The package source named 'NuGet' is either invalid or not available

Question

I have installed Nuget 1.2 yesterday, and today, while I was trying to install Entity Framework package, I have encountered the following problem in the Package Manager Console:

PM> install-package entityframework
Install-Package : The package source named 'NuGet official package source [https://go.microsoft.com/fwlink/?LinkID=206669]' is either
invalid or not available and thus is currently unreachable.
At line:1 char:16
+ install-package <<<< entityframework
+ CategoryInfo : NotSpecified: (:) [Install-Package], InvalidOperationException
+ FullyQualifiedErrorId : NuGetCmdletUnhandledException,NuGet.PowerShell.Commands.InstallPackageCommand

I can surft the internet and I don't think anything is preventing Visual Studio to access the internet.

Answer 1

Actually, there was something that was preventing Visual Studio from accessing the internet. Seems like VS had some problems with my proxy server.
So I used a solution that has been introduced in here and everything works fine now.