Network models

- [Narrator] I'll start with the TCP/IP model. It consists of five layers that provide a set of rules for network devices to follow. I like to start at the bottom and work my way up as this is usually how I troubleshoot. The physical layer consists of the physical hardware that makes connections. This will be cables, their connectors, and everything related to how light travels over fiber or electricity moves over a cable. So when I reference a layer one issue, that is going to be a physical issue like a bad cable. The data link layer, layer two, deals with MAC addressing. It structures its data into frames, and each frame has a header that contains various information like which MAC address it's sourced from, and which MAC address it's destined to. As information travels down the model it is encapsulated and handed to the next layer for processing. When a router wants to send a packet it encapsulates it and hands it to layer two to be delivered to the next device's MAC address. As a note, as packets move up the model they are decapsulated. Switches are usually associated with transporting information at layer two. Layer three is the network layer, which deals with IP addressing. IP addressing is usually described like the postal system. IP addressing is a means for me to send a letter from my house in Texas to my friend in LA, and have it actually be delivered on the internet. Just like the addresses of your house it must be globally unique. Devices associated with layer three are generally routers. Routers in essence play hot potato with packets. Once they know which direction they should go, they pass them to the person directly next to them and move on to the next packet. Layer four is the transport layer, which deals with Transmission and Control Protocol and User Datagram Protocol. TCP and UDP are examined in more detail in a following chapter. But a high level, they specify ports which range from one to 65,535 to transmit information. Specific port numbers can map to applications like unencrypted HTTP which uses TCP port 80 or TFTP which uses UDP port 69. Layer five, the final layer, is application. This deals with how software running on PCs and servers communicate with lower layer protocols. For example, how a browser uses HTTP to pull information from a web server. The data structure used to move information has different names depending on where in the model it is. At layer two it's known as a frame, at layer three it's a packet, and at layer four it's a segment. There is another model, an arguably even more popular one known as the Open Systems Interconnection. Conceptually it's the same as the TCP/IP model with the bottom four layers matching one for one. It's the higher layer protocols that happen to be broken out into additional layers. The majority of networking configuration and troubleshooting is done in the bottom four layers. Understanding an utilizing these models is important for modern engineers as they serve as a common language when discussing where issues may be happening in a network.