Common Network Ports

 

Jerry Grugin

 

Overview:

When it comes to the security plus examination, it is a good idea to memorize the following information about ports and how information is transferred over these ports.  There is no getting around it.  You will need to know these things.  Just go ahead and memorize these things.

In computer networking, a port is an application-specific or process-specific software construct serving as a communications endpoint in a computer's host operating system. A port is associated with an IP address of the host, as well as the type of protocol used for communication. The purpose of ports is to uniquely identify different applications or processes running on a single computer and thereby enable them to share a single physical connection to a packet-switched network like the Internet.

The protocols that primarily use ports are the Transport Layer protocols, such as the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) of the Internet Protocol Suite. A port is identified for each address and protocol by a 16-bit number, commonly known as the port number. The port number, added to a computer's IP address, completes the destination address for a communications session. That is, data packets are routed across the network to a specific destination IP address, and then, upon reaching the destination computer, are further routed to the specific process bound to the destination port number.

Note that it is the combination of IP address and port number together that must be globally unique. Thus, different IP addresses or protocols may use the same port number for communication; e.g., on a given host or interface UDP and TCP may use the same port number, or on a host with two interfaces, both addresses may be associated with a port having the same number.

Of the thousands of enumerated ports, about 250 well-known ports are reserved by convention to identify specific service types on a host. In the client-server model of application architecture, ports are used to provide a multiplexing service on each server-side port number that network clients connect to for service initiation, after which communication can be reestablished on other connection-specific port numbers.

Examples

An example for the use of ports is the Internet mail system. 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). The SMTP service application usually 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 e-mail client applications on user's 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.

SPECIFIC PORT NUMBERS (MEMORIZE THIS)

FTP -   FILE TRANSFER PROTOCOL -     tcp/21(control)-           tcp/20(data in active mode)

SSH -  SECURE SHELL -    tcp/22

SCP -   SECURE COPY -      tcp/22

SFTP - SECURE FILE TRANSFER PROTOCOL -            tcp/22

TELNET-        tcp/23

HTTP- HYPERTEXT TRANSFER PROTOCOL-   tcp/90

HTTPS- HYPERTEXT TRANSFER PROTOCOL SECURE-        tcp/443

FTP OVER SSL-        tcp/990(control)          tcp/989(data in active mode)

NetBIOS- NETWORK BASIC INPUT/OUTPUT SYSTEM-         tcp/139(session service)

TFTP-  TRIVIAL FILE TRANSFER PROTOCOL-            udp/69