1) Protocols that are designed for LANs are often exposed to the public Internet. This means that it can spread from one LAN to another by scanning the entire p

1) Protocols that are designed for LANs are often exposed to the public Internet. This means that it can spread from one LAN to another by scanning the entire public IP address range for the vulnerable service. This happened in 2003 with the Blaster worm. https://en.wikipedia.org/wiki/Blaster_(computer_worm) https://en.wikipedia.org/wiki/Blaster_(computer_worm) 2) LANs can be very big (thousands of computers) and span multiple buildings or cities, and many LANs only have firewalls at the "border" with the Internet (instead of per-machine). Windows machines in particular, tend to have shared network drives (CIFS or SMB) enabled so you can log into any computer and continue working (and to avoid viruses spread by floppies and USB keys).