You’ve probably miss-typed something above that bit of code or created your own class called IPAddress. If you’re using the .net one, that function should be available.
Have you tried using System.Net.IPAddress just in case?
System.Net.IPAddress ipaddress = System.Net.IPAddress.Parse("127.0.0.1"); //127.0.0.1 as an example
The docs on Microsoft’s site have a complete example which works fine on my machine.