Windows 7 / Networking


For a network service to receive incoming communications, it must listen for communications on a specific TCP or UDP port. When troubleshooting network problems, you might want to view the ports on which your computer listens for incoming connections to verify that a service is properly configured and that the port number has not changed from the default.

Netstat (Netstat.exe) is a useful command-line tool for identifying network services and the ports they listen on. Listing the ports a computer listens on is useful for verifying that a network service is using the expected port. It is common practice to change the port numbers that services listen on, and Netstat can quickly identify nonstandard listening ports.

To view open ports and active incoming connections, open a command prompt and run the following command.

netstat -a -n -o

Netstat will display a list of listening ports as well as outgoing connections and the Process Identifiers (PIDs) associated with each listener or connection.

Notice that the line in bold is listening for incoming connections on TCP port 3389, which Remote Desktop uses. Because the Foreign Address column shows an IPv4 address, you can tell that a user is connected to the computer using Remote Desktop from a computer with the IP address of If you notice that a computer is listening for incoming connections on unexpected ports, you can use the value in the PID column to identify the process. Tools such as the Processes tab in Task Manager can reveal which process is associated with a PID.

Note To identify processes by PID in Task Manager, select the Processes tab. On the View menu, click Select Columns. Select the PID (Process Identifier) check box and then click OK.

Alternatively, if you can open a command prompt with elevated privileges, you can use the -b parameter to resolve applications associated with active connections. The following example demonstrates that using the -b parameter shows the associated process in brackets before each connection.

netstat -a -n -o -b

TCPView, a free download from Microsoft, provides similar functionality with a graphical interface. TCPView is described later in this tutorial.

[Previous] [Contents] [Next]

In this tutorial:

  1. Troubleshooting Network Issues
  2. Tools for Troubleshooting
  3. Table-1 Network Troubleshooting Tools
  4. Arp
  5. How to Identify a Problem with the ARP Cacher
  6. How to Clear the ARP Cache
  7. Event Viewer
  8. IPConfig
  9. Nblookup
  10. Nbtstat
  11. Net
  12. How to View Shared Folders on the Local Computer
  13. How to View Shared Folders on Another Computer
  14. Netstat
  15. Network Monitor
  16. Nslookup
  17. Verifying that the Default DNS Server Resolves Correctly
  18. Verifying that a Specific DNS Server Resolves Correctly
  19. Verifying Specific Types of Addresses
  20. Using TCP for DNS Lookups
  21. PathPing
  22. PathPing Output
  23. Routing Loops
  24. Performance Problems
  25. Possible Connectivity Issues
  26. No Connectivity Issues
  27. Performance Monitor
  28. Data Collector Sets
  29. Windows Resource Monitor
  30. Ping
  31. PortQry
  32. Identifying the TCP Port for a Service
  33. Windows 7 Testing Service Connectivity
  34. Determining Available Remote Management Protocols
  35. Why PortQry Is Great
  36. Route
  37. Task Manager
  38. TCPView
  39. Telnet Client
  40. Testing Service Connectivity
  41. Test TCP
  42. Windows Network Diagnostics
  43. The Process of Troubleshooting Network Problems
  44. How to Troubleshoot Network Connectivity Problems
  45. How to Troubleshoot Application Connectivity Problems
  46. Default Port Assignments for Common Services and Tasks
  47. How to Troubleshoot Name Resolution Problems
  48. How to Verify Connectivity to a DNS Server
  49. How to Use the Hosts File
  50. How to Troubleshoot Performance Problems and Intermittent Connectivity Issues
  51. How to Troubleshoot Joining or Logging on to a Domain
  52. How to Verify Requirements for Joining a Domain
  53. How to Troubleshoot Network Discovery
  54. How to Troubleshoot File and Printer Sharing
  55. How to Troubleshoot File and Printer Sharing from the Client
  56. How to Troubleshoot File and Printer Sharing from the Server
  57. How to Troubleshoot Wireless Networks
  58. Network Diagnostics
  59. How to Troubleshoot Firewall Problems