What is a port?

A port complements an IP address, such as 192.168.1.1:5000. The number after the colon is the port.

What are ports used for?

Ports are required , in order to be able to specifically assign the data sent to an IP address to an application or service.

What ports are there es?

Ports can range from 1 to 65535, where standard ports (0 to 1023) and sometimes also user ports (from 1024 to 49151) are mandatory.

1. Ports belong to the IP address

We have already explained in detail what an IP address is in a separate article, so here it is At this point just a brief summary: So that data can be transmitted between two end devices on the Internet at all, every end device directly connected to the Internet needs a public IP address.

It doesn’t matter whether the device is a router, PC or smartphone. The data is sent from the source computer to the target computer via the Internet.

Non-public IP addresses: PCs or other end devices such as network printers, game consoles, tablets or smartphones that are connected to the Internet via a router use the internal Data traffic non-public, private IP addresses from the address ranges 10.0.0.0 to 10.255.255.255, 172.16.0.0 to 172.31.255.255 or 192.168.0.0 to 192.168.255.255. Only the router receives a public IP address from the Internet Service Provider and assumes the role of a gateway in the local network and thus forwards data from the local network to the Internet and vice versa. This technique is called Network Address Translation, or NAT for short.

On the other hand, not every user is familiar with the term “port” and little is known about the purpose. Ports play a central role here, together with the IP address for Internet traffic.

1.1. An example of the need for ports

Imagine the following typical scenario. Music runs on the PC via the app from a streaming service such as Spotify. The Firefox web browser has several pages open, loads an ISO image from another server via FTP and you are using the Personal Information Manager (PIM) Microsoft Outlook to retrieve emails from an IMAP server. If the data for this PC were only forwarded via the IP address, the system would be overwhelmed and could no longer assign the data packets to the individual applications.

Conversely, when you access a website on the Internet with a web browser, you are basically just connecting to a different IP (destination server). In addition to a web server for websites, this server also runs other services such as a mail server, FTP server, etc.. How is the target system supposed to know which of these services your request is intended for?

The solution to both problems is: ports.

2. This requires ports

With the help of ports, data packets are supplemented by a network port via the Internet protocol and they can be assigned to a specific application, the actual destination (app, program, service) of the requested data.

Based on our example, this means that the web browser calls up the private IP 192.168.1.47 via the public IP of the router with the random port 59840 to the destination server 162.125.19.131 on port 443 (https) sends. The web server on the target server “listens” on port 443 (i.e. the port is open) and can therefore correctly accept the request for the website. The data is sent back to the requesting system and its public IP (router) on port 59840, which means that the target system can then assign the data to the web browser.

Good to know: If the web browser or another program/service establishes multiple connections, additional ports are simply opened.

You can easily display the behavior from the command line with the netstat -a command.

You can see all active connections, the protocol used, the local address with port, the remote address with port and the corresponding status.

The Netstat command has other parameters, which we have described in the tip Netstat commands for analyzing ports and network connections.

3. Standard ports, user ports and dynamic ports

Ports are divided into different categories and made mandatory for specific services by Internet Assigned Numbers Authority (IANA). The ports are divided into three groups.

3.1. Default Ports

Standard ports (0 to 1023), which are also called system ports or well-known ports, bind ports to certain services such as websites (HHTP Port 80, HTTPS on port 443) or IMAP on port 143 or 993 (encrypted IMAP).

Unencrypted emails are processed via port 110 (POP3) and port 25 (SMTP). Standard ports are among the important ports.

137 as a NetBIOS name service is mainly found in Windows networks and is used to resolve NetBIOS names.

Well-known standard ports include:

port
protocol
service

20
TCP
FTP data transfer

21
TCP/UDP
FTP connection establishment and control

22
TCP/UDP
Secure Shell (SSH)

23
TCP
Telnet

25
TCP/UDP
Simple Mail Transfer Protocol (SMTP)

80
TCP
Hypertext Transfer Protocol (HTTP)

110
TCP

Post Office Protocol v3 (POP3)

115
TCP
Simple File Transfer Protocol (SFTP)

123
UDP
Network Time Protocol (NTP)

137
TCP/UDP
NetBIOS name service

143
TCP/UDP

Internet Message Access Protocol (IMAP)

161
UDP
Simple Network Management Protocol (SNMP)

194
TCP/UDP
Internet Relay Chat (IRC)

443
TCP
HTTPS (Hypertext Transfer Protocol over SSL/TLS)

465
TCP
SMTP over SSL

587
TCP
SMTP

993
TCP
Internet Message Access Protocol over TLS/SSL (IMAPS)

995
TCP
Post Office Protocol 3 over TLS/SSL (POP3S)

3.2. User Ports

The second group are User Ports or Registered Ports and cover the range from 1024 to 49151. There are also numerous port numbers for user ports, which are reserved for certain services or applications (including many for the management of networks), for example:

3.3. Dynamic Ports

The third group are dynamic ports from 49152 to 65535 and are assigned dynamically by the operating system. Windows uses these ports to connect to a web server, for example.

4. Show open ports

You can use the Netstat command in the Windows command promptto display all active Windows connections, but you cannot see which program or service is responsible for this.

Netstat only shows you which program or service is responsible for the connection with the parameter –b. The parameter requires administrator rights.

The Netstat command has other parameters, which we have described in the tip Netstat commands for analyzing ports and network connections.

In the Netstat list, you will see the status as “established, waiting” or “listening”.

Established tells you that an application has established a connection to a server service, such as the web browser or mail client.

Waiting, on the other hand, are services that are waiting for a server connection, but this does not yet exist or is not active.

The “Listen” status indicates open ports. These are server services on your system that accept requests from outside.

5. Danger from open ports

Open ports are always dangerous if you don’t know the source. Many Trojans or other types of malware wait for “commands from outside” and therefore listen primarily to dynamic ports.

In order to be able to analyze open ports better, free programs such as PortExpert or CurrPorts are available. Start one of these programs and wait for the analysis. You will then see all ports on your system and can also identify open ports.

To do this, click on “Show Local Connections” in PortExpert. For each entry, you can see in the lower area of ​​the program window whether it is an open port that is “listened to” and is basically waiting for “commands from outside”. Whether the process is desirable or not depends on the process. In our case, a local FTP server is running, so this behavior is desired.

See our tip Test ports: How to check if your ports are open for more ways to scan your network for open ports.

6. Protection against open ports

A firewall on the PC as well as the router protect against open ports to the outside. For security reasons, you should deactivate the function “UPnP” in your router. This enables client PCs to release server ports on the router without the user having to intervene. You can find out how to deactivate the function in the manual or on the manufacturer’s website for the router.

7. How to open ports

If you have to release ports under Windows or on the router, you will find help in our tip Release ports: This is how port release works on the firewall and on the router .

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