[Notorious]

Networking 101
Topologies, OSI model, the TCP/IP Suite


This part of Networking will be a little more technical than the first, I'll try not to make it a complete lecture though. We'll be going trough more basic networking concepts that form the basis of what we'll be doing later on. Nothing more to say, jump in.

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Topologies
Topologies? What the hell do they have to do with networks? Well, as with anything related to IT, there's a hardware side and a software side. Network topologies function on the physical, or hardware, part of the equation. Let's start.
First thing to know, different topologies (or network structures) have different pros and cons. It's because of these that we choose to use one over another and, in case of hacker-activity, it's because of these that we decide to attack in one way or in another.

Spoiler:

Bus Topology
The bus topology is the simplest one, consisting of various nodes that connect thanks to a single run of data which is called the backbone. To put it simply, and to explain the name, it's like this:
Imagine a road on which a bus goes up. People go from one stop the the other every time the bus passes. The same thing with data.
Easy right? Not so much when you think that it has zero redundancy since losing the road will stop everyone from using the bus and being able to go from one place to another.

Ring Topology
The ring topology is barely ever used since its pros and cons are outclassed by different types of topologies. Nonetheless, it is a topology that some people use and to know about it is a good thing.
Ring topologies are pretty true to their names, as much as the bus topology, since the "road" is a circle. Packets (data) are delivered according to the designated turn and/or availability of the transfer. The pro that this topology has compared to the bus topology is that it can have some redundancy when you make concentric rings (one ring inside another).

Star Topology
Much more used than the first two, the star topology is pretty simple in its theory. Imagine this:
There's only one market in a city. Everybody that lives in the city has to go there to have the food they want.
Translated into technical terms, a star topology has a central hub that every device connects to for their data. This is pretty secure compared to the other topologies since any device, except the hub, can go offline or crash and the rest will still work. The problem exists when the hub goes offline, at that point every node connected to it will be unusable.

Mesh Topology
The most secure of them all, it's designed to be almost indestructible. A mesh network is a more extended version of P2P connection. In a mesh network, every single node connects to every other node directly.
Look at it like this: you live in a friendly neighborhood and every house is facing each other with roads that connect at the center. Whenever someone needs some salt, they just go to any of the houses to ask for it. Even if one of the houses gets hit by an asteroid, there are all the others that have salt.
This is what a mesh topology means. It's usually used for projects that need to be secure in case any of the nodes are compromised or even destroyed.

Hybrid Topology
By far the easiest to understand.
You have a main road with a bus that connects to the market via side-road, to a friendly neighborhood via a second side-road, etc.
Mixing up these various topologies will create the hybrid.

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OSI Model
The OSI model is a framework that allows anything network-related to be designed around standard guidelines. If everyone follows them, it's easier to work together, ok? The OSI model has 7 layers that work together, letting data flow from layer 1 all the way to layer 7. It breaks down various aspects of a network into these layers allowing for administrators to address possible problems at the root and to develop solutions that work on certain levels rather than others. Each layer has its own function that I'll briefly explain here.

Spoiler:

Physical Layer - Layer 1
Can you touch it? If the answer is yes, guess what it's the physical layer. This consists of all networking hardware including routers, switches, cables and everything that comes to mind. Pretty much it addresses everything that you can feel which means cable length, cable type, connectors and so on. Network topologies work on layer 1, the Physical Layer.

Data Link Layer - Layer 2
This layer makes sure that the data is free of errors, it's made for proof-checking because, obviously, transferring wrong data is not effective and can also risk breaking a system. It also starts defining what the packet actually is: size, address and asynchronous delivery throughout the network. On this layer we can define MAC addresses for every component of our network, which are hardwired directly into the NIC (for the definition of NIC and MAC, go here to see my guide on connectivity devices)

Network Layer - Layer 3
This layer decides where the packets (error-free thanks to the data link layer) have to go. To put it simply, your GPS system that tells you where you have to be. It assigns a logical address to every device which can be changed by the network administrators. This address is called IP address and quite frankly, if you go on a forum like this, you've probably already heard about an IP address.

Transport Layer - Layer 4
This layer is where you'll find the TCP/IP protocol, which makes sure that packets got from one device to the other without encountering any errors. How does it do this? It just listens to both end of the communication and has receipts for whether the package was sent or if it was received. When one of the two is negative, it just retries. It does this in a smart manner though, by dividing big packets into smaller ones and then reconstructing it all at the receiving end. Why is this more efficient? Think about it.
(This is your mom, she makes sure you got from point A to point B. Pretty straight-forward)

Session Layer - Layer 5
This layer makes sure that between the client and the server there is an active session which means that the two are actually connected. If it weren't for this layer, you could be sending data into nothing for example. One session is maintained between two computers until the data transmission is over, which means every packet got to the client. While sessions are open there are three types of transmission types:

Spoiler:

Simplex - Data is transferred in only one direction
Half-Duplex - Data flows in both directions, but only one at a time
Full-Duplex - Data flows in both directions at the same time

Presentation Layer - Layer 6
Here the data is translated into a language understandable by layer 7 so that it can be presented correctly. The usual standard for data presentation is ASCII (American Standard Code for Information Technologies). There is also EBCDIC (Extended Binary Coded Decimal Interchange Code) that is/was used by IBM mainframe computers. These two aren't compatible which means that from one to another there would have to be a second translation. The most used one is ASCII, anyways.

Application Layer - Layer 7
This functions as a platform and intermediary between users and software allowing us to access network resources.
What does that mean? This layer deals with the techniques used to run software that require network services. For example:

Spoiler:

DNS - User to translate domain names into IPs
FTP - File Transfer Protocol
SMTP - Simple Mail Transfer Protocol
Telnet - Terminal emulation

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TCP/IP Suite
What? Don't just freak out after seeing the title. It's much easier than you think.
TCP is a communication protocol, literally Transmission Control Protocol, and is used to connect various different devices on a network. It uses the Client/Server model I already explained by providing the client with whatever they ask from the server.
@MrSecurity already explained how this works but let's give it a go via an understandable example and a technical one:
I want to talk with someone so I call their name. They then turn around and say "Yeah, what's up?". At that point I start talking to them since we established a connection.
The same thing happens with networking where the client requesting communication with the server (me calling the person) is called the SYN Packet. Once the server receives this packet (they hear their name being called) it'll send the client a SYN-ACK Packet which means it's ready to communicate with the client. When our happy client receives the second packet ("Hey, what's up?") it sends a third and final ACK Packet to start the TCP communication. Easy diagram which makes everything much simpler:

Code:

Client --> SYN Packet --> Server Client <-- ACK-SYN Packet <-- Server Client --> ACK Packet --> Server Client <--> TCP Requests <--> Server

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Found it interesting but have some questions? Ask me anything and I'll respond as quickly as possible if I feel like it.
Discussion is the basis of anything serious.

[Bish0pQ]

This reminds me of my CCNA lessons. This is a good start if you want to know some basics about networking. Of course there is way more to networking than just this, but this thread definitly gives a good understanding (hence the 101) of how they layers work together and what layer does what.

This will come in handy for people who want to know a little bit about networking. Thank you for sharing this, very much appreciated. Would love to see more threads like this.

[Notorious]

Would love to see more threads like this.

Already on it.

[Kyaa]

Thank you for the guide, worth reading. <3