ARPANET was the world's first operational packet switching network and predecessor to the global internet, developed by the US Department of Defense's Advanced Research Projects Agency during the Cold War. A hard bounce occurs when an email fails to be delivered due to a permanent issue like an invalid address, while a soft bounce happens when the message reaches the recipient's server but bounces back before delivery, possibly due to a full inbox. An intranet is a private computer network within an organization that uses firewalls, while the internet connects computer networks globally.