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Why does `GlobalScope.launch(Dispatchers.Main)` throw a `Fatal Exception: android.os.NetworkOnMainThreadException` and  `GlobalScope.async` doesn't?

How about `CoroutineScope(Dispatchers.Main).launch`?
See here: <https://codelabs.developers.google.com/codelabs/kotlin-coroutines/#0>

See this explanation
<https://stackoverflow.com/a/46226519/420412>

What does "You MUST NOT forget about the coroutine you’ve started with async." mean?

if nobody never call `await()` you will never know about this error

it actually explained in comments to this answer

&gt; Can you elaborate on “You MUST NOT forget about the coroutine you’ve started with async”? Are there gotchas that one wouldn’t expect for example?

&gt; An uncaught exception inside the async code is stored inside the resulting Deferred and is not delivered anywhere else, it will get silently dropped unless processed.
&gt; If you forget the result of async than it will finish and will be garbage collected. However, if it crashes due to some bug in your code, you’ll never learn about that. That is why

Do they mean like this?

```
val addJob = GlobalScope.async {
    val hostAddress: String
    val ip = InetAddress.getByName(hostName)

    hostAddress = ip.hostAddress

    addIPToMap(hostAddress)
}

runBlocking {
    addJob.join()
}
```

yes, this is an example when you never handle error and do not know that your code is probably broken

if you need result from async, use `.await()` instaed of join, to handle result or errors

And in case of `launch` you should wrap code to try/catch or `runCatching` and handle result of operation, otherwise in case of error you application will crash

Returning to your original problem, `NetworkOnMainThreadException` happens there because you do network requesst (lookup ip address) on the main thread, to avoid this use `<http://Dispatchers.IO|Dispatchers.IO>`