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New Member

VLSM Design


In attached file,can anyone kindly help if the IP Addressing scheme is ok or not. This is how I have to configure my network. I already have the IP Scheme working on my central site and somehow due to my application design, I have to use the subnet from the same IP Scheme.

Now the problem is I cannot change the subnet mask on the Central Site, I must have to use the default subnet mask.

Now the design given in the diagram is possible or I have to think some other solution?


Mansoor Hafeez


Re: VLSM Design

Hi Mansoor,

Typically, with VLSM, you should use non-overlapping subnets.

In your case, I see that your have used over-lapping subnets between your central and remote sites e.g. you are using at the central site and at your remote site. This will not work terribly efficiently.

Whenever a host sitting on the network tries to access a host in the network, it will send out an ARP for it. Your router will most likely respond to this ARP (since proxy-arp is enabled by default) and so you will be able to get to your remote site. So your setup will work .. it's just that it is not very optimal...

This is not really the best way to do this. Since you are using RFC1918 private addressing, address shortage should not be an issue. So I suggest you pick another address range altogether e.g. if you chose 192.168.100/24 and then split it up into /29 subnets, you could assign one of the following to each of the VLANs at your remote site..

and so on..

Hope that helps.

Pls remember to rate posts.



New Member

Re: VLSM Design

Hi Paresh

You are right, I have tested the configuration and it is working but i know it is not optimal.

The problem is my application design. Actually it checks the source addresses and they must be from the same network number thats why I must have to use the same network number.

I have tested before with different network numbers and IP connectivity was ok but the applications were unable to communicate.

One thing, if I use overlapping subnets and somehow by mistake if I have the same IP address on both sites, will this create any problem?




Re: VLSM Design

Hi Mansoor,

If you are happy with the way it is working, I suppose there is no reason to change it.

As for duplicate IPs, they should be avoided at all costs since the hosts with the duplicate IPs will find it incredibly difficult to communicate with other hosts. For example, if you assign to one host at the remote site and also a host at the central site, those hosts at both sites will not be able to communicate with the hosts at the other site... The bottom line is: avoid this at any cost.

Pls remember to rate posts.