internal-ip 
Get your internal IPv4 or IPv6 address
CLI
$ npm install --global internal-ip
$ internal-ip --help
  Usage
    $ internal-ip
  Options
    -4, --ipv4  Return the IPv4 address (default)
    -6, --ipv6  Return the IPv6 address
  Example
    $ internal-ip
    192.168.0.123
    $ internal-ip -6
    fe80::200:f8ff:fe21:67cf
API
$ npm install --save internal-ip
var internalIp = require('internal-ip');
internalIp.v4();
//=> '192.168.0.123'
internalIp.v6();
//=> 'fe80::200:f8ff:fe21:67cf'
Related
See public-ip or ipify to get your external IP address.
License
MIT © Sindre Sorhus