Im installing webmin cp and it needs its own web-server in different port than normal. I have Debian+Apache on my server working just fine. How do i create web-server in port :81 to webmin?

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Webmin will automatically create the necessary ports and bind to them appropriately. I've never (ever) had to manually configure apache for webmin on any os (and I've installed it on quite a few).
When running the setup script (assuming it's not prepackaged for your OS), what does it tell you?

Ah, im installing the first time so i thinked it needs to be configure... Thanks for the tip, i just keep pushing enter.

It now says:

Webmin uses its own password protected web server to provide access
to the administration programs. The setup script needs to know :
- What port to run the web server on. There must not be another
web server already using this port.
- The login name required to access the web server.
- The password required to access the web server.
- If the webserver should use SSL (if your system supports it).
- Whether to start webmin at boot time.

Web server port (default 10000):

Webmin works now smoothly...

Yes, like you have already seen we can configure the apache such that it listen to a different port

Hope you have got your answer for your question..:-D .if you need any help just let me know.

Yeah, it just wants to know what port to listen on, it does everything for you :)