Javascript to find modems.

findmodem.js

function findModems(baseIP='192.168.0', lowIP=0, highIP=255, maxInFlight=20, timeout=4000) {
	var currIP = lowIP;
	var inFlight = 0;
	var modems = [];
	var path = '/ws/';

	return new Promise( resolve => {
		function tryOne(ip) {
			++inFlight;
			var socket = new WebSocket('ws://' + baseIP + ip + path);
			var timer = setTimeout(() => {
				try {
					socket.close();
				}catch(e){
					//
				}
				--inFlight;
				next();
			}, timeout);
			socket.onopen = () => {
				clearTimeout(timer);
				modems.push(socket.url);
				--inFlight;
				next();
			};
			socket.onerror = () => {
				clearTimeout(timer);
				--inFlight;
				next();
			};
		}

		function next() {
			while (currIP <= highIP && inFlight < maxInFlight) tryOne(currIP++);
			if (inFlight === 0) resolve(modems);
		}

		next();
	});
}

findModems('192.168.0.', 1, 255, 20, 4000).then(modems => {
	if (!modems) console.log('No modems found! :( ');
	modems.forEach(m => {
		console.log('Modem found at ' + m);
	});
});