Go function syntax explanation

I am reading one book.

One function came out:

> func (ip IP) DefaultMask() IPMask

Source code for this function is located inside the net package:

func (ip IP) DefaultMask() IPMask {
	if ip = ip.To4(); ip == nil {
		return nil
	}
	switch true {
	case ip[0] < 0x80:
		return classAMask
	case ip[0] < 0xC0:
		return classBMask
	default:
		return classCMask
	}
}

Question is: what is (ip IP)?

https://golang.org/ref/spec#Function_declarations

"Method declarations" - the way to define a method for receiver base type.

This syntax allows you to call DefaultMask() from an IP type, similar to how member functions work in other languages:

ip.DefaultMask()

The (ip IP) basically represents the "thiscall" argument.