DevCtl numbers

I want some DevCtl for my network DD.

Now I am using the following definitions tentatively:

<dcmd_xxxx.h>
#define DCMD_XXXX_FUNC0 _DION(DCMD_NET, 80)
#define DCMD_XXXX_FUNC1 _DIOTF(DCMD_NET, 81)

But I think I should not use 80 & 81 arbitrarily.
Please teach me, how shoud I add my original DevCtls.
Are there any reserved numbers for user defined DevCtls?