IP Addressing on Cisco

Discontiguous Networks

Discontiguous are those networks where the IP addressing is not well designed. Discontiguous nets can create routing problems, for the reason that there is more than one summary route inside the routing table that is used to reach the subnets of a net. Some routing protocols will not be able to route data traffic unless manually configured since those routing protocols automatically summarize routes by default.

Disabling Automatic Summarization

Typically, automatic summarization is good. Nevertheless, when we have discontiguous subnets, we have to run the following command for both Routing Information Protocol version 2 (RIPv2) and EIGRP to disable automatic summarization: “Router(config-router)#no auto-summary”.

Variable Length Subnet Mask (VLSM)

The net architect uses variable length subnet-mask to generate the subnet scheme for the future network. Using variable length subnet-mask reduces the requirement that all subnets of the same network have the same amount of IP host addresses and the same prefix length. Variable length subnet-mask provides a more effective IP address space. VLSM also permits routers to summarize paths on networks that are not the same as the classed networks.

Classless InterDomain Routing (CIDR)

When variable length subnet-mask is used in the IP addressing scheme, the architect must use a routing protocol that supports Classless InterDomain Routing. Classful routing protocols do not care about the subnet mask or the prefix length information in routing updates. These protocols depend on the default subnet masks to define the net portion of the IP addresses.

Classless routing protocols always send the prefix length together with the route data in routing updates. These protocols allow routers to decide the network subnet of the address without using the default masks.

CIDR and Summarization

A hierarchal net design is intended to make route summarization more efficient and to decrease routing protocol processing. Route summarization is also called route aggregation. It is the method that routers use to advertise a set of contiguous addresses as a single entry with a smaller, less specific subnet mask or prefix.

Because CIDR disregards the drawback of classful boundaries, it allows summarization with VLSMs that are smaller than the default classful mask. A network address with a prefix length smaller than the default classed prefix length is called a supernet. A good example of a supernet address is 170.16.0.0/14. The default prefix for the Class B 170.16.0.0 address is 16 bits. Using 255.248.0.0 for the subnetmask, four contiguous Class B addresses can be summarized into one routing table entry.

This kind of summarization helps decrease the amount of entries in routing updates and lowers the amount of entries in local routing tables. The outcome is faster routing table lookups.