Question : why a web server behind an adsl modem is not seen by neighbour computers using modem's external IP?

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?"

Answer : why a web server behind an adsl modem is not seen by neighbour computers using modem's external IP?

Because your isp does not provide loopback routing.