A Virtual Private Network (VPN) can be viewed as a private network which is connected through a public network. Virtual Private Networks (VPNs) are used to connect LANs together across the Internet. Using Virtual Private Network (VPN) technologies, remote users can connect to enterprise network securely over the public internet as if their computers are physically connected to the network.
Virtual Private Network (VPN) connections use either Point-to-Point Tunneling Protocol (PPTP) or Layer Two Tunneling Protocol/Internet Protocol security (L2TP/IPSec) over internet. Internet connections are usually cheaper than leased line, Dial-up, ISDN or similar type of connections. Since Internet is the connection medium, Virtual Private Network (VPN) can save huge telecom costs.
Point-to-Point Tunneling Protocol (PPTP)
PPTP was created by Microsoft and available since Windows NT 4.0 Routing and Remote Access Services. Point-to-Point Tunneling Protocol (PPTP) encrypts the data it encapsulates, but the header is not encrypted. Since the VPN header is not encrypted, an eavesdropper can read the VPN header but the data is somewhat secure since the contents are encrypted.
Layer 2 Tunneling Protocol (L2TP)
Layer 2 Tunneling Protocol (L2TP) is another VPN tunneling protocol which is used together with Internet Protocol Security (IPSec). IPSec encrypts the entire L2TP packet. A advantage of L2TP over PPTP is that eavesdroppers cannot identify that a VPN is in use, because IPSec encrypts the L2TP header information also. Hence L2TP/IPSec protocol is much more secure than Point-to-Point Tunneling Protocol (PPTP).