OOTB the WebMatrix runs on a local Computer only for development reasons.
Once the application is finished you need to publish it to a hosting provider.
On top of this, it allows only your localhost to access the Web Application
How to overcome this :
4. steps to complete
1. Add row in applicationhost.config file. Located in the USER Directory (change IP and Port to your settings).
<binding protocol="http" bindingInformation="*:60485:10.0.0.1" />
IIS Express maintains a user-specific “applicationhost.config” file in the %userprofile%\documents\IISexpress\config” folder.
2. Run this command to give access to your IIS Express running on the server.
netsh http add urlacl url=http://10.0.0.1:60485/ user=everyone
3. Run this command to give access to your IISExpress running using the Firewall rule for the spec. port.
netsh advfirewall firewall add rule name="IISExpressWeb" dir=in protocol=tcp localport=60485 profile=private remoteip=localsubnet action=allow
4. Next create a service to start the IIS Express automatically, using the SC.EXE command.
Or run it from an IIS intranet sever (see my other post on How To)
Keep in mind that this is not a MS supported method, and all risks are on your side.
Fun to try anyhow !