When creating IP ranges in Cerberus, I was unable to create a /30 network with only 1 IP address marked as usable by VMs. The scheme I tried to achieve was a x.x.x.0/30 network with address x.x.x.1/30 being used by another device already and x.x.x.2/30 would be given to a VM.
Specifing x.x.x.2 in both "First IP" and "Last IP" fields resulted in blocking the "add" button by Cerberus (which is kinda understandable, since making a 1-address IP range looks like a weird idea).
It's a common practice to use /30 subnets to create interconnect networks or small IP Access at the ISP I work at, so it would be really nice to have a way to add /30 subnets as 1-address IP ranges. As for now, I've used a /29 network.
Also, Kevin Meziere hinted me to the line of code that may be responsible for this behaviour and asked me to reference it here:
Latest release versions of FiFo packages and latest Joyent SmartOS platform image.