added info for when behind a NAT

content/instant-messaging/server-support-for-audio-video-calls.md

@@ -28,15 +28,15 @@ To make your server communicate make sure following ports are open in your firew
     :::console
     3478 (TURN)
     5349 (TURN + TLS)
-	49152-65535 (UDP endpoints for clients)
+    49152-65535 (UDP endpoints for clients)
 
 Set up Coturn
 ---
 
 First install Coturn
 
-	:::console
-	$ sudo apt-get update && sudo apt-get install coturn
+    :::console
+    $ sudo apt-get update && sudo apt-get install coturn
 
 After installing first make a backup of the existing configuration:
 
@@ -60,7 +60,8 @@ The file looks like this:
     listening-port=3478
     tls-listening-port=5349
 
-    # Your external IP, needed for some connections
+    # Your machine's IP addres
+    # This is either the external IP or, in the case you are behind a NAT, the IP of the machine in the NAT
     listening-ip=CUSTOMIZE THIS
 
     # Your domain name
@@ -92,10 +93,15 @@ The file looks like this:
 
 Now time for some config. You need to edit `/etc/tunserver.conf` in a few places.
  
-First, add your external IP-address to `listening-ip`. If you don't know it, you can find out using the following command:
+First, add your servers' IP-address to `listening-ip`. 
+
+In case you are behind a NAT, for example when you are hosting from home and are [making use of port forwarding]({filename}../fundamentals/port_forwarding.md) this will be a local IP adress. If your machine is directly exposed to the internet, that is the external IP-addres. 
+
+If you don't know it, you can find out using the following command:
 
     :::console
-    $ curl https://ifconfig.co
+    $ ip --oneline addr show primary | grep -E '(en|eth)'
+   
 
 After that make sure `realm` points to the domain name of the server.