I'm having trouble creating the subnet 100.72.3.0/23 because it keeps showing up as an invalid range. I know that the CIDR block itself is valid since it covers 100.72.2.0 to 100.72.3.255, and it's within the virtual network range I'm using. I can successfully create the subnet 100.72.6.0/23 without any issues, but I can't seem to do the same for 100.72.5.0/23. Can anyone explain why this happens? If possible, please share any documentation that might help clarify.
3 Answers
When you're setting up a virtual network, you really need to use custom private IP address spaces, and these should follow the guidelines outlined in RFC 1918. There's a lot of detail on Microsoft's documentation about this if you're interested in diving deeper!
The address 100.72.3.0 isn't valid for your situation because it's assigned to ISPs for backend NAT. You probably want to stick with private IP ranges. You can refer to RFC 6598 for more details about carrier-grade NAT. Just to note, I don’t get why 100.72.6.0/23 works, though—it might be that Microsoft has internal reservations for that range.
You can't just create subnets anywhere, especially with a /23 range. For that, the third octet needs to align with specific values like 0, 2, 4, 6, and so on. So that's probably why you're hitting a wall there. If you're interested, there's tons of info out there about networking guidelines!
Ah, that makes sense! I didn't realize the octet alignment was that strict. Thanks for the heads-up!
Lol, but your own source suggests otherwise! Might wanna double-check that before claiming it's wrong.