host parameter in bigip_pool_member

The

host

parameter is the F5 device the want to pass the module task to, in this case is a variable passed it it.

With Ansible

gather_facts

is set to 'on' (it is on by default) then you have access to a load of variables you can use in your playbooks.

With

gather_facts

you should get a dictionary variable set named

ansible_default_ipv4

which would look like the following:

"ansible_default_ipv4": {
    "address": "10.0.0.1",
    "alias": "eth0",
    "gateway": "10.0.0.254",
    "interface": "eth0",
    "macaddress": "REDACTED",
    "mtu": 1500,
    "netmask": "255.255.255.0",
    "network": "REDACTED",
    "type": "ether"
},

You can then use

ansible_default_ipv4.address

or

ansible_default_ipv4['address']

to reference the address index of the dictionary.

In your playbooks you will need to use

"{{ansible_default_ipv4.address}}"

or

"{{ansible_default_ipv4['address']}}"

See Ansible 2.6 Playbook Vaiables for more information.