Every system connected to the internet has a number of ports open on it. Ports are basically virtual doors that allow the inflow and outflow of data packets. Without the opening of ports, no data communication can take place on a particular system. Typically, each time a client established a new connection over the network, a randomly chosen port number gets opened. Similarly, each time a service is enabled on a server, it automatically opens a predefined port number and listens for any clients who might want to establish a connection, Typically, Port number are of three different types:
1. Well-known Port numbers.
2. Registered port numbers.
3. Dynamic/private port numbers.
Well-known ports are those that range in number from 0 to 1023. Each Port in this range usually has a specific service running on it. In fact, an internationally accepted port-number-to-services listing (Known as a request for comments or RFC 1700) fixes all service(like FTP, SMTP, POP and other) to their respective predefined port numbers.
It is quite possible for a system administrator to choose to run any service on a port other than a predefined port.
Registered ports are those that range in number from 1024 to 49151. Ports in this range are not bound to any specific services. In fact, networking utilities such as your browser, e-mail client, and FTP client open random ports within this range to initiate communication with a remote server. Port numbers within this range are what enable you to surf the net, check your e-mail, and the like. That’s why if you issue the netstat –a command and discover that a number of ports in this range are open, there’s probably nothing to worry about.
When we close the application, probably find that the port follows suit, closing automatically.
Dynamic or Private ports are those that range in number from 49152 to 65535. This range of port numbers is rarely used by normal applications. Typically, on Most occasions, port numbers in this range are used by malicious programs like Trojans, Keyloggers, or spyware tools. However, on certain occasions, even legitimate applications use ports in this high range.