VLANs and Subnet Masks

[Zach Smith]

I got a wierd question.  I currently have an odd situation and am wondering what the reprocussions are to changing a subnet mask on a vlan.

I have a core switch with an interface VLAN 5 which is 172.20.0.1 255.255.0.0

This VLAN happens to be for all of my servers.  This VLAN interface was supposed to have been setup as 172.20.0.1 255.255.252.0.  So my servers are given IPs ranging from 172.20.0.2 - 172.20.3.254 with a subnet mask of 255.255.252.0.  (default GW of the switch - 172.20.0.1)  I have some random systems that have been given an IP in this range but the subnet mask is 255.255.0.0 like on the VLAN interface on the core switch.  I have a need to create a 172.20.4.0/22 network and since my VLAN interface on my core switch is 172.20.0.1/16 I don't believe I can do this so I plan on changing the VLAN interface to 172.20.0.1 255.255.252.0 like it was supposed to have been upon creation.  I'm confident that all of my servers that are properly IPd with a /22 subnet mask won't see any change/outage.  However I'm concerned about my devices that are given a /16 subnet mask.  Will these devices have any outage if I change the VLAN interface subnet mask to /22 ? 

My hypothesis is the following - the devices in the 172.20.0.0/22 network with an incorrect /16 subnet mask won't be able to communicate to my new network of 172.20.4.0/22 but will still communicate to everything else just as they do today.

I will obviously change everything to the /22 but some of these systems are high profile and if they'll see an outage i'll need to plan accordingly.

Thanks for the help.

[Bilal Nawaz]

Hello, It will be too much of a task (if there are many servers) with high risk to change over networks (you'd have to be pretty quick to change IP's of all the servers to have minimal down time - maybe there's a script to do this), it would be far easier sticking with the 172.20.0.0/22 network.

From your 172.20.0.0 255.255.0.0 network, if your servers and gateway have IP within the 172.20.0.0/22 range, I do not see a problem in changing masks of the interface - the problem will be with anything outside of the 172.20.0.0/22 range. But you've stated that you haven't, so you shouldnt run int to problems.
The reachability will still be the same - but please ensure you change the masks on the servers too - not urgently necessary, but will need to be done for sure.

As soon as you change the mask on the interface and servers, also created the 172.20.4.0/22 network, you will have reachability to that network. Otherwise all other networks will be reachable.

Hope this helps

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