We connect to Internet using Zoom X4 modem.
Our LAN consist of several computers (windows platform).
We made one of them a web server (Apache).
Mapped ports in modem using Advanced Setup/Virtual Server.
The server is seen from outside our LAN using modem's external IP.
Computers from LAN access the served pages with the LAN IP of the webserver.
But I couldn't find an answer to "Why those computers (from LAN) cannot get to the web server using the modem's external IP?"