Common IP ports

There are common port numbers that are used by computers. First 1024 ports are reserved for common use and these are standard (count starts with 0). The rest of the ports can be used for other applications. The port numbers are not limitless, the upper limit for them is 65535.

These ports are also open for communication between computers and closed after the communication is over.

Ports that are over 1024 are not reserved, and can be used by other applications.

(I am not a network engineer, and don’t know what IPv6 implementation is.)

What are the common port numbers? Let’s list a few.

20,21 FTP

22 SSH

23 Telnet 

25 SMTP (to send emails)

53 DNS to resolve names to IP address and back

80 HTTP

110 POP3 protocol to receive emails

443 HTTPS (encrypted HTTP traffic)

995 encrypted POP3

YouTube video

Study Guide: Common Computer Port Numbers

Quiz:

What is the typical range of reserved port numbers, and what is their general purpose?

Explain the role of port numbers in computer communication using an analogy if it helps clarify your explanation.

What is the fundamental difference in terms of allocation between port numbers below 1024 and those above 1024?

Name three common applications and their associated well-known port numbers as listed in the source material.

Briefly describe the function of the DNS protocol and the port number it utilizes.

What is the primary difference between HTTP and HTTPS, and what port number does HTTPS use?

Explain the purpose of the SMTP protocol and the associated port number mentioned in the text.

Describe the functions of the POP3 and encrypted POP3 protocols, including their respective port numbers.

What is the significance of port numbers being opened and closed during communication?

What is the upper limit for the total number of available port numbers on a computer?

Answer Key:

The first 1024 ports (0-1023) are reserved for common or well-known services and applications. These are standard ports used for fundamental internet protocols.

Port numbers are like apartment numbers in a large building (the computer's IP address). They specify which specific application or service within the computer should receive the incoming network traffic.

Port numbers below 1024 are reserved for standard, well-known applications, while port numbers above 1024 are not reserved and can be used by other applications.

FTP uses ports 20 and 21, SSH uses port 22, and HTTP uses port 80. (Any three from the list are acceptable).

DNS (Domain Name System) resolves human-readable domain names (like google.com) into numerical IP addresses (like 192.168.1.1) and vice versa. It uses port 53.

HTTP (Hypertext Transfer Protocol) is the standard protocol for transferring files (like web pages) on the internet. HTTPS (Hypertext Transfer Protocol Secure) is the encrypted version of HTTP and uses port 443 for secure communication.

SMTP (Simple Mail Transfer Protocol) is used for sending emails and utilizes port 25.

POP3 (Post Office Protocol version 3) is a protocol used to receive emails and typically uses port 110. Encrypted POP3 provides a secure way to receive emails and uses port 995.

Opening a port allows a specific application to listen for and receive network communication. Closing the port after communication ends enhances security by preventing unauthorized access.

The upper limit for the total number of available port numbers on a computer is 65535.

Essay Format Questions:

Discuss the importance of reserved port numbers in establishing and maintaining consistent network communication. Consider the implications if these standards did not exist.

Explain the relationship between IP addresses and port numbers in the context of network communication. How do these two concepts work together to ensure data reaches the correct destination and application?

Analyze the security implications of open and closed port numbers. What risks are associated with leaving unnecessary ports open, and how can these risks be mitigated?

Compare and contrast the functions and security considerations of HTTP and HTTPS. In what scenarios is the use of HTTPS particularly critical?

Based on the provided list, categorize the common port numbers by their general function (e.g., file transfer, email, name resolution, web access). Discuss the significance of having distinct ports for these different types of network traffic.

Glossary of Key Terms:

Port Number: A numerical identifier assigned to a specific process or application running on a computer. It allows multiple network services to run on the same computer.

Reserved Ports (Well-Known Ports): Port numbers ranging from 0 to 1023 that are typically assigned to common and standardized network protocols and services.

FTP (File Transfer Protocol): A standard network protocol used for transferring files between a client and a server on a computer network.

SSH (Secure Shell): A cryptographic network protocol for operating network services securely over an unsecured network.

Telnet: An application protocol used on the Internet or local area network to provide a bidirectional interactive text-oriented communication facility using a virtual terminal connection.

SMTP (Simple Mail Transfer Protocol): An internet standard for electronic mail (email) transmission.

DNS (Domain Name System): A hierarchical and decentralized naming system for computers, services, or other resources connected to the Internet or a private network. It translates domain names into IP addresses.

HTTP (Hypertext Transfer Protocol): An application-layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. It is the foundation of data communication for the World Wide Web.

POP3 (Post Office Protocol version 3): An application-layer Internet standard protocol used by local email clients to retrieve email from a remote server.

HTTPS (Hypertext Transfer Protocol Secure): A secure version of HTTP, where the communication is encrypted to protect the confidentiality and integrity of the data being transmitted.

Encryption: The process of encoding information in such a way that only authorized parties can understand it.

IP Address: A numerical label assigned to each device connected to a computer network that uses the Internet Protocol for communication. It identifies the host and its location on the network.

Frequently Asked Questions about Computer Ports

Q1: What are computer port numbers and what is their basic function?

Computer port numbers are virtual endpoints that facilitate communication between applications and services running on different computers (or even the same computer). Think of them as numbered doors on a building (the computer) that specific services use to send and receive data. When data arrives at a computer, the port number in the network header indicates which application or service should handle that data. This allows multiple applications to use the same network connection simultaneously without their data becoming mixed.

Q2: What is the significance of the port number range 0 to 1023?

The port numbers ranging from 0 to 1023 are known as "well-known ports" or "system ports." These ports are reserved for common and widely used network protocols and services. These assignments are typically standardized by the Internet Assigned Numbers Authority (IANA). Using these reserved ports ensures that clients connecting to standard services know which port to use. For example, web servers consistently listen on port 80 (HTTP) or 443 (HTTPS).

Q3: What is the range of available port numbers and how are the ports above 1023 different?

The total range of port numbers is from 0 to 65535. Ports above 1023, specifically from 1024 to 65535, are generally considered "ephemeral ports" or "dynamic ports" and "registered ports." Registered ports (1024-49151) can be registered with IANA to avoid conflicts, but are not as strictly controlled as well-known ports. Ephemeral or dynamic ports (49152-65535) are typically used by client applications when they initiate a connection to a server. These ports are usually assigned automatically by the operating system for the duration of the connection and are released when the connection closes.

Q4: How are computer ports used in communication between computers?

When two computers communicate over a network, the initiating computer (the client) will typically use a randomly assigned ephemeral port for its end of the connection and will connect to a specific well-known or registered port on the destination computer (the server) that is associated with the desired service. For instance, when you browse a website, your computer might use a port in the dynamic range to connect to the web server listening on port 80 or 443. Both the source and destination IP addresses and port numbers form a unique socket pair that identifies the specific connection.

Q5: What does it mean for a port to be "open" or "closed"?

A port is considered "open" if an application or service is actively listening for incoming connections on that specific port. This means that the operating system will accept network traffic directed to that port and forward it to the listening application. A "closed" port indicates that no application is currently listening on that port. If a computer receives traffic destined for a closed port, it will typically reject the connection attempt.

Q6: Why are some specific port numbers like 20, 21, 22, 23, 25, 53, 80, 110, 443, and 995 important?

These are examples of well-known ports that are associated with fundamental internet protocols and services:

20, 21 (FTP): File Transfer Protocol, used for transferring files between computers. Port 20 is typically used for data connections, and port 21 for control connections.

22 (SSH): Secure Shell, used for secure command-line access and other secure network services.

23 (Telnet): Telnet, an older, unencrypted protocol for remote command-line access (less commonly used today due to security concerns).

25 (SMTP): Simple Mail Transfer Protocol, used for sending outgoing email.

53 (DNS): Domain Name System, used for translating domain names (like "example.com") into IP addresses and vice versa.

80 (HTTP): Hypertext Transfer Protocol, the foundation of the World Wide Web, used for unencrypted web traffic.

110 (POP3): Post Office Protocol version 3, a protocol used by email clients to retrieve email from a mail server.

443 (HTTPS): HTTP Secure, the secure, encrypted version of HTTP, crucial for protecting sensitive data transmitted over the web.

995 (Encrypted POP3): POP3 over SSL/TLS, a secure, encrypted method for receiving emails.

Q7: Are the port numbers above 1024 always fixed for specific applications?

No, port numbers above 1024 are not permanently reserved in the same way as well-known ports. While some applications might commonly use certain registered ports, there's no strict guarantee. For example, a specific game server might default to a particular port above 1024, but it could potentially be configured to use a different one. Similarly, when a client application initiates a connection, the operating system dynamically assigns an ephemeral port from the available range.

Q8: Is there a limit to the number of available port numbers?

Yes, the total number of available port numbers is limited to the range of 0 to 65535. This means there are a finite number of virtual endpoints available for communication on a single IP address. While this number is generally sufficient for most use cases, high-demand servers handling a massive number of concurrent connections might need to manage their port usage efficiently.