ASP.NET 5 has a number of architectural changes that make the core web framework much leaner and more modular. ASP.NET 5 is no longer based on System.Web.dll, but is instead based on a set of granular and well factored NuGet packages allowing you to optimize your app to have just what you need. This leaner architecture brings lots of benefits to your ASP.NET projects, but it causes a small problem: Before you can use certain classes/APIs, you have to find out which NuGet package might contain the type, and import the package into your project. For example, in this Stackoverflow thread, the developer ppumkin had trouble to use the IPAddress class in ASP.NET 5 for lack of the NuGet package.
The “totally unofficial” ASP.NET 5 Reverse Package Search tool can save your time.
Just type in the class/API, e.g. IPAddress and you’ll get a list of NuGet packages that are good hints for you to import. According to Eilon in the StackOverflow thread, this tool is not 100% reliable, but it’s better than guessing.