Using a port number, we can recognize the exact process by which an Internet or other network message will be forwarded when it arrives at a server. A host in a network can receive traffic from different hosts at the same time and send traffic to different hosts. So, how does the system recognize which data belongs to which application? TCP and UDP solve this problem using port numbers in their headers.
Port numbers range from 0 to 65535, but port numbers 0 to 1023 are reserved for privileged services and designated as well-known port numbers. TCP and UDP on the receiving host know which application to send the data to based on the port numbers received in the headers.
Each TCP or UDP session on the source host assigns a random port number above the range of 1024. So that returning traffic from the destination can be identified as belonging to the originating application. A combination of the IP address, Protocol (TCP or UDP) and the Port number forms a socket at both the sending and receiving hosts. Because each socket is unique, an application can send and receive data to and from multiple hosts.
The figure above illustrates two hosts communicating with the third host using TCP. Hosts A and C send traffic to host B at the same time. Hosts A and C send traffic destined for Port 80 but from different source ports. Host B can handle both connections simultaneously because of the combination of IP address, port numbers, and Protocols that make each connection different.
Well-known Port Numbers
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The File Transfer Protocol (FTP) provides a framework to transfer files between networked computers, mostly like Hypertext Transfer Protocol (HTTP) does through a web browser. The FTP server provides the ability to easily transfer files from one system to another. With little networking knowledge, anybody can set up the FTP server. FTP control handled on TCP port 21 and its data transfer can use TCP port 20 as well as dynamic ports depending on the specific configuration. FTP ports 20 and 21 must both be open on the network for successful file transfers.
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Port 22 has reserved for SSH connection. Using SSH we can manage network device remotely. Typically we used SSH as a secure substitute to Telnet which does not support secure connections. TCP, UDP and SCTP protocol can use port 22.
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Port 23 has reserved for Telnet connection. Telnet is one of the oldest Internet protocols and the most popular program for remote access. It is the primary method used to manage network devices remotely. Like SSH it does not provide a secure connection, it simply provides a basic unsecured connection. Many lower level network devices support Telnet and not SSH as it required some additional processing. Caution is important when connecting to a device using Telnet over a public network as the login credentials will be transmitted in the clear.
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SMTP has two primary functions, transfer mail (email) from source to destination between mail servers and end users email to a mail system. Usually an outgoing server uses port 25, the default SMTP port. Port 587 and port 465 is also used for SMTP to avoid the blockage.
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Port 53 is reserved for DNS services. The DNS translate domain names into IP addresses; typically it is used for network routing. It converts the alphabetic names into numeric IP addresses. For example, when a Web address (URL) is typed into a browser, DNS servers return the IP address of the Web server associated with that name.
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UDP port number 67 is the destination port of a server, and UDP port number 68 is used by the client. DHCP service uses UDP and required two ports for both server and clients end. It is assign IP address automatically to the client. A DHCP server can be set up by an administrator or engineer with a pool of IP addresses that are available. When a client device is turned on it will send a request for assigning an IP address to the local DHCP server, the local server then assigns an IP address to a client device. This assignment is not on a permanent basis, all IP addresses are assigned . If an address renewal is not requested and the lease expires the address will be put back into the poll for assignment.
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TFTP uses UDP port 69 to establish network connections. TFTP offers a method of file transfer without the session establishment. It is used where user authentication and directory visibility are not required. TFTP is used by devices to upgrade software and firmware, this includes Cisco and other network vendors’ equipment.
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HTTP is the most commonly used protocols over the Internet and private networks. It is the main protocol for all web browsers and is thus used by any client that uses files on these servers. Port number 80 assigned to commonly used internet communication protocol, HTTP. It is the port from which a computer listen and sends Web client-based communication and messages from a Web server and is used to send and receive HTML pages or data. A port can be specified in the range from 0-65535. However, the server administrator configures the server so that only one port number can be recognized. By default, the port number for a Web server is 80 and Port 8080 is also used for HTTP services.
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Port 110 – this is the default POP3 port and port 109 is the default port for POP1 and POP2. POP is application layer standard protocol defines an email server (the POP server) and a way for local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection. Port 995 is also used for POP when need to use if someone want to connect using POP3 securely.
