We have about 500 public IP addresses and is about to run dry. Most of the hosts are configured to get the IP from DHCP.

But there some that was configured with static IP.

The problem

Now I would like to free those IP addresses, where the host no longer exist.

For the DHCP IP's it is easy; There I can just look in the DHCP server and check when each IP was last issued. If not within X months, then I declare it as free and if the host should come online again, it will just get another IP.

But what about for those hosts that used static IP's?

I can ping the IP and if it doesn't reply then maybe is it free.

But what if I give the IP to a new host, because I think it is free, and the old host one day comes online? Problem!


How do you clean up your IP's in such a situation and prevent "ghost hosts" from coming back online and claim their IP address again?


Hopefully, you're tracking your static IP address assignments somehow. Most shops have some type of document (like a spreadsheet) that keeps track of any static assignments. If you don't, there are free IP address managers available on the Internet (SolarWinds comes to mind), that can help you inventory what you have on your network.

As far as DHCP assignments go, if you're using the Windows DHCP server, you can enable address conflict detection so that it will ping an address to see if it is in use before assignment. I think other DHCP servers have similar capability. Of course, your client and hardware firewalls must allow pings for this feature to work.

As for static address clients that connect after your IP inventory, they will receive an address conflict notification when they rejoin the network, if their address is already in use. Hopefully, they'll have the sense to switch over to DHCP (or call support). Unfortunately, you have to apply more sophisticated techniques (such as network access protection or 802.1x) to prevent these guys from connecting in the first place.

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