For anything to communicate with Gatekeeper, it must know this secret, and know how to use it, to be able to negotiate a connection. Firewall A firewall is a barrier between your network and the Internet, through which only authorised traffic can pass. To create the rules by which your firewall allows and disallows traffic, simply select the different Internet services, IP addresses and hosts you wish to permit or deny. Most firewalls screen traffic between a company's internal network and the Internet, however firewalls can also secure on part of a network from another. For instance securing your corporate accounting department or your network from your subsidiary's network.
Many client applications exist to make the process easy.
Typically members of a group will share common characteristics, such as being in the same department of a company. If a rule applies to a group then the rule then applies to all members of that group. You might have 'Administrators' , 'Users' and 'Dial-in' groups, each allowing different access rights. Users can belong to one, several, all or no groups. This stores some info about where certain machines are.