firewall 
Software that prevents unauthorized users from logging in to a private network (usually one that's connected to the public Internet). It can also be used to keep users inside the firewall from accessing computers outside the firewall. A firewall could be a dedicated computer equipped with security measures such as a dial-back feature, or it could be software-based protection or a combination of both. The firewall screens incoming server requests to make sure they come from authorized sources. For instance, a company might authorize outside users from certain IP addresses to access its intranet.