Installation of server does not accept subnet CIDR

Question

I try to install a server from scratch with a fixed IP address. For installation I use the provided ISO of 18.04.3 and the installation agent of Ubuntu.

First I thought that my understanding of the subnet CIDR was wrong, but after I received credible information from https://networkengineering.stackexchange.com/questions/63499/how-to-interpret-subnet-mask I am sure that this is more a problem within Ubuntu.

When I enter 255.255.252.0/22 as subnet CIDR and 10.16.192.252 as the IP address I get the error message that 10.16.192.251 is not contained in 255.255.252.0/22. But a subnet calculator tells me that 10.16.192.1 - 10.16.195.254 would be part of the subnet.

Solution

The prompt wants a network address with CIDR prefix. That is, if you are using 10.16.192.251/22 for your network (255.255.252.0 subnet mask, /22 CIDR mask), you need to first calculate the network address (the first IP of the range, usually a .0 with ones like this in the last subnet, but could be a different third octet, in this case 10.16.192.0 is the network address) and then put that in for subnet with the CIDR mask.

Therefore, use 10.16.192.0/22 for the Subnet field. This is used to autocalculate Broadcast address and Gateway address on a 'default' network, but in the case here also makes sure that the Address you enter is in fact in the subnet you're looking to add it to.

Comments

[Thomas Ward]

subnet does NOT mean the subnet mask. It means the network segment your device is on, and its gateway. So, 10.16.192.0/22 for the Subnet. NOT the subnet mask (which is defined by the /22 portion)

[Marged]

@ThomasWard This worked. Do you want to create an answer ?

[Marged]

255.255.252.0 is rejected by the installer, it insists on a CIDR notation.

[chili555]

Please try 10.16.192.0/22 in the subnet field. Thank you, @ThomasWard

[u15p7fgy863eiq5]

Why can't Ubuntu's installer calculate it itself? All other static IP GUI wizards can handle it themselves.

[bendecko]

subnet-calculator.com

[Thomas Ward]

@u15p7fgy863eiq5 Because ask the Server team? Might be a feature you should request.