Same IP Subnet across multiple sites

Answered Question
Mar 11th, 2010

Hello,

Our clients have recently purchased some equipment. This equipment will only work if the server / pc that is used to control it is on the same IP subnet. The complication is that the server / pc will be located at one site, but the equipment will be located at various different sites with different ip subnets.

My question is, is it possible and if so how can I have the same subnet present at multiple sites.

Our remote sites are connected via private circuits, we control layer 3 devices on our LAN but other layer 3 devices are controlled by 3rd party.

Thank you in advance.

I have this problem too.
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Correct Answer by Paolo Bevilacqua about 6 years 10 months ago

You have identified an extremely poorly written piece of software.

I was referrering to the their "engineers" defending this choice as uneducated, not to you.

Personally I would either try get it working with NAT, or drop that software altogether.

Another technique you can use, in set the mask large enough to software that does idiotic subnet check is happy.

Router will still be able to forward by mean of proxy-arp.

There is no reason on earth by which application software can decide the network design, especially when it dictates  against best practices universally adopted.

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Paolo Bevilacqua Thu, 03/11/2010 - 01:53

It is never true that equipment works only when located in the same subnet. That is a statement made by uneducated enginner with little understanding of IP networking. I have disporved in many many case. Even when based on UDP broadcast, the router is able to forward these correctly.

So please state what makes you think that is the case.

You must NEVER have the same subnet spread across WAN sites for professional results.

shiliwala Thu, 03/11/2010 - 02:41

Hi, Thanks for your quick reply.

The statement I have made is based on extensive testing and not "an uneductaed network engineer", howvever I should have been much more clearer with my initial post.

Although I have identified the UDP broadcasts that are used between the device and the server which in turn I have allowed the router to forward (and works) but there is an inbuilt limitation in the actual control software which will not allow the two devices to communicate with each other unless they are on the same subnet.

The software checks to make sure that the two devices are on the same IP network, if they are not it does not work with a big warning telling you so. I have also had in depth dicussions with with the technical engineers of these products and they also state that the devices need to be on the same IP subnet. 

I know this may not be good practice but I am also sure this can be acheived.

Thanks.

Correct Answer
Paolo Bevilacqua Thu, 03/11/2010 - 02:52

You have identified an extremely poorly written piece of software.

I was referrering to the their "engineers" defending this choice as uneducated, not to you.

Personally I would either try get it working with NAT, or drop that software altogether.

Another technique you can use, in set the mask large enough to software that does idiotic subnet check is happy.

Router will still be able to forward by mean of proxy-arp.

There is no reason on earth by which application software can decide the network design, especially when it dictates  against best practices universally adopted.

shiliwala Thu, 03/11/2010 - 02:58

I agree entirely with all your points, however unfortunately we have a business need to get this working. I will try your idea of setting a larger mask.

Thanks.

shiliwala Thu, 03/11/2010 - 07:54

Thanks for your tip, I have got them working on different subnets by changing the mask on both devices to cover both subnets, putting an ip helper and opening up the broadcast ports on our router.

Paolo Bevilacqua Thu, 03/11/2010 - 08:37

You welcome, see ingenous networking fixes application dumbness.

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