Classless Inter-Domain Routing (CIDR)

The IETF introduced RFC 1517 in 1993, introducing the classless inter-domain routing (CIDR). The CIDR replaced the old classful network assignments. The classful address now becomes obsolete due to Classless Address scheme. The CIDR network address is determined by the subnet mask instead of the value of the first octet of the address. The network portion and host portion of the IP address are also determined by the subnet mask calling the network prefix. The network prefix is also known as prefix lengths such as /16, /17, /25, and /30 etc.

The ISPs are no bond only to  /8, /16, or /24 subnet mask. They can now assign IP addresses more efficiently using any prefix length. Now the ISPs can assign IP address blocks according to the requirement of the customers. from a few hosts to hundreds or thousands of hosts. The CIDR also reduces routing tables size and manages the IPv4 address space more efficiently using Route summarization and supernetting.

Route Summarization and Supernetting

The route summarizations, also known as prefix aggregation. The multiple routes are summarized into a single route reduce the size of routing tables. For example, one summary static route can change several specific static route statements. The figure below illustrates the route summarization. Router1 has 5 different routes. Each network has a different IP address network.  All networks can be summarized into a single network to Router0.

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172.16.0.0/21 summarized or aggregated route includes all the networks belonging to Router2, Router3, Router4, and Router5. To summarize this type of route required suppernetting. A supernet summarizes multiple network addresses with a mask that is smaller than the classful mask. Supernetting is required when the route summarization mask is a smaller value than the default traditional classful mask. The supernet is always a route summary, but a route summary is not always a supernet. The procedure to determine a summary route is the following:

  • Convert all network addresses into binary format.
  • Count the number of far-left matching bits to identify the prefix length or subnet mask for the summarized route
  • Copy the matching bits and then add zero bits to the remaining places to determine the summarized network address.

Now, this address and subnet mask can be used as a summary route for all the networks. We can configure Summary routes for both static routes and classless routing protocols. The figure below illustrates the summary routing procedure:

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Static Routing CIDR Example

The smaller routing tables make the routing table lookup process easy fast and efficient because there are fewer routes to search. So if we use a single static route instead of multiple static routes, the size of the routing table has reduced. A single static route can represent dozens, hundreds, or even thousands of routes with efficient way. This is possible to configure a summary static route using CIDR.

In the Figure below, Router0 has configured to reach the identified networks in the topology. Though acceptable, it would be more efficient to configure a summary static route.

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Figure 2 show route aggregation using CIDR summarization. The four static route entries reduced to 172.16.0.0/21 entry. The below example removes the six static route entries and replaces them with a summary static route.

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Classless Routing Protocol Example

In the classful routing protocols, the receiving router automatically applies the default subnet mask to the network address in the routing table. If the topology in the figure contained a classful routing protocol, then Router0 would only install 172.16.0.0/16 in the routing table. Variable Length Subnet Mask(VLSM) and supernet routes needed classless routing protocol such as RIPv2, OSPF and EIGRP. Classless routing protocols advertise network addresses with their associated subnet masks. When a supernet route is in a routing table, such as a static route, a classful routing protocol does not include that route in its updates.