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What is a server port? 

When we refer to the server port, we usually refer to the virtual port defined in the TCP/IP protocol, rather than the physical port connected to other network devices on the backplane of the computer. The TCP/IP protocol, the most basic communication protocol in network services, defines port numbers from 0 to 65535 in order to identify different applications. The transport layer protocol in the TCP/IP can decide which application to receive data based on different port numbers. If the IP address is compared to the house number, the port is the room number of each room in the building.

Ports can be divided into 3 categories:

1) Well Known Ports: From 0 to 1023, they are tightly bound to some services. Usually communication on these ports clearly indicates the protocol of a service. For example: port 80 is actually always HTTP traffic.

2) Registered Ports: from 1024 to 49151. They are loosely bound to some services. That is, there are many services bound to these ports, and these ports are also used for many other purposes. For example: many systems handle dynamic ports starting around 1024.

3) Dynamic and/or Private Ports: from 49152 to 65535. In theory, these ports should not be assigned to services. In practice, machines usually assign dynamic ports from 1024 onwards.

List of Server Ports 

Some of the most important and commonly used ports and corresponding services are listed below:

Port 7 Echo, Port 20/21 FTP, Port 22 SSH, Port 23 Telnet, Port 25 SNMP, Port 53 DNS, Port 80 HTTP, Port 110 POP3, Port 143 Imap 4, Port 464 Kerberos, Port 636 LDAP, Port 1194 OpenVPN

Port 7: Echo

Echo is a common command in the shell, usually used to output a string. This command can also be used to test the port on the server.

Port 20 21: FTP

FTP(File transfer protocol) is a standard communication protocol, used to download, upload, and transfer files from a server to a client on a computer network. FTP enables the transfer of files back and forth between computers or through the cloud. FTP uses a client-server model architecture, with separate connections for control commands and data transfer. Port 20 is used by FTP for data transfer, while port 21 is for control connection.

Long before operating systems had a graphical user interface, FTP client applications were widely available in the form of command-line programs. To this day, most operating systems still ship with an ftp client by default. Over the past few decades, many dedicated FTP clients and automation utilities have been developed for desktops, servers, mobile devices, and hardware, and are widely used on the Internet and other networks.

Port 22: SSH(Secure Shell), SCP(Secure Copy) and SFTP(Secure File Transfer Protocol)

SSH is developed by the Network Working Group of the IETF; SSH is a security protocol based on the application layer. SSH is a widely used secure login protocol, a protocol designed to provide security for remote login sessions and other network services, replacing the previously insecure Telnet protocol. Since the data packets transmitted over the network are encrypted according to the SSH protocol, the risk of information leakage is effectively reduced.

SSH was originally created in the form of a command-line program on UNIX systems, and then quickly became widely used, launching corresponding applications on other operating platforms. At present, almost all UNIX platforms—including HP-UX, Linux, AIX, Solaris, Digital UNIX, Irix, and other system platforms can run SSH .

SCP is a protocol used for data transmission with remote hosts, which is equivalent to the encrypted Copy command. In terms of data transmission protocol and authentication method, SCP is consistent with SSH.

SFTP is an FTP protocol encapsulated by SSH, which is equivalent to an encrypted FTP protocol. Its function is the same as that of FTP, except that the transmitted data is encrypted.

Port 23: Telnet

Telnet is an application layer protocol belonging to one of the TCP/IP protocol families. It provides a two-way, text string-based command line interface interaction function in the form of a virtual terminal. Telnet was once widely used for remote control of servers, allowing users to manipulate remote hosts from their local machines. However, due to its poor security of transmitting messages in plaintext, it has been gradually replaced by SSH.

Port 25: SMTP

SMTP is a protocol that provides reliable and efficient email transmission. SMTP is a mail service built on the FTP file transfer service. It is mainly used for mail information transfer between systems and provides notifications about incoming letters. But SMTP is usually used to send email messages, not to receive them.

Port 53: DNS(Domain Name System)

The Domain Name System is a service of the Internet that translates human-readable domain names (eg, www.xxx.com) into machine-readable IP addresses (eg, 192.0.2.44). All computers on the Internet, from smartphones or laptops to servers that serve up a large amount of retail website content, use numbers to find and communicate with each other. These numbers are called IP addresses. However, we don’t have to remember and type such numbers to visit a website. Instead, users enter a domain name(such as example.com), which will then look up through DNS and return the corresponding IP address.

Port 80: HTTP(Hypertext Transfer Protocol)

HTTP is a simple request-response protocol, which usually runs on top of TCP and belongs to the application layer protocol. HTTP was originally designed to provide a way to publish and receive HTML pages.

HTTP has the following characteristics:

Simple and fast: When a client requests a service from the server, it only needs to transmit the request method and path, so the communication speed is very fast.

Flexible: HTTP allows the transfer of data objects of any type.

Connectionless: Only one request is processed per HTTP connection. After the server processes the client’s request and receives the client’s response, it disconnects. In this way, transmission time can be saved.

Stateless: The HTTP protocol is a stateless protocol, which means that the server does not care about the context of each connection, which simplifies transaction processing and makes the server respond faster.

Because of these excellent features, HTTP has become the basis of data communication in the World Wide Web for decades, and almost any website is accessed through the HTTP protocol.

Port 110: POP3(Post Office Protocol, version 3)

POP3 is primarily used to support remote management of e-mail on a server using a client. POP3 supports offline mail processing and is the first offline protocol standard for Internet e-mail. It allows customers to store mail from the server to the local host (i.e. their own computer), and delete the mail stored on the mail server at the same time.

Port 143: IMAP4    (Internet Message Access Protocol)

Similar to POP, IMAP is a mail acquisition protocol, which can obtain mail information, download mail from the mail server. Different from POP, the operations of the email client will be fed back to the server, and the operations you perform on the mail (such as: moving the mail, marking it as read, etc.), the mail on the server will also take corresponding actions. Also, IMAP can download only the subject of the message, and then download the entire content of the message if necessary.

Port 464: Kerberos

Kerberos is a computer network authentication protocol that allows an entity to communicate in a non-secure network environment to prove its identity to another entity in a secure manner. MIT implemented and released a suite of free software of the same name under this agreement. It is designed primarily for the client-server model and provides a range of interactive authentication – both the user and the server can verify each other’s identities. The Kerberos protocol can protect network entities from eavesdropping and repeated attacks. Many computer systems use Kerberos-based authentication.

Port 636: LDAP (Lightweight Directory Access Protocol)

LDAP is an open, neutral, industry-standard application protocol that provides access control and maintains directory information for distributed information through the IP protocol. It was born in 1988, even before the birth of the World Wide Web. LDAP adopts a tree-shaped user directory and a tree-shaped storage structure, which is intuitive for organizational management modeling, and is very fast in querying, so it has been widely used and supported in the following decades.

Port 1194: OpenVPN

VPN stands for “Virtual Private Network”. VPN allows users to connect to a virtual private network, and then connect to the public network to send or receive data through this network. This brings more features, better security, and is often used to manage resources which are not  accessed through the public network.  Encryption is common but not an inherent part of VPN connections.

OpenVPN is a long-established VPN protocol, first released in 2001, and has become one of the most popular and widely used protocols. It is an open source protocol, which means that programmers can modify the protocol as well as scrutinize the source code for possible vulnerabilities. OpenVPN uses SSL technology and is available on almost all platforms.