I've been studying my CCDA a few days now and the term Contiguous Subnet and another term who's name escapes me now, but probably it's opposite keeps creeping up.

Can anybody explain this to me - I look forward to your responses.

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This ain't so easy to explain briefly but I'll try...

Contiguous subnet - 10.0.0.0 255.0.0.0
The whole class A address block is in this subnet, so it is contiguous.

Non-Contiguous subnet - 10.10.0.0 255.255.0.0
This class A block has been broken into smaller subnets and therefore is non-contiguous.

This becomes very important if your routing protocol does not support VLSM.

Hope that brief answer helped!

Yankee

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To understand Contiguous Subnet, first try to understand route summarization and supernetting, or try to find what we will benefit of supernetting. Once you got what is supernetting you will be able to understand Contigous Subnet.

Please refer to page 206 of "CCDA Exam Certification Guide"

Contigous Subnet will help you decrease number of entries in your routing table.

Hope I had given the hint. More light please.

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Thanks for the Info, sorry Yankee I may have to disagree with you - although Ive been wrong before.

I've done some research with a chap I work with and he says he's come across dis -contiguous (probably same as non-contiguous) subnets once before.

It was a subnet which didnt have "continous" 1's in the binary equivalent of the mask.

e.g. Contiguous subnet -
255.240.0.0
or
11111111.11110000.0.0

dis-contiguous subnet -

255.232.0.0
or
11111111.11101000.0.0

Unfortunately he can't remember why they were using it!!

Thanks to you both for answering - although normally I raise more questions than I ask!!

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Mdbilal Is correct in his post.

Contiguous Subnets must match the binary bit value of their address up to the subnetmask value. Any bit value past (to the left of) the Subnetmask can be different from each other and still be contiguous. To be considered contiguous,
the subnets only have to match up to the subnetmask value.

These are contiguous subnets

200.100.50.0 network
200.100.150.0 network
255.255.0.0 subnetmask

These are not contiguous subnets

200.100.50.0 network
200.100.150.0 network
255.255.255.0 subnetmask

Contiguous Subnets are used in route summarization and supernetting.

Contiguous Subnets depend on bit value matching at the determined subnetmask.
In other words, the 0s and 1s up to the subnetmask value must match. After the value they may be different.
So for example, Contiguous Subnets address may be several different Class C networks with subnetmask moved far enough to the left to where the address match in binary value. The binary values Class C addresses of 200.100.200.0, 200.100.100.0, and 200.100.10.0, and 200.100.0.0 would be contiguous up to 200.100 therefore a mask of 255.255.0.0 could be applied to them.

The binary values would be this:

11001000.01100100.00000001.00000000 for 200.100.1.0
11001000.01100100.00001001.00000000 for 200.100.10.0
11001000.01100100.01100100.00000000 for 200.100.100.0
11001000.01100100.11001000.00000000 for 200.100.200.0 11111111.11111111.00000000.00000000 for a 255.255.0.0 Subnetmask

Notice the 0s and 1s are the same between the network address up to the
point where the 1s in the subnetmask field stop.

In this example a router using route summarization could advertise just network address of 200.100.0.0 with a mask 255.255.0.0 rather than having to advertise all four Class C networks individually. The advantage to this in now you only have to advertise 1 network rather than 4 separate networks and this reduces your routing updates size and possibly the frequency of updates.

The previous address would be all contiguous with each other if the bit value to the left of the Subnetmask value is the same.

If the subnetmask value

11001000.01100100.00000001.00000000 for 200.100.1.0
11001000.01100100.00001001.00000000 for 200.100.10.0
11001000.01100100.01100100.00000000 for 200.100.100.0
11001000.01100100.11001000.00000000 for 200.100.200.0
11111111.11111111.11100000.00000000 for a 255.255.224.0 Subnetmask

These address are no longer contiguous with a subnetmask that is not matching
the contiguous portion of the Class C networks. This would be a invalid route summary/CIDR address mask for these networks as all the 1s and 0s bits to the left of the subnetmask is not the same.

Subnets with no common bits can not be contiguous as no subnetmask value
can be applied to them to make them contiguous.

11001000.01100100.01100100.00000000 200.100.100.0
01001000.01100100.01100100.00000000 72.100.100.0


Steve_NTS
CCNA, CCDA, CNA 4.11, Network +, A+

[This message has been edited by Steve_NTS (edited 12-01-2000).]

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Mat,
We don't disagree. My post was oversimplified and not well stated by me.

Both you and Steve did a good job of stating the "bit by bit" explanation at the global sort of level, but what it boils down to is summarization in individual networks. What I was trying to say and did so very poorly was that contiguous networks can be grouped (summarized) and non-contiguous can't. Of course the reason they can or can not be summarized is as you describe with the bits, but no network engineer draws out all that bit stuff. Simply glancing at the network addresses and subnet masks is sufficient info to identify what is contiguous or not.

Sorry for the confusion,

Yankee

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Thanks to everybody that replied - I think I've got it now!!!

I'm English and can't even understand my own language :-)

Thanks again for your answers, now for my next question - Why do they call orange jam ................

No doubt I will be back with another question very soon - Good luck with all your studying.
Mat P

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Yankee,
You are really outstanding in the way you explain as well as the way you agree with the facts. Its so nice of you.
Loving
Bilal

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I am glad to see such a great response from people to a good posted question. People often forget what it was like when they were
learning things for the first time and
stuggled with concepts that now seem second nature to them. This is a good example of what forum should be like.

Thanks to everyone that provides input to the questions posted here, especially the time and hard work that Yankee and many others have put into helping others.

Steve_NTS
CCNA, CCDA, CNA 4.11, Network+, A+


[This message has been edited by Steve_NTS (edited 12-04-2000).]

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