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New Member

IP subnetting question

This is a silly question but I need to get some clarification on this subnetting question.

Given a network address 11.253.26.0/24 from a provider I want to have 3 networks with the following number of hosts.

10, 35, & 95.....what would the networks and subnet masks be using VLSM.

I know that the all ones and subnet 0 may be used but givrn this generic question and not knowing what the existing ip stack capabilities are, and as a fundamental design rule, I would not use subnet 0 and the all ones subnet.

I say the networks shoud be:

11.253.26.32/27

11.253.26.64/26

11.253.26.128/25

Am I wrong for saying this or should the first network be 11.253.26.0/28

6 REPLIES
New Member

Re: IP subnetting question

If your provider knows your network with /24 mask you are able to divide it several subnets just by defining next hop (e.g different ethernet interfaces) on your border router.

So that, it is also possible to use subnet zero and others. Bear in mind that your definitions had not be overlapped.

New Member

Re: IP subnetting question

Those subnets work for what you are wanting to do. You can use subnet 0 nad the all ones subnet with Cisco products if needed. You have to be careful of the routing protocol you use if any, since you are using VLSM.

New Member

Re: IP subnetting question

Here's a follow up question. I was asked the basic question to determine what the networks and subnet masks would be using VLSM given the above network from the provider. I wasn't given any information about routing protocols or anything else. Just the basic question. I was told that my answers were incorrect. I suggested that as a basic design practice to not use the all ones subnet and subnet 0 without knowing that information.

New Member

Re: IP subnetting question

Yes you are correct: - but remember if these are internal nets you can youse the private address scheme:

10.0.0.0/8

172.16.0.0/16.

192.168.123.0/24

You can use 11.253.26.0/28 (network) which will yield 14 hosts

11.253.26.1- 14 - hosts

.15 Broadcast

Then you can jump to 11.253.26.64/26

hosts 65-126

127 b'cast

11.253.26.128/25

hosts 129-254

255 b'cast

hope this helps.

C

New Member

Re: IP subnetting question

Shouldn't first network with with 10 hosts be 11.253.26.16/28. Typo, I bet.

New Member

Re: IP subnetting question

The important thing to note here is that you have been allocated a 24-bit masked subnet of a class A network, not a class C network. Consequently this means that you are free to use the whole subnet for your own use without having to worry about the subnet 0 and all ones subnets (using the same masking, these would be 11.0.0.0/24 and 11.255.255.0/24).

Regarding routing, you provider is likely to just be routing any traffic in the 11.253.26.0 network to your router. The handling of any routing throughout further subnets and the provision of a default route out to your provider would then be done by you.

If I was allocating the subnets, I would be tempted to do the following:

11.253.26.128 /25, giving 126 hosts in the range .129 - .254

11.253.26.64 /26, giving 62 hosts in the range .65 - .126

11.253.26.32 /28, giving 14 hosts in the range .33 - .46

This is basically as you calculated, but with a smaller mask for the final subnet in case you need to add further subnets. The final subnet could be placed at 11.253.26.48 /28, but there would then be no room for this subnet to expand in the future. By placing it at the .32 position, the subnet could be expanded in the future to handle 30 hosts simply by changing the subnet mask (11.253.26.32 /27).

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