Currently my Apache server is set up like so
ServerName www.example.com
ServerAlias example.com
DocumentRoot /var/www
Using local measures only (/etc/hosts instead of a DNS) I found that this can indeed work.
First, change your /etc/hosts file to have a mapping of your desired website name(s) (www.example.com), and target IP address (192.168.1.1). I used my local IP address.
IPAddress Hostname Alias
----------- -------------------------- ------------------
192.168.1.1 www.example.com example.com
192.168.1.1 www.john.example.com john.example.com
192.168.1.1 www.jane.example.com jane.example.com
Your web browser will check your /etc/hosts file before looking at the world wide web.
Next go through all your Apache config files (httpd.conf, apache2.conf, ports.conf, conf.d/*) and make sure in exactly one file the command NameVirtualHost *:80 is issued (it doesn't have to be port :80 but if it is issued more than once, you will get this problem). Mine was issued in /etc/apache2/ports.conf, so put yours there if you have to. Finally, update your Apache configuration file (mine was at /etc/apache2/sites-available/default) like so.
ServerName www.example.com
ServerAlias example.com
DocumentRoot /var/www
ServerName www.john.example.com
ServerAlias john.example.com
DocumentRoot /var/www/john
ServerName www.jane.example.com
ServerAlias jane.example.com
DocumentRoot /var/www/jane
As a final step, you may need to add the websites to Apache by issuing the below commands (this step is not necessary, if you give all websites into sites-available/default and not into separate files for individual websites).
# a2ensite www.example.com
# a2ensite www.john.example.com
# a2ensite www.jane.example.com
After doing this, john.example.com will go to /var/www/john. That directory will then act as the root directory, and john will no longer have access to www, and, therefore, have no access to /var/www/jane.
Likewise, after doing this, jane.example.com will go to /var/www/jane. That directory will then act as the root directory, and jane will no longer have access to www, and, therefore, have no access to /var/www/john.
With symbolic links turned off --by default-- neither directories will be able to access each other