I'm taking over IT responsibilities for a previous IT guy.

We have a 50mb cable modem connection from Comcast along with 5 static IP addresses:

XXX.XXX.XXX.180
XXX.XXX.XXX.181
XXX.XXX.XXX.182
XXX.XXX.XXX.183
XXX.XXX.XXX.184

We are in the process of replacing our firewall machine. Currently the firewall box is the only thing connected to the cable modem. However the cable modem has multiple ethernet ports on it, similarly to a router.

I have assembled a new firewall machine and its time to start testing and configuring it. So that means that I also need it plugged into the cable modem (remember it has multiple ethernet ports on it).

So now with multiple computer plugged into the cable modem, how does the cable modem know where to route the traffic? If some request on the internet is made to XXX.XXX.XXX.181, which goes to our cable modem, how does the cable modem know which connected computer that traffic is supposed to be sent?

Looking at the web interface for the cable modem, there doesn't seem to be anything special setup on it with regards to routing or NATing IP addresses. Is that because when there is only one computer connected to the modem, all traffic is sent to it by default? Now that I am going to (temporarily) have multiple computers plugged into the cable modem, do I need to specify routing or NAT rules on the modem itself?

I am going to speak to Comcast about this next, but I figured I'd ask here first just so I can get a better grasp on how this type of thing generally plays out.