As long as your router is receiving a public IP address on the WAN side you will be able to do it by setting up port forwarding, which tells your router, for example, "port tcp/8000 goes to private IP A.B.C.D:tcp/80".
You will test it by the address http://A.B.C.D:8000/ from a PC in another place of the Internet, not the same as your LAN, to be sure.
However, your ISP may be giving you a "dynamic IP address" which means that it changes every N hours or so. Tough luck to keep it stable. You might want to try a dynamic DNS service like changeip.
Also, your ISP may be "protecting" its network with a firewall, which will not let you set ports up like this. (It will let you, it it won't work).
Just consider this will NOT give you a highly available service.