I have a whole network range I would like to block which is 192.241.0.0. In order to block everything in this range I need to know the subnet mask/CIDR.

Until now I have been blocking with a large list of addresses and blocking with my own list of individual addresses.

I don't really need a tutorial on subnet masks because like anything math related I have learned about it and forgotten it many times.

I don't really feel like choosing every CIDR from 1 to 32 and waiting to see if works or not.

I think I must be missing something very basic here. I figured it would be easy to block an address range but I must know the subnet mask first.

whois will tell you the size of the allocation, which is probably what you want.

There is also an ICMP message type called "Address Mask Request", but a) it's not mandatory for hosts to respond to these requests, and b) the network is probably subnetted anyway, so knowing the mask of any individual host is of no value.

https://www.whois.com/whois/192.241.0.0

/21

Doesn't mean it is CIDRed smaller, however.

You have the answer, but I’m curious (read nosy) about why want to block that netblock and what you’re blocking it from.
None of my business, of course…