Cisco CCNA Discontiguous Addressing


If you create VLSM network, sometimes you may find that the backbone connecting buildings together is a different class of network.  This is called discontiguous addressing.

By default routing protocols will not work across discontiguous networks.  By using the “no auto-summary” command on the network boundaries, routing protocols will be able do work across a discontiguous addressed network.

Cisco CCNA VLSM Question



Network 1: – Equates to, block size of 64

Network 2: – Equates to, block size of 32

Network 3: – Equates to, block size of 16

Serial link 1: – Equates to, block size of 4

Serial link 2: – Equates to, block size of 4


Cisco CCNA What is Route Summarization?


Route summarization forms a supernet.  This is an IP network that is formed from the combination of two or more networks (or subnets) with a common Classless Inter-Domain Routing (CIDR) routing prefix.  In other words, it is a block of contiguous subnetworks that are addressed as a single subnet.

An example is that subnets through can be summarized as

Be careful not to over summarize as it can cause black holes in routing.

Cisco CCNA Route Summarization


Summarization is not always possible because unless you design and implement the network with summarization in mind, you will not find contiguous boundaries in which to implement summarization.

Route summarization can significantly reduce the size of routing tables if implemented correctly.  If not implemented correctly it can cause problems such as black holes if the routes are over summarized.

Cisco CCNA Implementing Summarization


Summarization is easy if you just know your block sizes….

Cisco CCNA To finsd a Summary Address…


Since this is a block size of 32, the mask would be

Know your powers of 2 and this goes a lot faster.


Cisco CCNA Route Summarization Examples


Answers to examples:

Cisco CCNA Summary Question


The subnet is and the mask is

This is a block of 16 in the third octet

The router will forward packets with a destination of the following addresses:   (Network address, not forwarded)   (beginning of host range)           (end of host range)           (broadcast address, forwarded)


Cisco CCNA Summary