Error building Player: SystemException: System.Net.Sockets are supported only on Unity iOS Pro. Referenced from assembly 'Assembly-CSharp'.
The solution is to use a plugin call Good Old Sockets which you can find right in the Assets Store of Unity3D. This library require only to change the namespace you are using from System.Net.Sockets to LostPolygon.System.Net.Sockets. The price is 25$ at this time and their license is very open, you can use and distribute this library without extra cost.
I had very quick response for support from LostPolygon both in mail and in Skype.