Need help setting up 2 set of LAN with same gateway

Dear all..

How to setting up 2 set of LAN that share one gateway?

Current local IP range : 172.16.101.1-172.16.101.255
Current subnet : 255.255.255.0
Current gateway : 172.16.101.254

I want to create another new local IP set range to 255 using same gateway,but how am i gonna do it?

Below are simple diagrams of my networks:

LAN'S > Switch > Firewall > Modem

What should I set in my firewall settings?
Is there anything I should add in my network?

Any suggestions welcome..
Please help..I'm so dumb in network

I am guessing you mean the same gateway device when you say same gateway ? (i.e. you don't mean the same gateway address).

Anyhow, if the gateway has another interface, you can setup a new IP Subnet and give the other interface a GW address for that network.

What Gateway is it. I know that a WatchGuard firewall has a great feature that allows you to add additional Secondary Network IP's to an interface which would then allow you to have two Gateway addresses (for different ip subnets) on a single interface.

Another way would be to add a router to add an additional IP Subnet and route the traffic through to your gateway instead of using the same gateway device.

Is your current subnet running out of address ? (i.e. is 254 hosts no longer enough) Is this why you need a new subnet ? Another option would be to re-address your current subnet using a 255.255.252.0 subnet giving 1022 address ?

Cheers