Get Public/External IP Address

There are some URLs which are able to return the public/external IP address of your machines.

Get the public/external IP via command line using http services (any one of the followings):
$ curl http://checkip.amazonaws.com
$ curl http://whatismyip.akamai.com

Get the public/external IP via command line using dns services (any one of the followings):
$ dig myip.opendns.com @resolver1.opendns.com +short
$ dig whoami.akamai.net @ns1-1.akamaitech.net +short