Scanning Your System for Open Ports

Before we talk about scanning your system for open ports, I think we would benefit from a brief overview of ports in general. A port is an access point on an IP address that an application can use for communicating with an application on another network device. An analogy for a port is a particular unit number in an apartment building. The address of the apartment building is like the IP address, and each unit in it has its own "port number."

There are two types of ports: TCP and UDP. TCP stands for "Transmission Control Protocol." Under TCP, when computers want to exchange information a link is established from one computer to the other, and that link remains open for the duration of the exchange. Once the exchange is finished, the link is then disconnected.

UDP stands for "User Datagram Protocol." Under UDP, when computers want to exchange information, the sending computer wraps the data into a package, and the package is dropped onto the network, addressed to the receiving computer. Devices in between the sender and receiver take on the task of relaying the package to its ultimate destination.

Each type of port is numbered from 0 through 65535 (that would make for quite a large apartment building!). You can see what ports your system has open by using the netstat command-line utility. The netstat command accepts several switches, but I typically specify "-aon". These switches cause the utility to display all connections and listening ports in numerical form, including the owning process ID associated with each connection. The following shows the partial output of executing the command "netstat -aon":

Proto  Local Address       Foreign Address      State           PID
TCP    0.0.0.0:4242        0.0.0.0:0            LISTENING       1896
TCP    0.0.0.0:5357        0.0.0.0:0            LISTENING       4
TCP    0.0.0.0:5500        0.0.0.0:0            LISTENING       5920
TCP    0.0.0.0:9090        0.0.0.0:0            LISTENING       1372
TCP    0.0.0.0:9998        0.0.0.0:0            LISTENING       4460
TCP    0.0.0.0:11456       0.0.0.0:0            LISTENING       1612
TCP    0.0.0.0:17500       0.0.0.0:0            LISTENING       2540
TCP    0.0.0.0:30101       0.0.0.0:0            LISTENING       4460
TCP    0.0.0.0:47232       0.0.0.0:0            LISTENING       2940
TCP    0.0.0.0:47233       0.0.0.0:0            LISTENING       4
TCP    0.0.0.0:49732       0.0.0.0:0            LISTENING       4
TCP    0.0.0.0:49735       0.0.0.0:0            LISTENING       4
TCP    127.0.0.1:1051      127.0.0.1:5354       ESTABLISHED     1824
TCP    127.0.0.1:1052      127.0.0.1:5354       ESTABLISHED     1824
UDP    0.0.0.0:3702        *:*                                  2180
UDP    0.0.0.0:4500        *:*                                  1120
UDP    0.0.0.0:5355        *:*                                  1500
UDP    0.0.0.0:17500       *:*                                  2540
UDP    0.0.0.0:53301       *:*                                  1704
UDP    0.0.0.0:60429       *:*                                  1096
UDP    0.0.0.0:60431       *:*                                  1096
UDP    0.0.0.0:60959       *:*                                  2180
UDP    0.0.0.0:64927       *:*                                  1612
UDP    127.0.0.1:1900      *:*                                  2180
UDP    127.0.0.1:49307     *:*                                  5784
UDP    127.0.0.1:49308     *:*                                  5784
UDP    127.0.0.1:52417     *:*                                  5684
UDP    127.0.0.1:53299     *:*                                  1824
UDP    127.0.0.1:53300     *:*                                  1824
UDP    127.0.0.1:57665     *:*                                  804
UDP    127.0.0.1:58065     *:*                                  1120
UDP    127.0.0.1:60428     *:*                                  2180
UDP    127.0.0.1:60977     *:*                                  1500

The first column indicates whether the port is using TCP or UDP. The second column gives the IP:port address on the local machine. The third column gives the IP:port address on the remote machine. The last two columns indicate the state of the connection and the process ID of the process using the port.

 This tip (9986) applies to Windows 7, 8, and 10.