From https://github.com/rsp/scripts
Usage: externalip [METHOD]
Source: https://rawgit.com/rsp/scripts/master/externalip
Get external IP address that is visible by servers that you connect to over Internet.
Note: this is not necessarily the same IP as the one that is used by your computer's network interface (for that use internalip).
METHOD can be: dns (default), http, https, ftp, telnet.
The http and https methods are using the whatismyip.akamai.com service that showed the best performance in all my tests (from Warsaw, Poland).
Use the externalip-benchmark script to find the best server for your location.