If I own two domains, for example domain.com and domain.net and
domain.net is a sub-domain to domain.com (www.domain.com/domain.net).
How can I make it work like, if I want those who type in www.domain.net
to end up in www.domain.com/domain.net ?

_HG_

Ask your provider to add DNS records for them. Have you tried .htaccess?

Will .htaccess do it?
Do you have an example code that does the trick ?

_HG_

One way I have done it is to have a hosted domain, www.domain.com/, create a folder called domain.net in the root of the web site, then take an unhosted domain, www.domain.net that is forwarded to http://www.domain.com/domain.net/.

You arrive at the end result.

[Edit} I had to restart the browser to edit lines I had already written.