What is a Port?

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In plain English, this simply means that a port is an endpoint through which data flows back and forth between two computers over a network. A computer has 65535 ports available to share information. These port numbers are based on a 16-bit number, which is where we derive the total number of available ports (0 to 65535).

These port numbers are assigned to specific services by IANA, which is responsible for maintaining the official port number designations. They designate port numbers based on three ranges:

System (or well known) Ports (0-1023)
User Ports (1024-49151)
Dynamic and Private Ports (49152-65535)

Common Ports and Associated Programs
As we can see, many of these ports are easily recognizable once the service is associated with the port number.

Port 20: FTP
Port 21: FTP
Port 22: SSH
Port 23: Telnet
Port 25: SMTP
Port 26: Common Alternate SMTP port
Port 37: cPanel’s time servers (tcp out)
Port 53: DNS/Named
Port 80: HTTP
Port 110: POP3
Port 123: NTP
Port 143: IMAP
Port 443: HTTPS (SSL)
Port 465: SMTPs (Secure SMTP)
Port 993: IMAPs (Secure IMAP)
Port 995: POP3s (Secure POP3)
Port 1167: Guardian (buagent)
Port 1433: Microsoft SQL Server
Port 2073: cPanel Razor
Port 2077: cPanel WebDAV
Port 2078: cPanel WebDAV Secure port
Port 2079: cPanel CalDAV and CardDAV
Port 2080: cPanel CalDAV and CardDAV Secure port
Port 2082: cPanel non-secure port
Port 2083: cPanel secure port
Port 2086: WHM non-secure port
Port 2087: WHM secure port
Port 2089: cp license
Port 2095: cPanel webmail non-secure port
Port 2096: cPanel webmail secure port
Port 2443: Nodeworx/SiteWorx
Port 3306: MySQL
Port 3389: Remote Desktop Protocol (rdp)
Port 4643: Virtuozzo Control Panel
Port 6660-6669: IRC
Port 8000: SHOUTcast
Port 8080: Tomcat
Port 8306: Plesk MySQL
Port 8443: Plesk
Port 49152:65534: Standard Passive FTP ports

TCP/UDP Protocol
The next step in the connection chain is the protocol used to connect to a service. A computer’s IP address is stored in a DNS record by an ISP (Internet Service Provider), contributing to the transfer of information.

When a computer requests to connect to another computer (or server), it uses the IP address listed in the DNS records to locate the other computer on the network. The service associated with the relevant port then uses one of these connection protocols to transmit the relevant information back to the requester. The open ports on the server listen for the unique communication requests on specific ports. The unique ports are associated with various software or services noted above.

For instance, when you open a browser and type in https://wesbytes.com, the https request is routed through an ISP or other internet provider, which then does a DNS lookup for that domain. Once the location of the IP is found, your https request is then routed to a server owned by Liquid Web. When the request hits the server, the service that is listening on port 443, in this case, Port 443 HTTPS (SSL), recognizes the request for a secure connection to the server and returns the webpage stored on the server securely.

Another example would be if you attempt to connect to an FTP server here at Liquid Web. The software (FileZilla for example) reaches out to the server’s IP address on port 21, the port the FTP service listens on. The request arrives, and if the port is open, the connection is allowed through the firewall. The FTP server then responds to the connection request, and if the user authentication is successful, your computer is now connected to the server via FTP.

Firewalls
After this, the firewall comes into play. The firewall monitors the traffic coming into and out of the server. The information stored in the firewall rules either allows or denies access based on the incoming TCP/UDP info and recognizes if the port is open or closed. Using the above FTP request as an example, if a firewall rule is set up to allow traffic to pass over port 21, the FTP request is forwarded to the FTP server where the connection is allowed. If port 21 is closed or blocked by a firewall rule, the request is denied and the connection will fail.

Services
Finally, if the request is allowed through the firewall, the service that listens to the specifically designated port receives the request and responds in kind. Granted, this is an oversimplification of the chain of events, but it demonstrates the overall process of how a request is related to a unique port. An IP address may have multiple services listening on any one of the ports listed above. When we connect to a server, we are connecting to an IP address and a port.

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