Skype is a software application that allows users to make voice calls over the Internet. Calls to other users within the Skype service are free, while calls to both traditional landline telephones and mobile phones can be made for a fee using a debit-based user account system. Skype has also become popular for its additional features which include instant messaging, file transfer, and video conferencing. The network is operated by a company called Skype Limited, headquartered in Luxembourg and partly owned by eBay.

Unlike other VoIP services, the Skype company does not run servers, but makes use of background processing on computers running Skype software-the original name proposed, Sky peer-to-peer (see below) reflects this.

This SonicWALL signature identifies one of several modes of network communications from Skype, but not all. In order to block Skype, you must all signatures in the application, as well as SIDs 5, and 7.