Fri Oct 20 2017 10:42:15 UTC
If an infinite amount of monkeys were to write code, one would make the same mistake like you did.
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ID Product Comp Assignee Status Resolution Summary Changed
27485 Class Li System.N bugzilla RESO DUPL UnicastAddress.IPv4Mask throws NotImplementedException when coding Xamarin.Forms app from Visual Studio 2013 on Windows 2016-05-25
One bug found.

as

File a new bug in the "System.Net.Http" component of the "Class Libraries" product