I have a sonicwall firewall with a dmz interface of 192.168.45.254, and i have one device connected at 192.168.45.232.
Now i need to add one more subnet (192.168.0.x) on this interface for a different device, so i am connecting a Cisco L3 3550 Switch.
I need to setup this config properly, so that i can still have my device at 192.168.45.232 and then also have another device at 192.168.0.x and they can go online.
You need to use VLAN's no matter what. If both subnets/VLAN's need to be on the same interface of the firewall and 3350, then on the 3550 you need to configure the interface as a trunk with at least one of the VLAN's being tagged.
On the firewall you need to configure the interface so that both VLAN's are active on the interface and at least one of them needs to be tagged.