Error in references

Question

HI! I try to run diferent examples but get errors with some of the references. Like a yellow tag in the references on some of 'em like the System.Configuration I guess there some dll's missing but where can i find 'em`? using System.net I try to do something like getting an IPAddress. There is nothing called system.net in add references. I'm new to C# so please help? Yepp