var ip = require('ip-address');

function subnetFromNetmask(netmask) {
  var mask = new ip.Address6(netmask);

  return mask.getBitsBase2().match(/^(1+)/)[1].length;
}

function addressFromAddressAndNetmask(address, netmask) {
  return new ip.Address6(address + '/' + subnetFromNetmask(netmask));
}

var address = addressFromAddressAndNetmask('1:2:3:4::', 'ffff:ffff:ffff:ffff::');

document.write('<pre>');
document.write(address.correctForm() + address.subnet);
document.write('\n\n');
document.write('start ' + address.startAddress().correctForm());
document.write('\n');
document.write('end   ' + address.endAddress().correctForm());
document.write('</pre>');