Many operating systems block access to port 20000 by default, as they have a firewall activated as part of the installation process. This may prevent you from logging into http://yourserver:20000/ from another system on your network or over the Internet.
The simplest way to open up port 20000 is to use one of the Webmin firewall management modules, such as Linux Firewall, BSD Firewall or IPFilter Firewall. This of course assumes that Webmin is itself already installed and accessible.
The alternative is to login as root via SSH, and manually edit the firewall configuration file. On Redhat and derived systems, this is /etc/sysconfig/iptables, while on Debian it is /var/lib/iptables. The line you need to add is :
-A INPUT -p tcp -m tcp --dport 20000 -j ACCEPT
It should be added in the file just after similar lines which grant access to ports 80, 22 and so on.
Once this line has been added, you will need to apply the firewall configuration. This is typically done with the command /etc/init.d/iptables restart .