Virtual Private Network is a Remote Access method in which at the end its purpose is to be *as if you were inside the network*.

The general overview of VPN is that you as a client logon to the internet, and the VPN server aslo logs on to the internet, both of you and the VPN will have a real IP. After you connect to the VPN server and authenticate, you are assigned a private IP (usually, not a must) that usually is part of the internal network (again this is not a must).

When you want to communicate with any server in the internal network, an IP packet is first constructed using your internal assigned IP address, then its encrypted and placed inside another IP packet but using your internet (real) IP address, this goes normally from your machine to the VPN server normally using both machines internet (real) IP addresses, just like any 2 hosts would communicate on the internet, when it arrives to the VPN server, it removes the outer IP packet and get the inner one that is encrypted, here the VPN server decrypts it and passes it to the internal network, remember that this packet was constructed by the client using his internal IP address, so to hosts inside the network, it appears that it came from a machine with them inside their subnet, the VPN server works here as a bridge.

VPN main protocols are only two, PPTP and L2TP. These are the protocols that decide how a client and VPN server will work, these protocols use other protocols that are not related to VPN specificlly.

PPTP uses another protocol called Generic Routing Protocol GRE (IP Protocol 47), you can think of it as a supporting protocol althought its not like that exactly.

L2TP uses another protocol called IPSec, which is totaly not related to L2TP, IPSec is an extension to the normal IP protocol that adds security to IP packets, IPSec in turn uses another protocol called IKE which is used to setup the encryption between any two hosts communicating with each other

SSL is not related to this subject at all, any way SSL is used mainly on webservers that require secure connectios, its always used when you use the HTTPS protocol, sometimes you might notice when you logon to yahoo mail and hotmail a small yellow lock appears the the buttom of the internet explorer windows, at this time you are using SSL, SSL primary function is to encrypt data between you and the webserver

L2F, i actaully didn't hear about this, i have no idea about it

Hope that this helps you

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