While running a playbook on a host you can request some information about other nodes.

The task will look like this:

- name: build account ring
  command: swift-ring-builder account.builder add z1-\{\{ hostvars[item]['ansible_' + iface].ipv4.address }}:6002/sdb1 100
  with_items: groups.storages

Please note that I had to escape the { with a \ so you can actually see it. Remove it from your task.

The above task is extremely useful since it allows me to retrieve the IP address on a specific network card (the one referred in the iface variable) of a group of hosts. Basically I am executing the swift-ring-builder command where I register a couple of nodes. Here the hosts are called storages, thus somewhere on my host file I have a [storages] section that defines several machines.

Ansible Magic! This start at thing is pretty neat too!