R18. From a user's perspective, what is the difference between the download-and-delete mode and the download-and-keep mode in POP3? R19. Is it possible for an organization's Web server and mail server to have exactly the same alias for a hostname (for example, foo, com)? What would be the type for the RR that contains the hostname of the mail server? R20. Look over your received e-mails, and examine the header of a message sent from a user with a .edu e-mail address. Is it possible to determine from the header the IP address of the host from which the message was sent? Do the same for a message sent from a Gmail account. R26. In Section 2.7, the UDP server described needed only one socket, whereas the TCP server needed two sockets. Why? If the TCP server were to support n simultaneous connections, each from a different client host, how many sockets would the TCP server need