Before jumping into the Network Extension framework, you need to know a bit of theory about VPN and networking in general. If you are already familiar with networking and VPN, feel free to skip this article. If you are not, this article has a quick overview/refresher on the network protocols, it outlines the problem that VPN is aimed to solve, and then analyzes one of the existing VPN protocols.
Source: How Does VPN Work? | kean.blog