You can think of your computer like a house. You don’t want strangers being able to come and go into your house as they please. The same goes with your computer and network traffic. In the early days of computers, they were wide open — any system or device could send network traffic to any other system or device. This is one of the ways that large-scale malware attacks of the past, like Blaster in 2002, were able to spread themselves quickly and widely across networks.