000000 172 16 22 0 10101100 0001000 00010110 00000000 172 16 23 0 10101100 0001000 00010111 00000000 172 16 24 0 10101100 0001000 00011000 00000000 172 16 25 0 10101100 0001000 00011001 00000000 172 16 26 0 10101100 0001000 00011010 00000000 172 16 27 0 10101100 0001000 00011011 00000000 172 16 28 0 10101100 0001000 00011100 00000000 172 16 29 0 10101100 0001000 00011101 00000000 172 16 30 0 10101100 0001000 00011110 00000000 172 16 31 0 10101100 0001000 00011111 00000000 Copyright 2001 SYBEX Inc Alameda CA www sybex com 88 Chapter 3 IP Addressing Notice where the bits stop lining up in boldface Count only the bits that are on ones to get the network address 172 16 0 0 Now create the summary mask by counting all the bits that are in boldface up to the point where they stop lining up We have eight bits in the first octet eight bits in the second octet and four bits in the third octet That is a 20 or 255 255 240 0 Boy that sure seems like a pain in the pencil huh Try this shortcut Take the first number and the very last number and put them into binary 172 16 16 0 10101100 0001000 00010000 00000000 172 16 31 0 10101100 0001000 00011111 00000000 Can you see that we actually came up …