Hi
I have a computer running as a web server and a file server. I would like to know how incoming packets are mapped, how the computer identifies which packet are to be handled by the web serer.

Please, not to technical, I am a newbie at this.
Thank you

it's the TCP port number that the packet is sent to. Web servers generally listen on port 80 and 443, so a packet hitting the box for those port numbers will be connected to the process listening on that port.

You can imagine the "line" is connected to the box and there are more than 65 thousand "gates" can be configured to let the information enter. Those gates are named ports and every port has its own program to interpret the information arrived. There is a file /etc/services where you can find the common configuration of those ports (that means which program will listen to which port). TCP and UDP are something like two different languages
web servers generally listen on port 80 and 443 (443 is used for a different kind of connection).
When you want to connect to a host you always need to tell not only the name of that host but the port too. (usually default values used, taken from that /etc/services file).

Thank you, I must of had a brain fade, never thought of port number at the time