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Virtual Private networks (VPNs) can extend a local area network (LAN) over the internet to remote networks and remote client machines. A vpn uses the internet to route lan traffic from one location to another by encapsulating the data inside encrypted IP packets. The encrypted packets are unreadable by intermediary internet devices and can contain any kind of network communications - such as file and printer sharing, e-mail, remote procedure calls, and database access.

VPNs can be setup using server computers, firewalls or routers. Client access to the vpn can be made using client-side vpn software or by connecting to an isp that supports the vpn protocol.

VPNs solve the problem of accessing private servers over the internet through a combination of IP encapsulation, cryptographic authentication and data payload encryption.

IP Encapsulation
IP encapsulation provides a way to protect the data while in transit between the remote client and the private lan. computers outside the vpn should not be able to snoop on the traffic exchanged between the remote client and private server or be able to insert their own data into the communication stream. This is accomplished by creating what people refer to as a private and protected "tunnel" through the public internet. When an IP packet contains another IP packet this is called IP encapsulation, and it provides a mechanism to refer to a host within a private network when a direct network connection may not exist. When this is combined with data encryption then we've effectively created our virtual tunnel.

Cryptographic Authentication
Cryptographic Authentication is used to securely validate the identity of the remote client so that the private lan can determine what level of security should be applied to that user. VPNs use the authentication process to determine whether or not a remote user can participate in the encrypted tunnel, and for exchanging the public key that will subsequently be used for data encryption.

data Payload encryption
data Payload encryption uses a public key to encrypt the data field of the IP encapsulated packet. That is, data payload encryption is exactly like normal IP except that the data has been encrypted. It does not encrypt the header information, so details of the private network can be gleaned by analyzing the header information.

advantages and disadvantages
Compared to wide area networks (WANs), VPNs offer some advantages but, also, present some disadvantages.

 Advantages - cheaper than WANs - easier to setup than WANs Disadvantages - slower than WANs - less reliable than WANs - less secure than isolated WANs 

Example
Although there are a number of ways to configure a vpn here is an example of one scenario that is fairly common -- an employee wishes to work from home and exchange data between their home machine and a private web server on the corporate network. There are two important processes here -- the process of negotiating and building a vpn session, and the process of protecting and handling the data within an existing vpn connection. Here I'll briefly describe the latter and leave the former as a potential topic for a future article.

Suppose we have the following:
(a) a vpn client with a public IP address of 66.123.77.196 and a private IP address of 192.168.0.202 (provided by the corporation's DHCP server).
(b) a vpn server on the corporate network with two interfaces -- a public interface to the internet that uses 168.156.192.75 and an interface to the private network with an IP of 192.168.0.101
(c) a web server on the corporate network with an IP address of 192.168.0.102

Prior to creating a vpn session the client host has one interface and a connection to the internet through an isp. The client machine can communicate with any host on the internet but can not access the web server on the private network 192.168.0.X. After the vpn session has been created then the client host has 2 interfaces -- the original interface to the internet and a new vpn interface. The new vpn interface becomes the default gateway -- that is, all packets will initially travel through the new interface. However, the vpn interface is not a physical network card -- it doesn't physically connect to anything. The vpn interface is used to encrypt and encapsulate packets that are subsequently sent as the payload of a new, outer packet. It is the outer packet that is sent out over the internet (using the original interface) to the corporate vpn server.

The inner packet will use the client's private IP of 192.168.0.202 as the source IP address and the web server's private IP of 192.168.0.102 as the destination address. The vpn client encrypts the data field of the inner packet and this inner packet then becomes the payload of an outer packet. The outer packet uses the client's public IP of 66.123.77.196 as the source IP address and the public interface of the vpn server (168.156.192.75) as the destination IP. The IP encapsulated packet is then sent to the isp and out over the internet.

When the IP encapsulated packet reaches the vpn server at the edge of the private network it will unwrap the inner packet and decrypt its data field. Since the vpn server also has an interface to the private network it will then be able to forward the inner packet to the destination web server. When data is sent from the web server back to the client then the process is reversed -- that is, the vpn server handles the encryption/encapsulation and the vpn client is responsible for unwrapping/decoding.

Steve Leytus is a senior software engineer and develops applications for NutsAboutNets.com. For more information about low cost, PC-based diagnostic tools for installing, optimizing and trouble-shooting 802.11 (Wi-Fi) wireless networks please visit http://www.NutsAboutNets.com

If you are thinking about upgrading to a gas tankless water heater, you will need to conduct some research first. Knowing where to find it is important. Some of the areas of information you need to be concerned with include the cost and the features offered. Dont waste your time doing a search for information that isnt reliable.

One of the best places out there to find great information on gas tankless water heaters is the internet. There are many reputable home improvement stores that will feature information about them on their websites. Most offer a product search so that you can see the various brands and models available. One thing you need to be aware of is most of these sites will have more gas tankless water heaters featured on their website than available at their physical stores. In that case you will have to special order it.

Of course there are some downfalls to searching for gas tankless water heaters on the internet. It is not uncommon for many websites to carry outdated information on them. However this can work to your advantage because you may end up finding a better deal but keep in mind that some of the prices you see probably are no longer accurate.

To prevent this, contact the retail store after you have researched a particular model on their website. Talking to a store employee can help you find out about any sales going on and if the model is carried in the store. If not, they can assist you with getting it ordered. In some cases you will have to provide a credit card number but you wont be charged until you come in to pick up the gas tankless water heater. Calling definitely saves you time over driving to the store to find out they dont have it in stock or that it is out of your price range. Using the internet to find out about gas tankless water heaters is a great way to find valuable information while saving time.

Learn more information on tankless water heater and best gas tankless. eTanklessHotWaterHeater.com is a comprehensive resource for tankless water heater with information on reviews, installation and how it works.

Losing weight can feel like an overwhelming task. But it's not as hard as it seems - nor as complicated. Your weight loss can be fast and simple - if you choose the right diet or weight loss plan.

The problem today is not that we don't know how to lose weight - but that we don't know which method to choose. There are countless diets and weight loss plans out there - and many of them do work for some people. But not every diet will work for you. You don't want to waste valuable time and energy on a diet that doesn't work.

So how do you find a plan that will speed you towards your dream body? In order to find a diet that works best for you, look for these 3 factors:

#1 It Has A Proven History

If it worked for other people, the diet has a high probability of working for you too. Look for realistic sounding testimonials - which means they won't sound perfect since they are written by real people. In general, the more testimonials the better.

It's always more reliable when people are willing to give their full name on a testimonial however many people can be shy about this so don't expect it.

Before and after pictures are always nice to have although again, not necessary since lesser known diets that work well may not have the money to sponsor a before and after weight loss contest.

#2 It Fits Your Personal Style

While many diets may work - they may not work for you because they don't mesh with your personal style. Every person is different and has specific preferences, needs and experiences in the weight loss arena.

For example, if you don't like eating meat or dairy products, choosing a low carb diet plan might not be your best option. If you're always on the go and don't have time to prepare meals you might want a plan where the meals are prepackaged and easy to prepare. If you cannot stand to feel hungry when losing weight, you might want to a plan that lets you eat several mini-meals throughout the day.

So before making your final choice, make sure it fits best with your personal style. I've seen people lose weight at lightning speed (and even enjoy the process) when they choose a diet based on their personal style.

#3 It Gets You Excited

One of the most underrated keys to success in any weight loss plan is enthusiasm. Too often the thought of losing weight takes on a negative connotation in our minds. You have to get excited about the diet - you have to believe in it and be motivated to commit your time to trying it.

When you don't believe in the diet you don't believe that it will help you lose weight - and what you believe you will eventually manifest.

Make sure the diet plan you choose gets you excited and enthusiastic. Perhaps it introduces a totally new concept you've never thought of before. Perhaps you're salivating at the ultra-fast results it produces. Or even perhaps you can relate to the written testimonials. It doesn't matter as long as it gets you enthusiastic and excited about your weight loss.

So those are 3 key factors that will help you find a diet that works. When deciding on a weight loss plan, keep these 3 factors in mind and you're guaranteed to choose a diet that will work best for your body and give you incredible results!

Vienna Miller is a contributing writer to Diet Reviews - a website offering the latest diet reviews and weight loss plans that actually work.

For the latest weight loss tips and free diet articles visit http://www.freetobethin.com