[C++/CLI] How to call function from another Form

Hello!

I have 2 forms: DivIP (parent form) and VLSM (child form). 
All functions in DivIP form i use #include "iplib.h" (It's my header). 
Now in DivIP form, I call VLSM form:

public: System::Void button_Click(System::Object^  sender, System::EventArgs^  e) 
		 {
			Form^ f = gcnew VLSM(textBox_VLSM->Text);
			 f->Show();
		 }

In VLSM form, I can't use any functions in "iplib.h".
If I add #include "DivIP.h" into VLSM form, DivIP form print err "indentifier VLSM" at the position I call VLSM form.

So I want your help to make VLSM form can use all functions in "iplib.h"!
Thank you very much!

P/s: I'm a student, trying to study C++/CLI by myseft. Sorry for my bad English!