IP Routing In A Subnet Or Classless Addressing (CIDR) Environment
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Aggregated Routes and their Impact on Routing
Classless addressing is formally called Classless Inter-Domain Routing or CIDR. The name mentions routing and not addressing, and this is evidence that CIDR was introduced in large part to improve the efficiency of routing. This improvement occurs because classless networks use a multiple-level hierarchy. Each network can be broken down into subnetworks, sub-subnetworks, and so on. This means that when we are deciding how to route in a CIDR environment, we can also describe routes in a hierarchical manner. Many smaller networks can be described using a single, higher-level network description that represents them all to routers in the rest of the internet. This technique, sometimes called route aggregation, reduces routing table size.
Let's refer back to the detailed example I gave in the addressing section on CIDR. An ISP started with the block 126.96.36.199/15 and subdivided it multiple times to create smaller blocks for itself and its customers. To the customers and users of this block, these smaller blocks must be differentiated; the ISP obviously needs to know how to route traffic to the correct customer. To everyone else on the Internet, however, these details are unimportant in deciding how to route datagrams to anyone in that ISP's block. For example, suppose I am using a host with IP address 188.8.131.52 and I need to send to 184.108.40.206. My local router, and the main routers on the Internet, don't know where in the 220.127.116.11/15 block that address is, and they don't need to know either. They just know that anything with the first 15 bits containing the binary equivalent of 71.94 goes to the router(s) that handle(s) 18.104.22.168/15, which is the aggregated address of the entire block. They let the ISP's routers figure out which of its constituent subnetworks contains 22.214.171.124.
Contrast this to the way it would be in a classful environment. Each of the customers of this ISP would probably have one or more Class C address blocks. Each of these would require a separate router entry, and these blocks would have to be known by all routers on the Internet. Thus, instead of just one 126.96.36.199/15 entry, there would be dozens or even hundreds of entries for each customer network. In the classless scheme, only one entry exists, for the parent ISP.
CIDR provides benefits to routing but also increases complexity. Under CIDR, we cannot determine which bits are the network ID and which the host ID just from the IP address. To make matters worse, we can have networks, subnetworks, sub-subnetworks and so on that all have the same base address!
In our example above, 188.8.131.52/15 is the complete network, and subnetwork #0 is 184.108.40.206/16. They have a different prefix length (the number of network ID bits) but the same base address. If a router has more than one match for a network ID in this manner, it must use the match with the longest network identifier first, since it represents a more specific network description.
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