Some firewalls will not permit applications such as Skype (or any applications you have not specifically allowed) to access the internet. Generally, the first time Skype or another application tries to access the internet, you will be asked if you would like to allow that application access. If you accidentally clicked No on that message (or if your firewall never asked for permission to allow Skype access), you will not be able to use Skype until you configure your firewall to allow Skype to access the internet.
With most firewalls, this is easy to do. Keep in mind that all firewalls are a bit different, but the process is usually as follows:
For comprehensive instructions on using your firewall software, refer to the software developer’s website.
If you are using Windows Firewall and have just reinstalled or upgraded Skype, these tips show you how to update your Windows Firewall settings.
Looking for information on which ports need to be open to use Skype?
Learn more about Skype security.