Most IT departments have seperate subnets for development,testing, etc. Because of that, they usually VLAN them off from the rest of the network for security or testing purposes.
If you existing subnet is something like 10.10.15.0/24 where 10.10.15.1 is your firewall or gateway device, you can create something 10.10.16.0/24 where 10.10.16.1 is the firewall or gateway address for the new network.
10.10.16.1/24 would be a sub-interface on the same firewall or gateway device the rest of the LAN uses. This would allow you to have a seperate subnet on the same phyiscal network, but also be logically seperated yet still have internet access.
If you need devices from both networks to commnicate together you would either have to keep them in the same VLAN or trunk the VLANs through your switches and assign specific ports to multiple VLANs. This is common among data center networks, but can get hairy or may be not needed in corporate settings.
