Ansible: Get all the IP addresses of a group

I find the magic map extract here.

main_nodes_ips: "{{ groups['mainnodes'] | map('extract', hostvars, ['ansible_host']) | join(',') }}"
main_nodes_ips_with_port: "{{ groups['mainnodes'] | map('extract', hostvars, ['ansible_host']) | join(':3000,') }}:3000"

An alternative(idea comes from here):

main_nodes_ips: "{{ groups['mainnodes'] | map('extract', hostvars, ['ansible_eth0', 'ipv4', 'address']) | join(',') }}"

(Suppose the interface is eth0)