Not all VPN technologies types are listed here, but the most common are briefly discussed. Allow for some generalization as well, to cover all flavors of each technology would be near impossible.
L2TP stands for Layer 2 Tunnelling Protocol. Its major difference from most of the other VPN flavors is that it allows for Layer 2 packets to be transmitted over a VPN tunnel. For instance, ARP requests and DHCP requests will be transmitted over the tunnel. With this type of VPN the user usually gets an IP that is from a pool on the LAN in the destination network. L2TP often runs inside of IPSEC
Internet Protocol Security (IPsec) is a suite of protocols used to encrypt data over an IP network. It allows hosts that have the intention to set up a virtual private network amongst themselves the ability to negotiate, based on preset allowed criteria on either side, the authentication and encryption methods. It forms the outer tunnel for many of the other protocols used in VPNs.With standard IPsec, the networks are usually on different subnets and are limited to Layer 3 packets unless paired with other Layer 2 tunneling technologies such as L2TP.
The most common form of this VPN is used for SSL Portal or SSL Tunnel type VPNs. A way to think about it is a VPN specific to a browser session, with the browser as the client. You would set up an SSL VPN to have access to an application you want to be able to provide to remote users via a browser-originated VPN. You would type in the URL of the application, be met by the firewall or other device maintaining the SSL VPN, which will require you to authenticate. This will establish an SSL session and you will be able to interact with the application.
Not to be confused with SSL authentication methods used by VPNs which are just using certificates as part of the authentication procedures.
Further to the choice of technology and protocol, you will use, will be specific choices to authentication methods, encryption methods, and negotiation methods. Each of these has its advantages and disadvantages and will need to be weighed when making the final decision.
We will continue to publish some articles around VPN Technologies, and we are quite excited about our new series where we will be publishing some how-to articles. So stay tuned…