Link Layer

Welcome back! Let's continue from where we stopped in our previous post. The lowest level of the internet protocol suite is the link layer. This layer is the physical hardware layer of the protocol. This layer is the grandfather to all other layers. It bears the name link because it provides the link or protocol for communication to be possible amongst the layers. Each of the other layers use this link or protocol to communicate. Computers in the internet world are used by humans.Humans are referred to as clients, the computers they use are referred to as devices. Hence the name client device. So, each layer relates to the client device. In the process of providing a link, the link layer gives an address called network address. Hence, each client device on this link has a network address known as media access control address(MAC address). This address is normally found on the hardware of the link layer. Any device that has this address will be connected to the network. Remember the link layer carries a link and this link has data in it. This data is what the client device needs. This link layer of data is divided into small units known as frames. Each frame has a header, frame data, and frame footer. See it below

A link layer frame The frame header contains the MAC address and describes the type of data carried in the frame data section. One specific example of a link layer network is the Wi-Fi network. The Wi-Fi network provides the link for data to move to the client device through the other layers. Another specific example of a link layer is Ethernet.While the Wi-Fi uses wireless network connection, Ethernet uses wire(cable made of copper wires or fiber optics) for physical connection. All client devices like laptops, smartphones, get their link to the internet or network via the link layer. These devices can also be referred to as hosts. Hence, these hosts participate or follow the protocol of the rest layers. But certain networking devices operate at the link layer only. One example of this type of device is called a hub. A hub is a networking device that connects multiple devices on a local network. An Ethernet with ports for connecting devices is a hub. Remember these link layers receive frames of data and send it to client devices. The hub simply retransmits every frame it receives on one physical port to all its other ports. Another link layer example is a network switch. The network switch sends out frames that contain MAC addresses to the physical port where a device with the destination MAC address is connected.

Internet Layer

This layer is a layer of data with a protocol called internet protocol(IP). Every client device connected on this layer has an IP address. This number uniquely identifies the host(client device) on the internet. The type of data carried by the internet layer is called packet data. It is called a packet because it is enclosed in the link layer frame. The internet layer is also divided into the header which carries the information that describes the packet such as the IP version and the header length. The data section contains the payload data that the IP is carrying. See the diagram of both the link layer and the internet layer below

The internet layer makes use of two versions of Internet Protocol. They are; Internet Protocol Version 4 (IPv4) and Internet Protocol Version 6 (IPv6). IPv4 is the dominant version in use. The clear difference between IPv4 and IPv6 is the size of their IP address. An IPv4 address is 32 bits in length, whereas IPv6 address is 128 bits. A 32-bit IP address is typically represented in dotted decimal notation separated into four groups of 8 bits each. An example of IP address displayed in decimal notation is 192.168.1.23. Note that the 8-bit numbers are displayed in decimal rather than hexadecimal or binary. Client devices that are on the same local network and have the same leading bits of IP addresses are said to be on the same subnet. Computers on the same subnet are able to communicate with each other at the link layer. However, computers on different subnets must send their traffic(data) through a device called a router. A router connects subnets of different local networks to the internet layer for communication or transmission of traffic(data). Computers(hosts) on the same subnet have their IP address divided into two parts; the network prefix, which all the computers on the subnet share, and the host identifier, which is unique to each computer(host) on that same subnet. See the diagram below of an IP address with a 24-bit network prefix.

An example of IP address using a 24-bit network prefix

Next we shall discuss the other layers in our next lesson. See you there!