Dedicated hackers are constantly on the lookout for new systems to hack into and they sometimes do this by finding Internet ports (openings into a PC or network) that are open either because they are in use or simply have been left open for convenience by the user.

Port scanning and sniffing can provide hackers with information about computers connected to the Internet and their vulnerabilities to attack. Even if hackers do not gain access, port scanning is a method by which hackers can piggy-back off your Internet connection thereby increasing your Internet usage without your knowledge. If you are paying for your Internet use by volume, this can result in extra costs.

Sniffer software is also used by hackers to track data travelling over the Internet or a corporate network. Unauthorised sniffers can compromise a network's security because they are difficult to detect and can be inserted almost anywhere. In this way, hackers can capture unsecured data travelling over a network. This data could include sensitive information such as passwords or financial documents.

Use firewalls to help protect against unauthorised sniffers and to prevent successful port scanning.