How to resubnet my network?

lenivan
lenivan used Ask the Experts™
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I've run out of IP addresses and need to resubnet my network. I am currently on 255.255.255.0 and would like to change it to 255.255.254.0 which should give me a range of 192.168.122.1 - 192.168.123.254.

What are the specific steps I need to take in doing this? What do I need to do on my email server running Exchange?
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Commented:
If you just change the subnet mask on all your machines, you will be able to access the extra IPS.
I would consider using 255.255.0.0 though.  This way you will not run into this problem again.

Commented:
And just for clarity, what you are proposing is a supernet, not a subnet.
Assuming that none of your server IP addresses will be changing, all you should need to do is change the subnet in DHCP for the various DHCP clients, and manually on the devices with static addresses (servers/printers/switches/etc).

If you have any routing or network policies that run on ip blocks rather than subnets, you will need to change them to include the new addresses too.

I cannot think how it would upset the exchange server as long as the network mask is correct, as it will be staying on the same address.
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Commented:
Would I first backup then delete my current DCHP? Then, create a new DHCP with the new subnet? What would my address range be if I use 255.255.0.0?

Commented:
You could just set a very short TTL for your DHCP clients on the server (like 2 minutes or less), when you are ready to make the switchover, ensure that all of the old leases have expired and then change to the supernet and set the TTL to its previous setting.

Note: If you have file servers on DHCP, Windows Server will sometimes reset the network adapter momentarily while it renews its IP address, this can cause files to become corrupted if someone is in the middle of saving something, or transfering new data to a file server.

Another (more complex) way to do the change is to just try a brand new /23 subnet (rather than extending the current one from a subnet to a supernet). You would then be able to place a Proxy-ARP machine on the network to answer ARP for the old subnet for devices which have been moved to the new subnet. This will allow you to move any devices with Static IP addresses prior to changing all of the DHCP clients. The Proxy-ARP device will need to be configured as a router and to do Static NAT between the two subnets.

Commented:
To answer your second question: "would my address range be if I use 255.255.0.0?", you would be using a 16 bit network in that case which gives you about 65534 usable addresses. You probably don't want to do that though. If you set a big broad subnet and need to do a quick scan for a vulnerability on the network it could take several days depending on how many hosts and how sparsely they are spread out.

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