The IP address which the server connects from is borrowed temporarily from the Internet service provider s pool of available IP addresses. If you have a dial-up connection, however, you can have either static or dynamic IP, depending on your agreement with your Internet service provider. Static IP is more expensive, but also offers more options like setting up a Web server or Internet mail server on the company s local network. This is not possible using dynamic IP, because the routers on the Internet would not know where to deliver traffic for these servers, because their IP addresses are not fixed and consequently cannot be entered in the Internet s routing tables. In order to ensure that only authorised users get access to a network, e.g. to retrieve mail from an account on an Internet service provider s mail server, some form of authentication is normally used.
Authentication is the process of verifying the identity of an individual.
Prompting for usernames and passwords is an example of a basic authentication tool. This form of authentication goes by the name of PAP , Password Authentication Protocol. Internet service providers who rely on this kind of authentication provide their customers with a username and password together with the confirmation of their account. CHAP not only requires the client to authenticate itself when access is requested, but sends challenges at regular intervals to make sure the client hasn't been replaced by an intruder. In some cases, the number which the client dials from is used as authentication instead of username and password.