subnet question

Unanswered Question
Sep 7th, 2007

hey i just have a quick question. what is the definition of a subnet? i alway thought if 2 address are using different subnet masks they are in different subnets, and therefor can't ping each other.

i tryed 192.168.40.x 255.255.254.0 and 192.168.41.x 255.255.252.0 was able to ping each other. i now know why they can ping each other, but are they considered to be on same or different sebnets

I have this problem too.
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nataku316 Fri, 09/07/2007 - 07:52

no router, using a layer 2 switch. i know why they can ping each other, cause the subnet mask will cover each other, that is not the question.

the questions is what is the definition of a subnet? and are the two computer in the same or different subnets?

Tony.henry Sun, 09/09/2007 - 13:15

Nataku,

If I found this situation in my day to day work life I would say the network was poorly designed and start work rectifying the situation.

I was going to suggest that the best way to define a subnet was to use a network address. In your case both network addresses are 192.168.40.0/23&22. But that doesn't work all the time.

I think a better definition may be to say a subnet is the range of addresses a computer can communicate too with out having to resort to routing. Your example, doesn't require routing with either of your configured hosts. You know that.

A system in your second subnet configured with an address of 192.168.42.1 would not be able to communicate with devices configured in your first example mask. The packets will arrive ok but the receiving device will want to use a default gw to reply.

Hope that helps

Tony

Tony.henry Sun, 09/09/2007 - 13:16

Nataku,

If I found this situation in my day to day work life I would say the network was poorly designed and start work rectifying the situation.

I was going to suggest that the best way to define a subnet was to use a network address. In your case both network addresses are 192.168.40.0/23&22. But that doesn't work all the time.

I think a better definition may be to say a subnet is the range of addresses a computer can communicate too with out having to resort to routing. Your example, doesn't require routing with either of your configured hosts. You know that.

A system in your second subnet configured with an address of 192.168.42.1 would not be able to communicate with devices configured in your first example mask. The packets will arrive ok but the receiving device will want to use a default gw to reply.

Hope that helps

Tony

avillalva Sun, 09/09/2007 - 15:55

Hi,

A subnet is the term used to describe chopping up a classful network into a number of smaller networks.

the problems with your example are;

neither are subnets, 192.x.x.x is a class C network which means the classful boundary is 3 octets (i.e. 192.168.40.x). Your masks describe a supernet.

The other problem is that your networks share address space. 192.168.40.x 255.255.254.0 has a range from 192.168.40.0 to 192.168.41.255

192.168.41.x 255.255.252.0 has a range from 192.168.40.0 to 192.168.43.255

it's ok to use supernets in your ranges, but just make sure that they don't overlap.

hope that helps,

Andres

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