[rbak-nsp] How to change dhcp prifex to /15

[amarsaikhan.a]

We use SE1200.

Our current configuration:

context VOIP
!
no ip domain-lookup
!
interface VOIP_DHCP multibind
  description TR069_DHCP_interface
  ip address 172.16.0.1/16
  dhcp server interface
!

we would like change from 172.16.0.1/16 to 172.16.0.1/15

but invailed /15 mask.

How to recover this issue. 

Thank you
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