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| 2.4 |  | Route Summarization |
| 2.4.1 |  | An overview of route summarization |

The use of CIDR and VLSM not only prevents address waste, but it also promotes route aggregation, or summarization. Without route summarization, Internet backbone routing would likely have collapsed sometime before 1997.

The figure illustrates how route summarization reduces the burden on upstream routers. This complex hierarchy of variable sized networks and subnetworks is summarized at various points using a prefix address until the entire network is advertised as a single aggregate route of 192.168.48.0/20.

Recall that this kind of route summarization, or supernetting, is possible only if the network routers run a classless routing protocol, such as OSPF or EIGRP. Classless routing protocols carry the prefix length and subnet mask with the 32-bit address in routing updates. In the figure, the summary route that eventually reaches the provider contains a 20-bit prefix common to all of the addresses in the organization. This prefix is 192.168.48.0/20 or 11000000.10101000.00110000.00000000 with a subnet mask of 11111111.11111111.11110000.00000000. For summarization to work properly, addresses must be carefully assigned in a hierarchical fashion so that summarized addresses will share the same high order bits.