CIDR and Route Summarization
CIDR uses Variable Length Subnet Masks (VLSM) to allocate IP addresses to subnets according to
individual need rather than by class. This type of allocation allows the network/host boundary to occur
at any bit in the address. Networks can be further divided or subnetted into smaller and smaller subnets.
Just as the Internet was growing at an exponential rate in the early 1990s, so were the size of routing
tables that were maintained by Internet routers under classful IP addressing. CIDR allowed for prefix
aggregation, which you already know as route summarization. As we know about "Static Routing" that
you can create one static route for multiple networks. Internet routing tables were now able to benefit
from the same type of aggregation of routes. The ability for routes to be summarized as a single route
helps reduce the size of Internet routing tables.
In the figure, notice that ISP1 has four customers, each with a variable amount of IP address space.
However, all of the customer address space can be summarized into one advertisement to ISP2. The
192.168.0.0/20 summarized or aggregated route includes all the networks belonging to Customers A, B,
C, and D. This type of route is known as a supernet route. A supernet summarizes multiple network
addresses with a mask less than the classful mask.