That was simplest scenario for the post office (router), but what if some package could contain illegal contents or exceeds maximum allowed weight? To check contents and other parameters, post office will need to setup some special devices (like ones which is used in airports to check contents of you bags) to make necessary measures. If some illegal contents will be found - the package will be stopped and warning message will be sent back to the sender (or person will be sent to the 'special' place ).

• Lesson 3: How local firewall blocks some web sites
• Lesson 3.1 Communicator device - router
• Lesson 3.2 What is firewall and how it works

That is right but how do you imagine that without that:


To steal something from your PC the one need first to install some exploit. A little software which will perform all operations its owner needs.

OR may be you have some ideas how information could be stolen from your PC.