Sometimes the default Ansible Facts are not enough.
For example, Ansible doesn't provide "ansible_private_ipv4_address" which with private IP address.
With it, we don't need to worry about which IP on which NIC should be used.

But we can create an Ansible module and then use module provided 'Facts'

The steps:
[root@idc-server2 ~]# ifconfig

eth0      Link encap:Ethernet  HWaddr 1B:2B:3B:4B:5B:6B
          inet addr:172.16.1.2  Bcast:172.16.1.255  Mask:255.255.252.0
...

eth1      Link encap:Ethernet  HWaddr 1A:2A:3A:4A:5A:6A
          inet addr:100.100.100.100  Bcast:100.100.100.255  Mask:255.255.255.240
...

lo        Link encap:Local Loopback 
          inet addr:127.0.0.1  Mask:255.0.0.0
...

[root@idc-server1 ansible]# vim myfacts.yml

---
- hosts: idc-server2
  roles:
  - myfacts

[root@idc-server1 ansible]# mkdir -p roles/myfacts/{tasks,templates}

[root@idc-server1 ansible]# vim roles/myfacts/tasks/main.yml

---
- name: run myfacts module to get customized facts
  myfacts: get_facts=yes

- name: update file with the customized facts
  template: src=myfacts.txt.j2 dest=/tmp/myfacts.txt

[root@idc-server1 ansible]# vim roles/myfacts/templates/myfacts.txt.j2

ansible_private_ipv4_address : {{ ansible_private_ipv4_address }}

[root@idc-server1 ansible]# mkdir -p library/heylinux
[root@idc-server1 ansible]# vim library/heylinux/myfacts

#!/usr/bin/python

import json
import commands
import re

def get_ansible_private_ipv4_address():
    iprex = "(^192\.168)|(^10\.)|(^172\.1[6-9])|(^172\.2[0-9])|(^172\.3[0-1])"
    output = commands.getoutput("""/sbin/ifconfig |grep "Link encap" |awk '{print $1}' |grep -wv 'lo'""")
    nics = output.split('\n')
    for i in nics:
        ipaddr = commands.getoutput("""/sbin/ifconfig %s |grep -w "inet addr" |cut -d: -f2 | awk '{print $1}'""" % (i))
        if re.match(iprex,ipaddr):
            ansible_private_ipv4_address = ipaddr
            return ansible_private_ipv4_address

def main():
    global module
    module = AnsibleModule(
        argument_spec = dict(
            get_facts=dict(default="yes", required=False),
        ),
        supports_check_mode = True,
    )

    ansible_facts_dict = {
        "changed" : False,
        "ansible_facts": {
            }
    }

    if module.params['get_facts'] == 'yes':
        ansible_private_ipv4_address = get_ansible_private_ipv4_address()
        ansible_facts_dict['ansible_facts']['ansible_private_ipv4_address'] = ansible_private_ipv4_address

    print json.dumps(ansible_facts_dict)

from ansible.module_utils.basic import *
from ansible.module_utils.facts import *
main()

[root@idc-server1 ansible]# ansible-playbook -u root myfacts.yml -i hosts

PLAY [idc1-server2] ***************************************************************

GATHERING FACTS ***************************************************************
ok: [idc1-server2]

TASK: [myfacts | run myfacts module to get customized facts] **************
ok: [idc1-server2]

TASK: [myfacts | update file with the customized facts] *********************
changed: [idc1-server2]

PLAY RECAP ********************************************************************
idc1-server2                   : ok=3    changed=1    unreachable=0    failed=0  

[root@idc-server1 ansible]# ssh idc1-server2 'cat /tmp/myfacts.txt'

ansible_private_ipv4_address : 172.16.1.2