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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How is this done
| Action | Address Space | Binary Equivalent |
|---|---|---|
| ISP segments off 16 addresses of the original address | 209.16.0.0/16 becomes 209.16.16.0/20 | 11010001.00010000.00000000.00000000 11010001.00010000.0001 | 0000.00000000 |
| ISP splits this new address in half, yielding two address ranges | 209.16.16.0/21 209.16.24.0/21 | 11010001.00010000.00010 | 000.00000000 11010001.00010000.00011 | 000.00000000 |
| Based on a customer survey, 209.16.16.0/21 is given to a single customer | Yields 8 Class C addresses | |
| 209.16.24.0/21 is split up again | 209.16.24.0/22 209.16.28.0/23 209.16.30.0/23 | 11010001.00010000.000110 | 00.00000000 11010001.00010000.0001110 | 0.00000000 11010001.00010000.0001111 | 0.00000000 |
Therefore, customer A gets the Class C address range of 209.16.16.0 through 209.16.23.0.
Use the preceding addresses and count up in binary using the table and you will get a better picture of how this operates.
So CIDR is at the ISP and Class addressing is at the customer site. What does this buy us? Not necessarily anything (except a faster network with the ISP), but it does great things for the ISP’s routing tables and, therefore, the Internet routing tables. Whereas the ISP would have had 16 entries in the routing table, it now has 4. Whereas the Internet routing tables would have had 256 entries in the global routing table, they now have 1. Now multiply this by the number of ISPs worldwide and I think you begin to see the efficiencies of this protocol, and without it the explosion of the Internet routing tables.
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