One VLAN two IP ranges, how is this possible?

Paul Walsh
Paul Walsh used Ask the Experts™
Hi All,

We have two firewall devices with their own sperate internal ip ranges say subnet and subnet Both are set to act as dhcp servers. Both internally are connected to the same VLAN. I am assuming these work as the company that manages them have not reported there are problems and they have been in place this way for a good few years.  

If I plug my laptop into a switch port untagged on this vlan I receive an ip address from the first device. I cannot see the other device. If I statically assign a free IP on the other range (device 2) I cannot ping this device (the gateway) or basically do anything.

How is this possible? I am clearly missing something, but I was under the impression that each VLAN only has one IP range and not two.

Sorry if this is a simply answer but I really cant get my head around this.

Kind Regards,
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Software Engineer
Distinguished Expert 2018
You CAN run as many IP ranges on a network segment as you like.
The systems will only communicate among each other if the network settings match......
Any broadcast traffic will go all places to will DHCP ..., the first to respond will win the race as far as the requesting system is concerned.

This is NOT a secure or healthy network..   (any broadcast packet can still be observed by all systems).
So this is a networks that is destined to experience "interesting" often more or less random effects. (mostly the stuff that system managers  hate to solve..).

try to at all things lined up. Are you sure the 2 routers are in one LAN segment?... is the port for the 2nd device actually enabled?  was it meant as a failover  "rookie style"?
Paul WalshSystem Administrator



Sorry for the delay in getting back to you. It could very well be meant as a failover, the setup has been in place for ages and the people at the support company who put it in place have since moved on.... I think its time to start a fresh and do it correctly. Thanks for your help.
nociSoftware Engineer
Distinguished Expert 2018

Yes that is probably the best approach.

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