added section about IP, sockets, DNS and NAT

chapters/network/chapter.md

@@ -2,17 +2,16 @@
 
 
 *by [Arturo Castro](http://arturocastro.net)*
-
+*extensions by tpltnt*
 *corrections by Brannon Dorsey*
 
+This chapter gives a brief overview over the things you need to know to use computer networks with openFrameworks. Do not be afraid if you do not understand all the concepts. The idea is more to give you a basic understanding, some terminology and references so you can explore this topic and go deeper when you want (or need) to.
 
 ## IP and NAT
 
-Something important that you might know already: in order to connect applications through a TCP/IP transport network protocol, you usually need, an IP address and a port, the IP address is specific to each machine, the port to each application. So with an IP/port pair we can define an application running in a specific address along all Internet, almost. For example an application that is running a web server usually runs on port 80, if the machine is being executed in has the IP 15.6.8.2, 15.6.8.2:80 defines that web server among any other application running in any other machine in all the Internet. So if we want to connect two machines, usually all we need to know is the IP and port of the application running in the server and use it in the client to connect to it.
-
-There's an exception though. In most internal networks, like your home network for example, there's a router that connects the machines in that network to the Internet. These routers usually do something called NAT: Network Address Translation. NAT was invented because the IPv4 protocol has a limited number of IP addresses. Internally, the network uses a reserved range of addresses: 192.168.x.x/24 or 10.x.x.x/32, which are addresses that won't be found directly on the internet. When we try to connect to an external address it acts as a kind of proxy between your computer and the server that we want to connect. The router has it's own external address, and when it receives a response it translates the address and port in which it has received it to an internal one, the one from our computer and sends the packets back to us.
-
-While this is really practical, it means that if we have 2 computers behind NAT routers, it's impossible to open a connection between them (in principle) . There's ways to configure a router to send any packet sent to a specific port to the same internal address. There's also libraries like [ofxNice](https://github.com/arturoc/ofxNice) that allow you to do NAT transversal, but that will only work using UDP.
+In order to connect to a computer on the internet or your local network you need to be able to identify the machine. This is done by what is called an [IP address](https://en.wikipedia.org/wiki/IP_address "Wikipedia on IP addresses"). This is label is used as the source and destination address when sending data packets. These data packets encapsulate chunks of the information (like a file or a text string) you want to send (later more on that). In order to send data to a particular application, you also have to tell the machine where your data packets and the desired application meet. This meeting point is called a ["port"](https://en.wikipedia.org/wiki/Port_%28computer_networking%29 "Wikipedia on ports (for networking)"). For reference: The combination of an IP and a port is called a ["socket"](https://en.wikipedia.org/wiki/Network_socket "Wikipedia on network sockets"). IP only handles only the routing part of networking while other protocols handle other functionality. TCP and UDP for example handle the data transmission (see below). All this functionality is often talked about in terms of "layers" (of abstraction) and the [OSI model](https://en.wikipedia.org/wiki/OSI_model "Wikipedia on the OSI model").
+The IP protocol comes in two flavors: [IPv4](https://en.wikipedia.org/wiki/IPv4 "Wikipedia on IPv4") (still very common), where addresses look like "93.184.216.34" (so called ["dot-decima"](https://en.wikipedia.org/wiki/Dot-decimal_notation "Wikipedia on dot-decimal notation") or "dotted quad" notation) and [IPv6](https://en.wikipedia.org/wiki/IPv6 "Wikipedia on IPv6") (the current version), where addresses look like "2001:500:8c::53". An application ususally listens on a predetermined port and waits for data to arrive. For some applications (like webservers) these are (informally) [standardized](https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml "IANA list of port numbers") (like ports 80 and 443), others (like maybe yours) just picks a free port number. As long as you do not pick one already in use, this is not (too much) of a problem. No two processes can use ("bind to") the same port number (and protocol), so there is only one application listening on port 80 for TCP connections (but another one might listen on port 80 for UDP connections). This is usually enforced by the operating system. The combination of IP and port is written as "93.184.216.34:80" in the case of IPv4 and "[2001:500:8c::53]:80" in the case of IPv6. Since people are bad at remembering long strings of random looking numbers and characters, there is a system in place to map [domain names](https://en.wikipedia.org/wiki/Domain_name "Wikipedia on domain names") into IP addresses. It is called [Domain Name System or DNS](https://en.wikipedia.org/wiki/Domain_Name_System "Wikipedia on the Domain Name System"). Here the IP/port pair (or socket) is notated as "[example.net]:80".
+Not all IP addresses can be reached. There is a [range of IPv4 addressed](https://tools.ietf.org/html/rfc1918 "RFC 1918: Address Allocation for Private Internets") and a [range of IPv6 addresses](https://tools.ietf.org/html/rfc4193 "RFC 4193: Unique Local IPv6 Unicast Addresses") which are called "private". That means that your [router](https://en.wikipedia.org/wiki/Router_%28computing%29 "Wikipedia on routers") will not pass on packets coming from these ranges into the internet. Your computer at home probably has a private IPv4 address of "192.168.x.x" or "10.x.x.x". That means that machines in your local network can talk to it, but no one from the outside. Your router as a public IP address and translates the packets with your requests to outside resources (e.g. when you surf the web). The router does so called "Network Address Translation (NAT)"](https://en.wikipedia.org/wiki/Network_address_translation "Wikipedia on Network Address Translation"). It takes addresses from one address range (your private one) and translates them to another one (the public range) back and forth (for an established connection). Since your ["Internet Service Provider (ISP)"](https://en.wikipedia.org/wiki/Internet_service_provider "Wikipedia on Internet Service Providers") assigns the public IP to your router, you have litte control over it. This in itself is quite useful from a management perspective, but it also means that two devices behind NAT usually can not talk to each other (since they are ususally not part of DNS and so called ["port forwarding"](https://en.wikipedia.org/wiki/Port_forwarding "Wikipedia on port forwarding") is not enabled by default on your router). To get around this you either have to enable port forwarding and know the IP of your destination router or use addons like [ofxNice](https://github.com/arturoc/ofxNice) that allows you to do NAT transversal (but it will only work for UDP).
 
 
 ## TCP vs. UDP