Do Ihave to open a range of ports 5800-5901 for realVNC to work or is there any specific port number. And how can I do it
thanks

Taken straight from the RealVNC website:

How do I use VNC through my firewall?

Many organisations operate firewalls to reduce the risk of intrusion by malicious attackers via the Internet. These firewalls typically operate by only allowing connections in to machines in that organisation on specific ports. Which ports are permitted access depends upon the network protocol that uses the port and the degree of security it provides. VNC servers can accept incoming connections through firewalls in two main ways. Although the first is usually the simplest to arrange, we recommend using the SSH tunnelling method wherever VNC is to be used over an untrusted network such as the Internet.

Opening Ports - The simplest way to allow VNC connections in through your firewall is to configure your firewalling software to allow connections to the VNC ports. If N is the display number of a particular VNC server then it will accept connections on port 5900+N. Configuring your firewall to allow connections to this port will allow VNC to work. If you wish to use the in-built web server and Java VNC Viewer then you will also need to allow connections to port 5800+N. Unfortunately, because VNC traffic is not encrypted, this approach weakens the security provided by your firewall, and so is not advisable.

You can use any single port for VNC. The default on Windows is 5900 or 5800.