Port Numbers | IT Exam Solutions

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Port Numbers

Explain how TCP and UDP use port numbers.

Multiple Separate Communications

TCP and UDP transport layer protocols use port numbers to manage multiple, simultaneous conversations.
The source port number is associated with the originating application on the local host whereas the destination port number is associated with the destination application on the remote host.

Socket Pairs

The source and destination ports are placed within the segment.
The segments are then encapsulated within an IP packet.
The combination of the source IP address and source port number, or the destination IP address and destination port number is known as a socket.
Sockets enable multiple processes, running on a client, to distinguish themselves from each other, and multiple connections to a server process to be distinguished from each other.

Port Number Groups

Port Group	Number Range	Description
Well-known Ports	0 to 1,023	These port numbers are reserved for common or popular services and applications such as web browsers, email clients, and remote access clients. Defined well-known ports for common server applications enables clients to easily identify the associated service required.
Registered Ports	1,024 to 49,151	These port numbers are assigned by IANA to a requesting entity to use with specific processes or applications. These processes are primarily individual applications that a user has chosen to install, rather than common applications that would receive a well-known port number. For example, Cisco has registered port 1812 for its RADIUS server authentication process.
Private and/or Dynamic Ports	49,152 to 65,535	These ports are also known as ephemeral ports. The client’s OS usually assign port numbers dynamically when a connection to a service is initiated. The dynamic port is then used to identify the client application during communication.

Well-know port number

Port Number	Protocol	Application
20	TCP	File Transfer Protocol (FTP) – Data
21	TCP	File Transfer Protocol (FTP) – Control
22	TCP	Secure Shell (SSH)
23	TCP	Telnet
25	TCP	Simple Mail Transfer Protocol (SMTP)
53	UDP, TCP	Domain Name Service (DNS)
67	UDP	Dynamic Host Configuration Protocol (DHCP) – Server
68	UDP	Dynamic Host Configuration Protocol – Client
69	UDP	Trivial File Transfer Protocol (TFTP)
80	TCP	Hypertext Transfer Protocol (HTTP)
110	TCP	Post Office Protocol version 3 (POP3)
143	TCP	Internet Message Access Protocol (IMAP)
161	UDP	Simple Network Management Protocol (SNMP)
443	TCP	Hypertext Transfer Protocol Secure (HTTPS)

The netstat Command

Unexplained TCP connections can pose a major security threat. Netstat is an important tool to verify connections.

Other related topics

Topic Title	Topic Objective
Transportation of Data	Explain the purpose of the transport layer in managing the transportation of data in end-to-end communication.
TCP Overview	Explain characteristics of TCP.
UDP Overview	Explain characteristics of UDP.
Port Numbers	Explain how TCP and UDP use port numbers.
TCP Communication Process	Explain how TCP session establishment and termination processes facilitate reliable communication.
Reliability and Flow Control	Explain how TCP protocol data units are transmitted and acknowledged to guarantee delivery.
UDP Communication	Compare the operations of transport layer protocols in supporting end-to-end communication.

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