What is VPN?

Explain in a way that I can be easily understand the rationale of VPN.
Nusrat NuriyevAsked:
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AkinsdNetwork AdministratorCommented:
VPN (Virtual Private Network) is exactly what it means.
Are there a lot of companies which uses VPN?
It uses a tunnel concept to connect 2 networks over a public network (internet) but maintains privacy.
Many companies use this either to connect two or more sites together or to allow their workers to connect remotely without compromising privacy or safety.

Seems like it's super-mega-important thing if, it has been asked on each interview.
Yes it is very important. Every middle or big size company uses the technology. Even small business use it. Home users also use it.

Please bear in mind that VPN is a general name for the technology. It is not a name of a software. It is the same way you view a web browser. There are many programs that give you web browsing experience eg Internet explorer, Firefox, Safari, Chrome, Oprah etc
Same for VPN
There are softwares that use VPN technology eg Teamviewer, PCAnywhere, GoToMyPC, LogMeIn, GoToAssist etc

You may need to google the technolgy (Wikipedia, Youtube etc) to get detail information but the simple foundation is it is a Network, it is Private and it is Virtual. There are 2 main categories and several sub groups of VPN. These are based on the way they are applied.
The 2 main Categories are Site to Site VPN and Remote Access VPN
Site to site VPN links to sites or offices together.
Remote Access VPN links a single user (multiple single users) to a site or office

The picture below is an example of Remote Access VPN
Phil PhillipsDirector of DevOps & Quality AssuranceCommented:
A VPN is basically a way to securely connect private networks together over a public network (i.e. the Internet).  It's useful if you want devices to communicate over physically different sites as if they were all on the same private network.  A VPN can also allow a user to access a local intranet from the outside.
Larry Struckmeyer MVPCommented:
The software (including that used in router to router or firewall to firewall or mobile client to either) that is used to create the VPN encrypts the packets flowing over the public network so they are useless to anyone that intercepts them.   It allows for "secure" opening of files as if you were in the office on the private LAN.

There are some pitfalls.  First untrusted or unmanaged systems connecting can introduce malware to the system they connect to.  Second, opening many types of flat file databases, such as most accounting programs and Access type programs  will lead to corruption should the connection be broken while writing back to the database.

For this reason, and others, Terminal Services, now called Remote Desktop Services is a better choice,
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Nusrat NuriyevAuthor Commented:
What is VPN from OSI point of view?

Ok, tell me what is Terminal Services/Remote Desktop services? is it like UtraVNC? Or we are talking about different things?
Nusrat NuriyevAuthor Commented:
Are there a lot of companies which uses VPN?
Seems like it's super-mega-important thing if, it has been asked on each interview.
Fred MarshallPrincipalCommented:
Mostly Network layer.  i.e. the most common ones are.

Yes, there are a lot of companies (and other users) that use VPN.

I think that Remote Desktop is like UltraVNC but others may want to be more specific about it.
They can be encrypted.

As far as I'm concerned, 3rd party services like GoToMyPC and other Citrix products that are similar, LogMeIn, TeamViewer, etc. are also VPNs but VPNs that are supported by a 3rd party.

You might watch the short videos at: http://www.labnol.org/internet/https-vpn-explained/20030/
This gives a bit of perspective.
Nusrat NuriyevAuthor Commented:
For site to site there will be :

Office_1  <-> ISP_1 <-> Internet <-> ISP_2 <-> Office_2

Larry Struckmeyer MVPCommented:
in the middle of Office-ISP insert router/firewall as the best answer to VPN.  The tunnel is created between the router at one end and the similar/same at the other end.

Adding that RDP/Terminal Services, Go to My PC and all the rest are NOT VPN.  None of these allow you to open a document or a database on the pc you are connecting FROM, they all allow/force you to open the document/database on the pc/server you are connecting TO.  If no data ever leaves the office/pc in which it lives it is much more secure than if it is opening across the internet.  With RDP/TS/RDS and all the rest the only thing that travels over the connection is keystrokes and screen refreshes.  Yes, those bits are encrypted, but still, a far cry from a true VPN.
AkinsdNetwork AdministratorCommented:
Office_1  <-> ISP_1 <-> Internet <-> ISP_2 <-> Office_2


@ Larry
There are softwares that use VPN technology

Please understand that VPN is not a feature perse, it is a terminology (technology). That's where the confusion lies. Is Mobile App a feature or technology? It fits in both but the true description is technology. People have limited mobile applications to the Apps they use on their phones. That is exactly what people have done with VPN. They have limited it to the concept on routers and oversighted the bigger picture. Please don't lock yourself in a box
Those RDP / Terminal Services use VPN technology in order to make your connection private. I'll recommend you do a little more research on VPN. Like I said, it is not a feature perse, it is a technology. Most routers have VPN technology labeled as features for marketting purposes.

Ask this question
- Does the connection provide a Network?
- Is the connection private (encrypted)?
- Is the connection virtual?
If you can answer yes to all the above, then you have a VPN

The connection is established in 2 phases
- You have Phase 1, where you establish communication between two VPN capable devices. Policies are exchanged, compared and seleced here.
- Then Phase 2, where the connection is established. Encryption occurs in this stage.

And for your information, you can transfer files back and forth using Teamviewer and any of the RDP utilities mentioned earlier.

All these information are available on the internet if you look for them. You can start from here
Nusrat NuriyevAuthor Commented:
I read that article several times, but still could not properly answer what is VPN. Now, it's much more clearer.
AkinsdNetwork AdministratorCommented:
I'm glad we could help
All the best
Evrard AudetCommented:
A Virtual Private Network (VPN) is a network technology that creates a secure network connection over a public network such as the Internet or a private network owned by a service provider. Large corporations, educational institutions, and government agencies use VPN technology to enable remote users to securely connect to a private network.
Larry Struckmeyer MVPCommented:

Yes, I know that some/all of the RDP types of connections will allow you to transfer files from the remote system to the local system.  But transferring files over the connection, VPN or otherwise, is not the same as opening files over the connection.
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