Don't resolve localhost if on network (#2332)

* Don't resolve if on network

* Update WebpackDevServerUtils.js

packages/react-dev-utils/WebpackDevServerUtils.js

@@ -193,10 +193,25 @@ function resolveLoopback(proxy) {
   if (o.hostname !== 'localhost') {
     return proxy;
   }
-  try {
+  // Unfortunately, many languages (unlike node) do not yet support IPv6.
+  // This means even though localhost resolves to ::1, the application
+  // must fall back to IPv4 (on 127.0.0.1).
+  // We can re-enable this in a few years.
+  /*try {
     o.hostname = address.ipv6() ? '::1' : '127.0.0.1';
   } catch (_ignored) {
     o.hostname = '127.0.0.1';
+  }*/
+
+  try {
+    // Check if we're on a network; if we are, chances are we can resolve
+    // localhost. Otherwise, we can just be safe and assume localhost is
+    // IPv4 for maximum compatibility.
+    if (!address.ip()) {
+      o.hostname = '127.0.0.1';
+    }
+  } catch (_ignored) {
+    o.hostname = '127.0.0.1';
   }
   return url.format(o);
 }