Software (www.practicallaw.com/A36970) which prevents unauthorised access to intranets (www.practicallaw.com/A35512) and extranets (www.practicallaw.com/A36067) by guarding the gateway between a company’s private network and the public internet (www.practicallaw.com/A36314). A firewall scrutinises each network packet to determine whether it should be forwarded to the private network or denied access.