Configuring 31-bit subnet in DHCP on a 31-bit subnet interface

Question

I am using Sophos Firewall Home on a Protectli Vault FW4B. 5 VLANS are defined in Sophos and I am using CISCO SG-200 switch. I have dedicated one VLAN specifically for an IP camera in my home. 
 Everything is working great but I want to configure that particular IP camera VLAN interface as a 31-bit subnet so that only IP CAM can exist on that network. I am able to change the interface subnet through Network > Interfaces > VLAN as shown in the picture below

But when I go to the DHCP of that VLAN and try to change the mask to /31 (255.255.255.254) and START IP - END IP as shown below, it gives an error: 
 You cannot enter a network IP for Dynamic IP lease

Please tell me what am I doing wrong and how to configure it.

rfcat_vk · Accepted Answer

Hi, 
 you need more addresses because the interface needs an address as well as the camera, so you will need a /30. The DHCP server will need to be in the same range as the interface address. 
 ian

dirkkotte · Answer

With /31 subnet-mask you have x.x.x.0 as network-ip and x.x.x.1 as broadast-ip but no usable client-IP#s between them. 
 Using a /30 subnet, you have 2 usable IP's (x.x.x.1 and x.x.x.2) usable for gateway and client.

Configuring 31-bit subnet in DHCP on a 31-bit subnet interface

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