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I currently have a local network running on the range 192.168.50.0/24, but I am running out of IP addresses to use.

The plan is to change the network to 192.168.50.0/23 network, which will effectively double my address space, giving me a range of:

192.168.50.1 - 192.168.51.254

I would like to do this migration in a phased approach, if possible. There are many devices that are served by DHCP, while many others have been assigned static IP addresses (and require them to be static). Several devices with static IP addresses can only be changed by a 3rd party supplier, so it will have to be done during business hours, on a live and in-use network.

My question is - what will happen during the transition phase, while changing IP settings on the various devices? Will things break? Or will communication still be fine between the existing /24 network and the new /23 network? Traffic won't be crossing any routers, just several switches.

What is the best way of managing this transition on a live and in-use network, while trying to minimise potential downtime and other issues.

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2 Answers

up vote 1 down vote accepted

In the scenario you describe:

  • Machines that both have an IP within the overlap will be able to communicate with each other regardless of the difference in their subnet declaration
    • Any communication that relies on local broadcasts will not however work, as the two subnets will have different local broadcast addresses (global broadcasts on 255.255.255.255 will still work)
  • Machines that have an IP not within the overlap can only communicate with machines that know the new subnet (they could technically communicate to machines in the overlap, but the machines there won't understand how to send back packets)
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Well initially, all machines will be in the overlap region (192.168.50.x), even through the transition. I won't be moving any devices into the 192.168.1.x range until the transition is complete. –  Saajid Ismail Oct 10 '13 at 9:41
    
You should be fine then, with the only potential issue being the local broadcast address. Try with a couple of low-priority machines first so you can see what potential impact the differing broadcast addresses may cause –  Graham Wager Oct 10 '13 at 9:49
    
By "broadcast messages" I assume you are referring to messages addressed to 192.168.50.255? If so, none of our services use this broadcast address. DHCP clients though will broadcast on 255.255.255.255 when requesting an IP address, but this shouldn't be a problem either. Does all the above sound right? If so, then I should be good to go. –  Saajid Ismail Oct 10 '13 at 9:53
    
Yes, that should be fine –  Graham Wager Oct 10 '13 at 10:06
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You are not changing the gateway address nor the dhcp-server address, but you are changing the netmask and the broadcast address.

Thus, so long as the broadcast address and the netmask are not changed on each machine, those on the original subnet (192.168.50.0) will be unable to talk to those on the new subnet.

For machines served by a DHCP, this will last only until a reconnection, whether by reboot or new lease it does not matter. But after reconnection they will be served with the new Broadcast address and Netmask, and all will be fine.

For machines with static IPs the problem will instead persist until manual intervention provides them with correct, new description of your network.

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