There are two similar but distinct "ipaddr" modules.

• ansible.netcommon.ipaddr
• ansible.utils.ipaddr

ansible.netcommon.ipaddr is being deprecated, so you should almost always use ansible.utils.ipaddr.

[DEPRECATION WARNING]: 
Use 'ansible.utils.ipaddr' module instead. 
This feature will be removed from ansible.netcommon in a release after 2024-01-01.

You may need to install the ansible.utils collections.

ansible-galaxy collection install ansible.utils

ansible.utils.ipaddr('address/prefix') can be used to take an IP address/subnet mask and convert it into IP address/prefix.

---
- hosts: localhost
  tasks:
  - debug:
      msg: "{{ '10.11.12.13/255.255.255.0' | ansible.utils.ipaddr ('address/prefix') }}"
...

Running this playbook should return the following, converting subnet mask 255.255.255.0 into prefix /24.

ok: [localhost] => {
    "msg": "10.11.12.13/24"
}