Handy converter: https://www.ipaddressguide.com/cidr
E.g. for the CIDR to IP Range using a CIDR Range of 10.10.20.0/26
means 64 IP addresses – the first being 0 and the last 33.
You can calculate this simply by adding up how many you need to get to 32 – i.e. 6.
Then 2^6 = 64.
So, the first would be 10.10.20.0 and the last would be 10.10.20.63.