fixes network mistakes

modules/resources/exercise_networking.md

@@ -54,24 +54,35 @@ Who gave you this second address?
 Can the client ping both IP addresses?
 Can the client now ping outside of the network?
 
-There are **two** main things you need to do in order for the router to actually route.
+The *easiest* way to achieve routing between your internal network and the outside world is to enable [NAT](https://en.wikipedia.org/wiki/Network_address_translation) on your router.
+In order to do this, you need to do 2 things.
+First enable the kernel to actually forward packages, secondly `iptables` needs to do masquerading.
+You can do both these things with just to simple commands on you router.
 
 1. IP forwarding needs to be setup on the router
 2. NAT needs to be enabled
 
+From here on out all your clients *should* have internet access but you won't be able to `ping` your clients from outside your network.
+Your clients can ping each other and the internet at large but for clients **outside** of their mini network the router will **masquerade** the IP address.
+This means that from outside your network, you'll never be able to ping a specific client, just the router itself.
+
 ```bash
 sysctl net.ipv4.ip_forward=1
 iptables -t nat -A POSTROUTING -j MASQUERADE
 ```
 
 These settings won't save themselves so next time you reboot they'll be missing.
-There are multiple ways of setting this up.
+For the kernel option you should have a look at `/etc/sysctl.conf`.
-Have a look around and find me a good solution!
+In this file you can enable, disable or set kernel values.
+
+To save `iptables` rules have a look online but [this](http://www.faqs.org/docs/iptables/iptables-save.html) and [this](https://zertrin.org/projects/iptables-persistent/).
 
 ### Extra Challenge
 
 Your client machines are now behind a NAT.
 Can you think of a way to ssh into them?
+As you can only ping the router from outside of the network you'll have to setup [port forwarding](https://en.wikipedia.org/wiki/Port_forwarding).
+On Debian this is done with [iptables](https://serverfault.com/questions/532569/how-to-do-port-forwarding-redirecting-on-debian#532575).
 
 ## DHCP