To That End, A Private Network Can Be Extended Across The Internet

To that end, a private network can be extended across the Internet infrastructure.
Tunnelling requires three different protocols for it to work correctly. The first is the Carrier protocol, this is the protocol of network the information is being transported over such as IP when using the Internet infrastructure. Encapsulating protocol is the protocol that ensures the original data is encapsulated around the transported protocol and is hidden from the transportation protocol. Such encapsulating protocols are L2F, PPTP, L2TP, GRE, IPSec (Tyson [4], 2004). Lastly is the Passenger protocol, this is the network protocol relating to the encapsulated data such as IPX, NetBeui, IP protocols. Passenger protocol can ensure the original data works on a private network once it has been transferred from one point to another point on the Internet.
Tunnelling is achieved from using either IPSec or Generic Routing Encapsulation (GRE) which ensures the passenger protocol can be read at the different ends of passenger protocol interfaces. Basically before being transported in the tunnel over the Internet the encapsulated data is wrapped up and information describing what the data is (kind of meta data similar to that used in eXtensible Markup Language (XML)) is then stored and read for interface communication at the other end.
VPN Security Technical Background
The most crucial element of a VPN is the security it uses for point-to-point communication. As already discussed encryption algorithms are used to convert the original data into a secret message and undistinguishable to an unintended user.
VPN uses the following technologies to ensure secure point-to-point networks; Firewalls, Encryption, IPSec, GRE, AAA Server. There are more technologies used however the above are key to most VPN systems and will be discussed further here. The firewalls act as a filter and will only allow desired packets of information to pass from the Internet to private network interfaces. If undesired packets arrive they are simply stored in quarantine with an alert sent to the user or, they are deleted. The firewall prevents such unwanted technologies such as outside sabotage or espionage in the form of computer Virus's (virus's can cause unlimited harm to software and hardware components) or Trojan horses (again can cause unlimited harm to software and hardware but is disguised as a program that is trusted by the computer say a software update or video file). Encryption as already discussed can be in the form of symmetric-key or public-key encryption and provides varying levels of secrecy when transporting data from one private network to another private network via the Internet. The most reliable and secure method of encryption is that of public-key encryption (not including quantum cryptography however this is still under research) which is discussed at length by Singh (Singh, 2000).