To work correctly, Skype requires unrestricted outgoing TCP access to:
If your firewall restricts access to both of these, you will need to update your firewall settings before you can use Skype.
When you install Skype, a port above 1024 is chosen at random as the port for incoming connections. You can configure Skype to use a different port for incoming connections if you wish, but if you do, you must open the alternative port manually.
Learn how to manually open a port in Windows XP; Windows Vista and Windows 7; and Windows 8 (under the heading Open a port in Windows Firewall).
If the port chosen for incoming connections becomes unavailable, by default ports 80 and 443 will be used as alternatives. If another application (such as Apache HTTP server or IIS) uses these ports, you can either configure the application to use other ports, or you can configure Skype to not use these ports.
To configure the ports that Skype uses for incoming connections:
If another application (such as Apache HTTP Server or IIS) uses these ports, Skype may not function correctly. Either uncheck this box, or configure the other application to use different ports. To configure the other application, contact that application's developer.
Connecting to Skype through a proxy server? These tips can help.