os.networkInterfaces currently returns IP addresses and netmasks in the old-fashioned format of separate address and netmask:

{ address: '127.0.0.1',
  netmask: '255.0.0.0' }
{ address: '2001:db8::',
  netmask: 'ffff:ffff:ffff:ffff::' }

It's very uncommon to specify a IPv6 subnet mask like above. I think it'd generally be easier for users to work with these addresses if they were also available in CIDR form, maybe via a new cidr property, e.g. '127.0.0.1/8' and '2001:db8::/64'.