Answer to Question 1
A POP3 server provides basic mailbox services. A client needs a service such as a POP3 server because clients, in general, are not always connected to the Internet or to the network that provides their e-mail. Because clients are not always connected, they are not always available to receive incoming e-mail from an SMTP server. This is where POP3 comes in. A POP3 server is, or at least should be, always connected to the network so that it is always available to receive e-mail from an SMTP server. The POP3 server can temporarily store the client's e-mail in a mailbox so that the client can eventually access and download his or her mail. POP3 uses well-known port 110.
Answer to Question 2
First, a protocol is specified. In the example address, the protocol is HTTP. For an FTP server, the protocol is ftp://. Second, either the server's IP address or the server's Domain Name System (DNS) name is provided. Third, a port number may be provided. If the port number is the standard logical port for the requested service, then the number does not have to be provided. For example, an HTTP request using standard logical port 80 would not have to include 80 in the request URL. Finally, the directory and/or file being requested needs to be specified. Thus, a URL has the following format: protocol://server-identifier:port-identifier/directory/file.htm.
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