nverse of VLSM: whereas VLSM prescribes rules for subdividing networks, CIDR prescribes rules for referencing groups of networks with a single route statement. Aggregating routes may seem like a solution looking for a problem if you've only ever dealt with small or medium-sized corporate networks, but, rest assured, it provides a real benefit. Usually, small or medium-sized companies use one of the private IP net- works described by RFC 1918. If a company used the entire 10.0.0.0/8 network, and subdivided it to maximize efficiency, each of these subdivisions would technically be a subnet. While VLSM deals with subnets, CIDR deals with groups of major or classful networks. Figure 34-4 shows how a single route statement can reference eight Class C networks. The route is called an aggregate route or a summary route. 192.168.0.0/24 192.168.1.0/24 192.168.2.0/24 192.168.3.0/24 192.168.4.0/24 192.168.5.0/24 Even though these are natively Class C networks, 192.168.6.0/24 by using CIDR, we can reference them all with a single route entry. 192.168.7.0/24 192.168.0.0/20 192.168.8.0/24 192.168.9.0/24 192.168.10.0/24 192.168.11.0/24 192.168.12.0/24 192.168.13.0/24 192.168.14.0/24 192.168.15.0/24 Figure 34-4. CIDR route aggregation Chapter 34: IP Design On Internet-attached routers with full tables, the routing tables may contain hun- dreds of thousands of routes. Anyone can see them at any time by connecting to one of many publicly available route servers. To illustrate this point, here's a sample of the number of routes the AT&T IP Services Route Monitor shows as of this writing: route-server.ip.att.net route-server> sho ip bgp summary BGP router identifier 10.1.2.5, local AS number 65000 BGP table version is 1094053, main routing table version 1094053 182347 network entries using 18417047 bytes of memory 3464386 path entries using 166290528 bytes of memory 42502 BGP path attribute entries using 2380392 bytes of memory 38401 BGP AS-PATH entries using 997216 bytes of memory 4 BGP community entries using 96 bytes of memory 0 