To make a NetMeeting call you need the IP address of the computer you are calling. Your IP address changes every time you log onto the Net, unless you are on a network or using something like a cable modem. Since most people don't know what their IP address is at any given time, this can cause some problems. Most Internet telephony applications have developed a way of matching people to an IP address to solve this problem. In the case of NetMeeting, it uses directory servers.
Directory servers are in effect, big phone books.
When you first get online and log into a directory server, it looks up the email address you have entered into your NetMeeting information page, and it adds that address to the Directory Server. Then, the NetMeeting software looks at your IP address and secretly attaches that to your email address. When that happens, anyone can place a NetMeeting call to you using your email address, as long as they know which directory you're listed on. The NetMeeting program then goes to that directory, looks up the email address, gets the corresponding IP address and completes the call. It's like using 411.