A port is an extension of an address, similar to adding an apartment or room number to a street address. A letter with a street address will arrive at the correct apartment building, but without the apartment number, it will not be delivered to the correct recipient. Ports work in much the same way. A packet can be delivered to the correct IP address, but without the associated port, there is no way to determine which application should act on the packet.
Once the ports have been defined, it is possible for the different types of information that are sent to one IP address to then be sent to the appropriate applications. By using ports, a service running on a remote computer can determine what type of information a local client is requesting, can determine the protocol needed to send that information, and maintain simultaneous communication with a number of different clients.
For example, if a local computer attempts to connect to the website www.example.com whose IP address is 192.168.1.10, with a web server running on port 80, the local computer would connect to the remote computer using the socket address : 192.168.1.10:80
In order to maintain a level of standardization among the most commonly used ports, IANA (Internet Assigned Numbers Authority) has established that the ports numbered from 0 to 1024 are to be used for common services. The remaining ports up through 65535 are used for dynamic allocations or particular services.
The most commonly used ports as assigned by the IANA (Internet Assigned Numbers Authority) are listed here:
|5||rje||Remote Job Entry|
|15||netstat||Who is Up or NETSTAT|
|17||qotd||Quote of the Day|
|20||ftp-data||File Transfer [Default Data]|
|21||ftp||File Transfer [Control]|
|22||ssh||SSH Remote Login Protocol|
|25||smtp||Simple Mail Transfer Protocol|
|39||rlp||Resource Location Protocol|
|42||nameserver||Host Name Server|
|53||domain||Domain Name Server|
|67||bootps||Bootstrap Protocol Server|
|68||bootpc||Bootstrap Protocol Client|
|69||tftp||Trivial File Transfer|
|75||any private dial out service|
|77||any private RJE service|
|80||www-http||World Wide Web HTTP|
|101||hostname||NIC Host Name Server|
|102||iso-tsap||ISO-TSAP Class 0|
|110||pop3||Post Office Protocol – Version 3|
|117||uucp-path||UUCP Path Service|
|119||nntp||Network News Transfer Protocol|
|123||ntp||Network Time Protocol|
|137||netbios-ns||NETBIOS Name Service|
|138||netbios-dgm||NETBIOS Datagram Service|
|139||netbios-ssn||NETBIOS Session Service|
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