var urlPart1 = "http://192.168.1.";
var urlPart2 = "/secret_file.txt";

for(i = 0; i < arr.length; i++) {
    var xmlhttp = new XMLHttpRequest();

    xmlhttp.onreadystatechange = function() {
        if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
            if (xmlhttp.responseText == "secret_text") {
                weHaveOurAddress(i);
            }
        }
    };
    xmlhttp.open("GET", urlPart1 + i.toString() + urlPart2, true);
    xmlhttp.send();
}