How to Block VoIP Traffic

VoIP technology lets users make phone calls over the Internet.

Voice-over-IP services allow users to place and receive telephone calls using special hardware or software, with the signal using the Internet instead of traditional copper phone wire. VoIP services have gained in popularity because they are easy to deploy and can significantly reduce costs for some consumers. However, some network administrators (or even some cautious parents) may seek to restrict VoIP services on their networks, either to reduce bandwidth consumption, or to deliberately restrict users' access. Blocking many VoIP services is a straightforward matter.

Things You'll Need

• Administrative access to network router

Instructions

1. Identify which VoIP applications are in use and identify the TCP/UDP ports that each application uses. Port information is available in the product documentation, or check a list of standard ports through IANA or another authority.

2. Log into your router's administrative control panel. Each router model has a different procedure for accessing the control panel, so check the manufacturer's documentation for the specific procedure.

3. Block the ports that the VoIP software in question uses, through the router's control panel. This is often completed on an "Applications" tab--type in the name of the VoIP software and the port numbers it uses, then click "Save."

4. The router should block all traffic on the specified ports immediately, but reset the router by turning off the power and turning it back on again, to make sure that the process completes.

Tips & Warnings

• Communicate any restriction to VoIP traffic so that users are aware that this type of network activity is prohibited. Run a network logging tool to validate that users are not trying to use VoIP software against policy.

• Some administrators prefer to block all ports except for a small subset that are specifically approved for traffic. This helps to "catch" VoIP software that runs on configurable ports.

• Users focused on bypassing port restrictions can often circumvent normal barriers by changing the port number used for outbound traffic. This strategy is especially useful for VoIP apps that focus on user-to-user connections--both halves of the conversation can set their ports to an unblocked range and use the VoIP software.

References

• Practically Networked: Application Port List

Resources

• VoIP Troubleshooter
• IANA: Port Assignments