contiguous subnets or networks

Bolton07 · June 2011

'Route summarization groups contiguous subnets or networks using a single address.'

What does the term contiguous subnets or networks mean?

ccnxjr · June 2011

"contiguous subnets or networks" refers to a group of networks that are touching each other (stop giggling!).

In this context they refer to networks that can be logically grouped and share a similar CLASSFUL IP addressing scheme eg:
192.168.1.0/26
192.168.1.64/26
192.168.1.128/26
192.168.1.192/26
and don't pass through networks of a different class to reach each other.
Lets say, if a packet sent from the 192.168.1.64/26 network needed to pass through the 172.16.3.0/30 network in order to reach a host on the 192.168.1.192/26, then it would no longer be contiguous.

Bolton07 · June 2011

Would the last example be called a Discontiguous network then?

defaultroute · June 2011

Hi Bolton07,

The word contiguous means 'without a break' or one after the other. What they are saying there is that to summarize networks properly the network numbers need to come one after the other e.g. Take these numbers 1 then 2 then 3. you see how they come one after the other?

A contiguous network example might be:
192.168.0.0/24
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24

Do you see how the third octet goes 1,2 then 3 without a break? We can summarise all of those networks using 192.168.0.0/22. Don'[t worry about the math - trust me you can do this.

If it were non-contiguous networks then you might have:

192.168.1.0/24
192.168.4.0/24
192.168.17.0/24
192.168.240.0/24

In this case you would not be able to summarize the networks without including other networks in the process. Using a network statement like 192.168.0.0/16 will include all of those networks but crucially it also includes all networks from 192.168.0.0 to 192.168.255.0...this is bad

What we don't want to do when summarizing networks is to include MORE networks that we don't actually know how to get to. By advertising a summary address for all of those 192.168.0.0/16 networks we would cause traffic destined for all of the other networks in that sumary e.g. 192.168.0.0, 192.168.2.0, 192.168.3.0 etc to be dropped. Remember if the router doesn't know how to send that data it drops it.

Of course summarization in itself isn't the end of the story, a more specific network in the routing table will always be preferred

Hope I helped and I also hope the moderators don't drop this comment like they did my other one. I was really annoyed with that guys - it took me ages to write it! If you don't allow links then why allow the URL tag?

If they don't drop this comment then you can read about how to subnet and binary math stuff at my site and I'd love to see you there. Also techexams have loads of material which is brilliant for studying CCNA and CCNP so make sure you take a look at the links on the left.

Thanks guys for a fantastic forum

ccnxjr · June 2011

Discontiguous would be a fair description of the last example, yes.

Bolton07 · June 2011

Is Discontiguous the same as non-contiguous?

My understanding is that they have different meanings

ccnxjr · June 2011

I assume that "non-contiguous" may be used interchangeably with "Discontiguous".
Grammatically, I think "Discontiguous" makes more sense, suggesting separation, whereas non-contiguous implies absence.
This is open to debate, but one can make the argument that there is a difference based on the significance of the prefixes.
Where a Discontiguous network may have a sequential clasful subnets separated by a different class, eg:
192.168.1.64/26 ----172.16.3.0/30---192.168.1.128/26

Whereas non-contiguous network may have no sequence at all
192.168.1.64/26---172.16.3.0/30---10.1.7.0/24

*shrugg* just my opinion, subject to change with new information

contiguous subnets or networks

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