The gather_facts module must be set to true, which is the default setting of the gather_facts module. In other words, the gather_facts module must not be set to false.

The ansible_default_ipv4.address variable will return the default IPV4 address of your control node (that's your Ansible server). For example, let's say you have the following playbook.

---
- hosts: all
  tasks:
    - debug:
        msg: "{{ ansible_default_ipv4.address }}"

Running this playbook should return the IPv4 address of the control node.

TASK [debug]
ok: [server1.example.com] => {
    "msg: "10.1.2.3"
}

You could also use hostvars to return the distribution of the managed node (e.g. target system). For example, let's say you have the following playbook.

---
- hosts: all
  tasks:
    - debug:
        msg: "{{ hostvars[inventory_hostname].ansible_default_ipv4.address }}"

Running this playbook should return the IPv4 address of the managed node.

TASK [debug]
ok: [server1.example.com] => {
    "msg: "192.168.1.2"
}