|I do not believe that you can do that. |
In any case you would be defeating the purpose of having a router. If that is what you want to do then by pass the router altogether (put it back in its box) and connect your PC straight to your modem.
The WAN port talks to the external world using a single IP address in your case "189.143.xxx.xxx". This may be a static or dynamic IP address dependant on your ISP and your subscription to your ISP. This address is allocated by your ISPs DHCP service.
Internally your router has a DHCP which will allocate an IP address to each PC connected to it. This can be done statically by you instructing which IP address to allocate to which PC (Mac code) or to let the DHCP in the router do it dynamically. These local IP addresses will be in the range "192.168.xxx.xxx". This is how the router is then able to manage / channel the external communication to and from the right PC.
Your router will also have an inbuilt firewall which gives you added protection.
I am also puzzled by your comment about a 56k connection. It does not make sense as you appear to be on a broadband connection?
☺ When everything else fails, read the instructions.