#routing

ChatGPT deny access when connected via VPN (Virtual Private Network) such as WireGuard or OpenVPN, and you will be blocked with the message. Here is how to skip ChatGPT (or any other domain/IP address from WireGuard or OpenVPN on Linux.

Vom 6. Stock über Wisconsin in den Keller

Diese Mail befindet sich in meinem bis 1985 zurückreichenden Archiv mit allen ca. 150.000 zwischen damals und 2022 von mir gesendeten und empfangenen Mails.

Anscheinend fanden wir es damals (also Anfang 1991) bemerkenswert, dass eine Mail von der TU-Berlin in die Firma meiner damaligen Freundin in der Kantstraße nur 20 Minuten unterwegs war.

Und dass eine Mail von meinem ehemaligen Büro im 6. Stock des Informatik-Gebäudes zum Großrechner im Keller desselben Gebäudes über Wisconsin geleitet wurde, würde heute bestenfalls ein Stirnrunzeln hervorrufen. Damals dagegen war es etwas, von dem ich glaubte, ich müsste später meinen Enkeln davon berichten.

Hätte ich Enkel, würden sie heute wahrscheinlich nur mit den Schultern zucken oder gar nicht verstehen, wovon ich rede.

I have spent the last month going through the Cisco Network Academy learning the essentials of networking and communication technologies. I learn a lot easier when I write things down so here is my best attempt at giving you a thorough guide to networking and communication. Below is everything that I have learned so far - condensed for your ease.

Networks

The internet is a huge network of interconnected servers, hosts, routers, switches, peripherals and IoT gadgets. They are all connected via wired or wireless technologies such as Ethernet, Fibre Optic, Coaxial cable, Bluetooth, NFC, etc.

A LAN is the smallest type of networking and it usually consists of a host computer, a router and perhaps a printer or some other peripherals. These are often found in your home. A WAN is a wider network that can be accessed by many different LANs but is still not accessible to the wider network. You can think of a WAN as many LANS connected together. (Think of a University network).

Switches - these are devices that are responsible for the movement of data traffic on a local network (when the data is moving from one local host to another on the same network). 

Routers - these devices are responsible for the movement of traffic from one network to another (and this can include destinations across the internet).

Most home ISP routers will also include a default gateway server (which wraps up data packets in the IPV4 packet ready to send over the internet), a DCHP server (which is used to allocate local logical addresses for hosts on a local network) and a switch all at once.

Data Transmission

One of the most fascinating aspects of networking is how the data is transmitted across huge distances in such as a small amount of time. And it’s even more impressive when you learn that the medium over which the data travels is constantly changing throughout its journey. For example, you may send data to a server in Africa and it may travel through fibre-optic, ethernet, co-axial, satellite transmission to get to its destination. 

These various mediums of data transmission rely on a variety of different techniques to transmit this information. For example, ethernet and coaxial cabling relies on electronic impulses sent through the wiring to read and send binary data. The same can be said about fire-optic cables but instead they use light beams which also pulse quickly to create binary data.

You can use light at varying frequencies to work together at the same time without interfering with each other. For example, let’s say that we have a fibre optic cable - inside this cable there is one glass tube sending data back and forth to two computers. We could send just one flash of white light every tenth of a second or so and this would transmit data (albeit at a very slow rate) - but we have developed a technique called dense wavelength division multiplexing (DWDM) which splits the light into different frequency bands (for example, one light is blue light, the other red, the other yellow). This way, we can send three streams of data down the same tube which increases the data transmission rate by three times!

Packet Switching and Encapsulation

As previously mentioned, the internet is a network of a variety of different mediums and media which travel great distances and use a huge variety of different technologies. So how can we ensure that data gets to the right place and in one piece? We use protocols.

Protocols are a set of rules (usually set up by a authority like the IEEE) that dictate how data should be communicated across the internet. This way, even if the computer is ten years old it can still communicate with new computers because they understand the same rules of communication. Much like how humans have interpersonal ptorocols such as a hand shake, saying hi, eye contact, saying goodbye etc. 

There are a variety of protocols that make the internet work but here is a quick breakdown:

MAC / Ethernet - MAC protocol is used for the data communication between two NIC cards on the same network. For example, if you are sending information from your PC in the bedroom to a PC in your living room it is using the MAC protocol. 

IP/ TCP - The IP protocol is designed for data transmission over different networks and is mostly associated with the internet. TCP is a protocol that runs at the same time as the IP protocol - this protocols job is to ensure that data transmission has been received in order and no packets were dropped.

HTTP - HTTP is a protocol that is at the ‘Application’ layer of the protocol stack (something I will show you later). This protocol is designed to provide the client with information such as HTML and media. (basically anything you see, read or hear on a website).

Networking devices rely on a layered service architecture when transmitting data. What this means is that there are a number of layers that are co-existing that are in charge of different aspects of the transmission journey to ensure that it goes smoothly. 

Along its journey from your local computer to a destination server your data may encounter various mediums and will abide by different protocols along the way in a process known as ‘encapsulation’. Look at the image below: 

Here we have a ‘protocol stack’ diagram. The physical section of this diagram has dotted lines because although it does feature protocols (such as the ethernet protocol) it is usually hardware and we can ignore that for now. Let’s just assume that physical means hardware such as ethernet cables, NIC cards, ethernet ports etc.

The ‘link’ element of the stack would include the MAC protocol which uses ethernet to send data from one local host to another. 

So the computer will create an ethernet packet with a destination MAC address (computer 2) and a source destination address (computer 1) in its header. In the body (or payload) it will hold the actual information that is being shared. This packet is called an ethernet ‘frame’. You can see how the frame would look below: 

When we are sending information back and forth to to other computers on the internet this works just great. Usually (due to DCHP servers) the switch will have in its routing table the MAC address of most of the hosts that are directly connected. 

But if we wanted to send information to a computer that was on a different network then we would need to move up the layer architecture and use the IP/TCP procotol. So the computer will take the ethernet frame above and send it to the Gateway server (this server is responsible for encapsulation and de-encapsulation). 

When the ethernet frame gets to the gateway server the ethernet frame (above) is put into an IP container (imagine russian dolls). The IP frame now uses a IP address of the source computer as its source and the IP address of the destination IP is used as the destination. This IP packet wraps around the ethernet frame and keeps it safe as it travels across the internet.

Now as this router sends the IP packet to the destination it may not be able to do that in one “hop” (jump from one network to the next). So it looks for the next router on its journey and finds its MAC address. It takes its own MAC address and replaces the old source MAC address with its own and replaces the old source destination MAC address with the next router. The IP source and destination stay the same throughout the journey. So you can imagine that data travelling across the internet is basically from router to router and at each meeting they change the old MAC addresses to the news one required for the next hop.

Global and Local IP Addresses

There are two different types of IP address that we work with on a daily basis when using the internet. You may have noticed that the local IP address for your computer is or close to a friends computer at his house? Perhaps it looks like 192.168.0.119 or something similar to this. This is because when we are connected to a LAN network (for example, two laptops connected to your home ISP router) the two computers on this network are automatically assigned a local IP address that is the same for every router across the world. The range that is assigned for most home, small businesses is 192.168. 0.0 — 192.168. 255.255.

The ISP router that you are connected to actually has a public facing IP address that can be seen by other networks and the internet. This is what’s called a global IP address. This is what servers like google.com send data traffic to when you request a search on their website. It is the routers job to then distribute that information via MAC procotol to your computer. This process is called Network Address Translation (NAT) and is used by the router whenever it receives data from an external source.