When there is a speed limit on a river how is the speed measured ?

Is the speed relative to the water or the land ?
What about the difference between travelling upstream or downstream ?
What if the speed of the flowing water is the same as, or greater than the speed limit - does that mean you can't go on the water ?

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia��).