Is it possible to whitelist/blacklist an IP range and what the correct format is for this?

Yes, it's possible.

Example (whitelist an IP range)

To add IP address range you simply go to the IP Rules section > select Create New IP Rule option > enter the range > select if you want to add it to the block or bypass list > and click to create (add) new IP rule. For example, adding to the block list:

Please note that IP address ranges are supporting using CIDR notation for IPv4 addresses only.

CIDR IP addresses are composed of 2 sets of numbers:

1. The network address is written as a prefix, like you would see a normal IP address, for example 123.255.255.255.
2. The second part is the suffix which indicates how many bits are in the entire address, for example /12.

Putting it together, a CIDR IP address would look like this:

123.255.255.255/12

Important: We don't currently support CIDR in IPv6 at the moment.

Manually block IP address ranges

You can supply an IP CIDR range when blocking IP addresses. Shield won’t validate your range or compare it with other ranges on the IP lists for overlaps. You’ll need to take care when using ranges as you could unintentionally block thousands of visitors with an incorrect range specification.

If you’re unfamiliar with IP CIDR notation, we encourage you to do some research to understand it fully, before attempting to use it. We don’t offer support in building CIDR ranges – we’ll leave this to you. If you’re unsure, it’s best to leave Shield to manage your blacklist for you, completely automatically.

You can find more about this here.