Networks

The subnet mask is used to help define which part of the IP address is the "network address" and which is the "client address".

You should switch it if your IP scheme on the individual networks requires it. If you are using (for example) two typical class C addresses (for example 192.168.1.X and 192.168.5.X) then you'll likely want to use the 255.255.255.0 subnet mask for both. If you however had another network with a 10.X.X.X addressing scheme, then you may want to change your subnet mask to reflect this.

Hope this helps some. Good luck.

It would be easier answered if you told us what you were trying to accomplish, like why you need 2 separate subnets on the same switch. More details would be helpful too, such as what kind of switch, is it capable of VLANs, what kind of router, network addresses, can you use CIDR, etc..?

The way it is, there are too many solutions and questions.