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diff --git a/network-knowledge-base/dns.md b/network-knowledge-base/dns.md
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diff --git a/network-knowledge-base/port_forwarding.md b/network-knowledge-base/port_forwarding.md
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-# Port Forwarding how to
-
-You want to open something on your network to the world.
+# Port Forwarding Guide
# What is a port
-Ports solves the problem on how to make many applications communicate over
+Ports solve the problem of how to make many applications communicate over
the network simultaneously.
-A port is a number between 0 - 65,535 that gets assigned to any application that wants
-to communicate over the network. This number is then added to every packet
-that is transmitted by that application. The system knows that any respone
-packets with that port number are to be send to that one application.
+A port is **a number** between [1 - 65,535] that gets assigned by the OS
+to any application that wants to communicate over the network.
+This number is then added to every **packet** that is transmitted by that application.
+The system knows that any **respone** packets with that port number belong
+to that one application.
-# How firewall works
+
-A firewall knows direction and state of traffic.
+# How NAT / firewall works
-* It allows outgoing communication on any port.
-* But the incoming traffic gets dropped on all ports, unless it is a response
-to communication initialized from the inside.
+* It **allows outgoing** communication on any port.
+* But **drops incoming** traffic unless it is a response to communication
+ initialized from the inside.
+
+This is not really some security effort, the router just literally has no idea
+where to send it...
More details
-* When you visit some website you initialize the communication.
+NAT is implemented in your router.
+It allows communication between two networks.
+It makes your **LAN side** devices able to connect with the
+outside world - **WAN side,** through one public IP.
+All that the "the internet" sees is one device it communicates with,
+but the LAN side can have hundreds of them.
+
+
+
+When you visit some website you initialize the communication.
+
* Your browser picks a random port as the **source port** and sends a request at some IP
-using a well known https port 443 - the **destination port**
+using a well known https port 443 as the **destination port**
* Then the browser is waiting for a response at that random port.
-* This traffic goes through your firewall and all that info is kept in its state table.
-* This allows firewall to know that when packets start coming from that IP, with that
-source port number now being a destination port, it is a response and let it through.
+* This traffic goes through your router and all that info is kept for a time in its state table.
+* This allows it to know that when packets start coming from that IP, with that
+source port number now being the destination port, it is a response and it
+know where to send it.
+
+Youtube explanation videos if you want deeper dive:
+
+* [NAT - Network Address Translation.](https://www.youtube.com/watch?v=RG97rvw1eUo)
+* [Public IP vs. Private IP and Port Forwarding](https://www.youtube.com/watch?v=92b-jjBURkw)
+# Double NAT (CGNAT)
+
+
+
+**Bad News.**
+It is very likely that even when you do everything 100% correctly,
+you still wont get your ports open.
+The reason being that you are behind double NAT.
+**Your ISP** - internet service provider, has you behind its own NAT device
+and that WAN side of your router is not really "the internet" but ISPs LAN side.
+
+A way to try and check, is looking up your [public IP online](http://icanhazip.com/)
+then loging on your router and finding somewhere the IP address of your WAN interface.
+If they are the same then your are not behind double NAT and port forwarding
+will work straight away.
+If they differ and some local IP is there, then there is still a chance it will work,
+but you wont know till you try.
+
+But if you are failing to make port forwarding work, it's time to call your ISP
+and inquire about public IP, how much would it cost.
+It can be few € extra to your monthly bill, or a one time payment,
+or they just enable it for you for free.. you dunno till you call.
+
# Port forwarding
-What if you want to host something, lets say a minecraft server.
-You set it all up, you have your IP address known to others and they try to connect,
-but your firewall blocks them. Its a connection initialized from the outside.
+Finally. Right?!
-So you need to tell your router/firewall to let through traffic that comes to
+You want to host something, lets say a minecraft server.
+You set it all up, you give your public IP address to others and they try to connect,
+but your router blocks them. It's a connection initialized from the outside.
+
+So you need to tell your router/firewall to let through the traffic that comes to
minecraft default port - `25565` and where to send it on your LAN,
to the local IP of your minecraft server.
-### Examples of port forward rule
+#### Examples of port forward rule
+
+* [Asus](https://i.imgur.com/brs9Mr6.png)
+* [TPlink](https://i.imgur.com/FNS2xCj.png)
+* [ubiquiti](https://i.imgur.com/D04HVJc.png)
How to actually create that port forward rule depends on router/firewall model.
-It can be easy, it can be bit complicated.
Generally what to expect
-* would be called port forwarding or a virtual server
-* **IP address** is a core information, it is your LAN side machine IP on which
- your stuff runs.
-* Another essential is **the port** on which to expect traffic,
- sometimes called a service port or an external port
-* it might offer option for **internal port**, this can be often left empty
- if port on which your stuff on server is running is the same as the one you
- are opening to the world. But this gives you option to open port 3333 of firewall
- but on your LAN machine have port 80 actually being used.
-* **protocol** - TCP or UDP, if dunno **select both / all**, its safer for the initial setup and testing
+* It would be called port forwarding, or a virtual server, or be under NAT section.
+* **The port** on which to expect traffic is obviously a core information,
+ sometimes it is called a service port or an external port.
+* **IP address** is required, so that the router knows where to send traffic
+ that comes to that external port.
+* The setup might offer option for **internal port**,
+ this can be often left empty, or the same port number is put there.
+ It is there to give you option to run stuff on your LAN network on a different
+ port than the one you open to the world. Like your webserver is `80`,
+ but you open to the world port `12250` or whatever.
+* **The protocol** - TCP or UDP, if dunno **select both / all**
+ You don't need to fear you are opening too much, if there is no service
+ running on that port for that protocol it is same as a closed port.
-# Testing if it works
+# Testing if port forwarding works
-### Windows
+
-There are sites that will test if your port is open, but you need to run
-some service at that port.
+First you need to understand that unless there is some application running
+that answers on that port, all tests will come back as - closed port.
-* [Port Listener](https://www.rjlsoftware.com/software/utility/portlistener/)
-* [yougetsignal.com](https://www.yougetsignal.com/tools/open-ports/) or
- [portchecker.co](https://portchecker.co/) or
+For testing we can use websites that will test if a port is open at specified public IP.
+
+* [portchecker.co](https://portchecker.co/)
+* [yougetsignal.com](https://www.yougetsignal.com/tools/open-ports/)
+
+## Windows
+
+
+
+* [Find the ip address](https://www.youtube.com/results?search_query=how+to+find+ip+address+windows)
+ of the machine you are planning to use for the test.
+* Follow the instruction in Port forwarding section of this guide
+ and forward port `666` to the IP of that machine.
+* Download [Port Listener](https://www.rjlsoftware.com/software/utility/portlistener/).
+* Run Port Listener, set port `666`, press Start.
+ * If a windows firewall notification pops up with a question, answer yes.
+* Go to [portchecker.co](https://portchecker.co/), set the port to 666 and press Check.
+
+In windows it is also pretty useful knowing that you can go
+`Task Manager` > `Performance` > `Open Resource Monitor` > `Network` Tab
+
+There unroll `Listening Ports`
+and you should find there - `listener.exe` with port `666` and firewall status
+should be *allowed, not restricted*
+
+
+
+## Linux
+
+* find your ip address - `ip r`
+* Follow the instruction in Port forwarding section of this guide
+ and forward port `666` to the IP of that machine.
+* try running netcat - `nc `
+ * if it is not installed, get it for your distro,
+ for arch it's `openbsd-netcat`, for debian it's `netcat-openbsd`,
+ for fedora it's `netcat`
+* execute `sudo nc -vv -l -p 666`
+* Go to [portchecker.co](https://portchecker.co/), set the port to 666 and press Check.
-### Linux
+## UDP port test
-* netcat
+UDP is kinda special cuz it's session-less, so you need to actually communicate
+through it to test it.
+
+* on a linux machine on LAN - `sudo nc -vv -u -l -p 666`
+* on a linux machine somewhere out there - `nc -u the_public_ip_goes_here 666`
+* write something and it should appear on the other side