Web Site
https://vpnoverview.com/vpn-information/what-is-a-vpn/

how_vpn_work.pdf
https://www.communicat.com.au/wp-content/uploads/2013/04/how_vpn_work.pdf

Technical_Overview_of_Virtual_Private_NetworksVPNs.pdf
https://www.researchgate.net/publication/274929918_Technical_Overview_of_Virtual_Private_NetworksVPNs

Defining-a-VPN.pdf
http://scitechconnect.elsevier.com/wp-content/uploads/2013/09/Defining-a-VPN.pdf

CiscoVPN
https://www.cisco.com/c/dam/en_us/training-events/le21/le34/downloads/689/academy/2008/sessions/BRK-134T_VPNs_Simplified.pdf

How_Virtual_Private_Networks_Work
https://www.academia.edu/29644138/How_Virtual_Private_Networks_Work