Just as every house has a unique street name and number, all devices on the Internet have two numbers: one for public use that you can share with your friends to send them messages or see where they are (your public IP address), and another private number used by other computers connected directly to yours. In addition, just as some houses may not be visible from outside their neighborhood because of higher fences or trees in front yards blocking views down streets, many devices do not give out information about what is inside due to security reasons (like when you connect via password-protected WiFi).