You can access this page with any of these easy to remember url's:

ip4.me - IPv4 only
ip6.me - Dual stack IPv6 and IPv4 (preference depends on your OS/client)
ip6only.me - IPv6 only
whatismyv6.com - Dual stack IPv6 and IPv4 (preference depends on your OS/client)

For automated queries use /api/ on any of the urls for a simple plain text csv result that will not be affected by future html changes on the main page. Recommended API urls
(Don't forget the trailing slash to avoid unnessary 301 redirects):

ip4only.me/api/ - IPv4 only query
ip6.me/api/ - Dual stack IPv6 and IPv4 (preference depends on your OS/client)
ip6only.me/api/ - IPv6 only test

See API v1.1 documentation for more information.