CIDR and Summarization

The hierarchal network strategy is intended to make route summarization simpler and to cut routing protocol processing. Route summarization is also called route aggregation. It is a method of advertising a group of contiguous addresses as a single record with a smaller, less specific subnet mask.
Since classless inter-domain routing disregards the limitation of classful boundaries, it permits summarization with variable length subnet masks that are smaller than the default classful subnet mask. A network address with a smaller prefix length than the default classed prefix length is called a supernet. For example 172.16.0.0/14 is a supernet address. The default subnet mask for the Class B 172.16.0.0 address is 16 bits. Four contiguous Class B addresses can be summarized into one routing table record, if we use a /14 prefix.
This kind of summarization helps decrease the amount of records in routing updates and lowers the amount of entries in local routing tables. The outcome is faster and more efficient routing table lookups.
Prefix Addresses and Summarization
Classless routing protocols transmit the prefix length and subnet mask in routing updates.
A mixed hierarchy of variable-sized nets and subnets can be summarized at several points using a prefix address. For instance, a summary route can have a 14-bit prefix. The prefix: 172.16.0.0/14 with a subnet mask of: 255.252.0.0 summarizes the 172.16.0.0 /16, 172.17.0.0 /16, 172.18.0.0 /16, and 172.19.0.0 /16 subnets.
Route summarization decreases the load on upstream routers. For summarization to work efficiently, addresses must be wisely allocated in a hierarchical way so that summarized addresses share the same high-order bits.
A good network address plan is determined by numerous criteria:
1. The amount of computers and network appliances that are currently supported on the net
2. How much growth is expected
3. The amount of computers that must be accessible from nets that are not part of the local LAN or Intranet
4. The physical layout of the net
5. The routing and security policies that are implemented
Reachability of Hosts
Certain PC in the network must be accessible from nets that are not part of the local LAN or Intranet. To be reachable from the Internet, the servers must have a publicly registered IP address. Furthermore, there needs to be enough public addresses to use with NAT.
Security and Routing Policies
From time to time supplementary IP networks are needed to separate IP traffic for security or filtering purposes. When this is the case, different IP subnets are typically created. Wireless and IP telephones need separate IP nets.
The routing protocol choice affects how a net is addressed. A number of routing protocols do not support classless IP addressing. The default summarization applied in the routing protocol is also something that you should think about.