First I have subscribed to NO-IP.com so that I always have a DNS name and where the outside IP doesn't matter.
Typically all firewalls NAT (network address translation) one external internet IP to an entire internal subnet or just a few IPs. You can do this manually or by using the built in firewall DHCP server. I personally only use about 5 DHCP addresses and statically assign all of the rest of the devices in my subnet. So as an example you can:
Create a small subnet internally like 192.168.246.240/28 255.255.255.240
Your internal side of the router would have a gateway and DNS address of :
192.168.246.241 255.255.255.240
Your network devices would have IPs of:
192.168.246.242 to 192.168.246.254 (9 devices)
You could statically assign these or use DHCP.
I have configured my firewall to let UPD/TCP port 8XXXX in and out.
I have left my ZM's Web interface at port 80.
I have set my firewall to UDP/TCP port forward from source IP/Port of 80 to port 8XXXX outside. It doesn't matter what my outside IP is as I am using DNS. I also do another port forward outside in the internet at the NO-IP site by port forwarding port 80 to port 8XXXX. This way all I am concerned with is the DNS name and directory. IE to get to my ZM from the Internet I use:
HTTP://XXX.GOTO.ORG/ZM
The path looks like this:
ZM Server IP: 192.168.XXX.XXX port 80 ====>
Firewall internally open and port forwarding IP and port above to:
TCP/UDP port 8XXX ======>
Firewall external interface only allows an IN/OUT UDP/TCP port of 8XXX
Dynamic DNS gets IP and assigns it a DNS name.
I take another DNS name from the same service and port forward it the above DNS name IE:
XXX.GOTO-1.ORG port 80 === XXX.GOTO-2.ORG port 8XXX
Two sections of my firewall are involved.
Section one is for the port forwarding
Section two is for the externally allowed access.
I think Linksys or generic routers tend to all be the same.
Typically DSL or Broadband modems are the same. They usually have one ethernet interface internally. This ethernet interface would go to the WAN interface on your firewall router. The WAN interface would use NAT internally to the rest of the network. Another way to pass the ZM traffic is to create a "pinhole" thru your firewall. This pinhole would say to allow all incoming traffic on port 80 to your ZM server IP and port. I don't think though this is really safe. Yet another way is to create a DMZ. This is a separate network between your firewall and your internal network. Each side of the network can access the DMZ but the DMZ protects you from the outside of the network and vice versa.
Hope this helps some.