Node array of function using variables from enclosing scope

I am trying to do dns.reverse() on a list of ip using async.parallel().

The code is as follows:

var functions = [];
for (var i = 0; i < data.length; i++) {
    var ip = data[i].ip;
    var x = function(callback) {
        dns.reverse(ip, (err, hostnames) => {
            if (err) {
                log.error("Error resolving hostname for [" + ip + '] ' + err);
                return callback(null, err);
            }

            callback(null, hostnames);
        });
    };

    functions.push(x);
}

async.parallel(functions, (err, results) => {
    for(var i = 0; i < data.length; i++) {
        data[i]['hostnames'] = results[i];
    }

    handler(null, data);
});

What is happening is dns.reverse() is getting called with the same ip (the last one in data array) for all the calls. May be I am doing something wrong. Can somebody explain what is my mistake?

Thanks to @apokryfos I got a hint. To get the code working I just need to use let instead of var while declaring ip.

        var functions = [];
        for (var i = 0; i < data.length; i++) {
            let ip = data[i].ip;
            var x = function(callback) {
                dns.reverse(ip, (err, hostnames) => {
                    if (err) {
                        log.error("Error resolving hostname for [" + ip + '] ' + err);
                        return callback(null, err);
                    }

                    callback(null, hostnames);
                });
            };

            functions.push(x);
        }
        async.parallel(functions, (err, results) => {
            for(var i = 0; i < data.length; i++) {
                data[i]['hostnames'] = results[i];
            }

            handler(null, data);
        });

For anybody interested in understanding following might be helpful: How do JavaScript closures work?