Adding firewall rules with IP ranges in the Plasma Firewall is not possible, as it doesn't seem to allow backslash character in the IP address boxes.
STEPS TO REPRODUCE
1. Open system settings and go to Firewall section,
2. After opening the add rule pop-up window, check advanced option
3. Try typing anything in the source or destination box that contains IP range and backslash (/) character, like 192.168.1.0/24 for the entire local network IP addresses.
4. You won't be able to type the backslash character and that rule cannot be created from Plasma Firewall, even though UFW does support such rules
It is not possible to add a rule with IP ranges in the Plasma firewall, because it doesn't allow adding backslash characters in the IP source or destination boxes
As UFW does support adding such rules, Plasma Firewall should support adding them, too.
Linux/KDE Plasma: KDE Neon User 5.26
(available in About System)
KDE Plasma Version: 5.26.2
KDE Frameworks Version: 5.99
Qt Version: 5.15.7
Kernel: 5.15.0-52 generic 64bit
UFW version: 0.36.1-4build1 (From Ubuntu main repositories)
*** This bug has been marked as a duplicate of bug 434317 ***