of attempting to connect to a range of ports in sequence on a single host is commonly known as port scanning. This is usually associated either with malicious cracking attempts or with network administrators looking for possible vulnerabilities to help prevent such attacks. Port connection attempts are frequently monitored and logged by hosts. The technique of port knocking uses a series of port connections (knocks) from a client computer to enable a server connection. Examples An example of the use of ports is the delivery of email. A server used for sending and receiving email generally needs two services. The first service is used to transport email to and from other servers. This is accomplished with the Simple Mail Transfer Protocol (SMTP). A standard SMTP service application listens on TCP port 25 for incoming requests. The second service is usually either the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP) which is used by email client applications on users' personal computers to fetch email messages from the server. The POP service listens on TCP port number 110. Both services may be running on the same host computer, in which case the port number distinguishes the service that was requested by a remote computer, be it a user's computer or another mail server. While the listening port number of a server is well defined (IANA calls these the well-known ports), the client's port number is often chosen from the dynamic port range (see below). In some applications, the clients and the server each use specific port numbers assigned by the IANA. A good example of this is DHCP in which the client always uses UDP port 68 and the server always uses UDP port 67. Use in URLs Port numbers are sometimes seen in web or other uniform resource locators (URLs). By default, HTTP uses port 80 and HTTPS uses port 443, but a URL like http://www.example.com:8080/path/ specifies that the web browser connects instead to port 8080 of the HTTP server. History The concept of port numbers was established by the early developers of the ARPANET in informal cooperation of software authors and system administrators. The term port number was not yet in use. It was preceded by the use of the term socket number in the early development stages of the network. A socket number for a remote host was a 40-bit quantity. The first 32 bits