by Matthew G. Naugle
Wiley Computer Publishing, John Wiley & Sons, Inc.
ISBN: 0471196568 Pub Date: 11/01/98
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Illustrated TCP/IP
by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568 Pub Date: 11/01/98 |
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In this example, aggregation is somewhat less efficient, but you would not know it from the address. The following addresses appear to be contiguous:
But when we translate it to binary to find the common prefix to all of the addresses, we find a noncontiguous bit pattern:
The common prefix is 100011xx in the third octet. Why? Because we do not know where 145 or higher is? We have to see which ones have the same prefix and then use that. Any other numbers must be separate entries in the table. This would give us a route aggregation of 155.1.140.0/22, but this leaves out the 155.1.144.0 subnet. Depending on the range that this address is in, it could be listed in another route aggregation prefix. Since this is all the information we were given, however, 155.1.144.0 must be listed as a separate route: 155.1.144.0/24 (subnet mask of 255.255.255.0). This is due to this address not being within the range of the common prefix of the other addresses even though the decimal address is contiguous. Networks do not calculate routes in decimal!!! Humans do, and this is why we make mistakes.
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Another Look at Route Aggregation
When we translate it to binary to find the common prefix to all of the addresses, we find a non-contiguous bit pattern:
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You should also notice that this allows us to have one route entry instead of four. This may not seem like much, but when this concept is applied to a larger range of addresses (such as those on the Internet routing tables), one route entry is used to aggregate thousands of individual addresses.
The common prefix is 100011, which allows us to aggregate those routes to 155.1.140.0/14.
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