Reconfigure network switch and WAPs to new subnet and leave legacy devices on old subnet

Librarytech

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Feb 5, 2015
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I haven't had to deal with thinking about subnets in about 10 years and so I am rusty and need some help that I can't find an answer to:

Current setup: DHCP server 192.168.11.150, gateway 192.168.11.1, subnet 255.255.255.0

I need more IPs, so I want to change the subnet to 255.255.254.0 on my switches, wireless APs, firewall, and DHCP server. I would need to either delete and make a new scope or create a superscope on the DHCP server (Windows Server 2012).

Questions: can I leave all of my old/legacy devices with the 255.255.255.0 subnet and still get access to the DHCP server and general internet? Can any of those legacy devices see any new devices connect to the network and can the new devices see the old ones with the different subnets?

Thanks!
 
Solution
The subnet mask must be consistent for all devices on the same lan segment. It can partially work in some cases but the largest issue are related to the broadcast address being different . The key problem is devices will use a ARP command to get the mac of a device they think is on the same segment. If they think the device is on a different segment they send it to the gateway ip instead but the other device will attempt to use arp and send directly. It shouldn't work but sometimes does because of routers doing things like proxy arp and clients that tolerate sessions that have different mac addresses.

I would just change them all to the new subnet mask. Short term pain but no long term random issues.
The subnet mask must be consistent for all devices on the same lan segment. It can partially work in some cases but the largest issue are related to the broadcast address being different . The key problem is devices will use a ARP command to get the mac of a device they think is on the same segment. If they think the device is on a different segment they send it to the gateway ip instead but the other device will attempt to use arp and send directly. It shouldn't work but sometimes does because of routers doing things like proxy arp and clients that tolerate sessions that have different mac addresses.

I would just change them all to the new subnet mask. Short term pain but no long term random issues.
 
Solution