Special device which connects all PCs within LAN is called router. It is like a post office were all packages (parcels) come. Same like post officer, special program inside the router checks each package separately. i.e. the sender could specify the wrong receive address and in that case post office should return package with additional information about mistake, otherwise pass it further.

• 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