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