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How do I allow VNC through my firewall?

VNC Server will attempt to add itself as an exception during installation. If this is not possible for some reason, then you will need to do it manually.

Configuring a firewall is fairly simple, you need to open TCP port 5900 in the firewalls 'exception' list.

(1) Windows

Windows XP, Vista and 7 have a built-in firewall, which is on by default, and which you will need to configure.

(1a) Windows XP

Navigate through to Control Panel > Windows Firewall
Click on the 'Exceptions' tab, at the top of the window.
Click on the 'Add Port...' button
Name it "RealVNC" and set the "Port Number" to 5900, select "TCP" then click OK.
That's it!

(1b) Windows Vista

Navigate through to Control Panel > Windows Firewall
Click on "Allow a program through Windows Firewall", in the left-hand pane.
Click on the 'Add port...' button.
Name it "RealVNC" and set the "Port Number" to 5900, select "TCP" then click OK.
That's it!

(1c) Windows 7

Navigate through to Control Panel > Windows Firewall
Click on "Advanced Settings" in the left-hand pane.
Click on "Inbound Rules" and then "New rule".
Select "Port", click next then select "TCP" and enter "5900 as the local port.
Click next, and allow the connection.
Click next again, and ensure that "Domain", "Private" and "Public" are all ticked.
Name your firewall rule, then save it.
That's it!

(2) Mac OS X

Navigate through to System Preferences > Security
Click on the top "Firewall" tab
If "Allow all incoming connections" is enabled, you do not need to do anything further.
If not, make sure "Set access for specific services and applications" is enabled.
Click on the plus symbol at the bottom (+)
In the new Finder window, find the RealVNC folder, and then select 'VNC Server'
That's it!

(3) Third-party firewalls

If you are using a 'third party' firewall (Symantec, Kaspersky, Mcafee, Avast, AVG) then you will need to refer to your particular firewall's documentation for how to do this. The easiest option is probably just to search (using a search engine, such as Google) for 'How do I open a port in X' with "X" being the name of your firewall software.