Network Devices

To be able to understand a packet's journey through a network, you must be familiar with the basics of network devices. You’ve probably seen or heard the phrase “IoT”, which stands for Internet of Things.

It’s a reference to how drastically the internet has changed in the past few years. Today there is almost no limit to what can be connected to the world wide web such as: smartwatches, refrigerators, lightbulbs etc. Regular computers and laptops will perhaps one day become a minority of internet traffic.

In this article we will take a look at some of the key players of the internet that handle all that network traffic.

First things first - what even is a network? Two laptops directly connected to each other can be considered to be a network. It’s a tiny and restricted network, but it fits the definition.

However, most networks are grouped together by a similar function or physical location. Your home router with devices connected to it - is a network. The café with a WiFi hotspot - is a network. The classroom full of computers - is a network. You get the point.

On the other hand large corporations most likely have multiple networks, which is usually separated by job role. For instance, a company might have one network for the HR department and another for sales. There are numerous advantages for this such as ease of management, security and scalability.

Worth mentioning - the icons used for network devices are standardized, which means everybody in the networking field uses them. You’ll know what it is the next time you see the icons somewhere.

If you’re completely new to the networking field then the word “interface” can be misleading. Usually people think it’s referring to a user interface with buttons and graphical elements. However in the networking world it’s always used to describe a network port. It does not matter whether it’s physical or virtual. Everytime you read the word interface, think of a physical network port.

From a networking point of view, a host is simply any sort of end-device on the internet that has an IP address. In the old days only traditional computers were considered hosts, but today that includes smart TV’s, voice assistants, smartwatches etc. A host is any device which might be the original initiation of traffic or the final destination of traffic.

Network traffic where two hosts communicate with each other is often labeled as client/server computing. The client is the one requesting a service such as a web page and the server is the one to respond and deliver the web page to the client.

A node is a very generic term, which usually is a reference to anything connected to a network. That would usually mean a switch, router or firewall. It's not uncommon to hear this phrase: “Oh no a node is not responding”.

The main purpose of a network switch is to facilitate communication within networks. Switches operate at layer 2 of the OSI model, where it looks at the layer 2 header to enable hop-to-hop delivery. The layer 2 header contains a source and destination MAC address.

To enable host to host communication, the switch keeps a MAC record of each host that is connected to it. The records are stored in the MAC Address Table. In order to forward a frame to the correct host, the switch will look up the destination MAC address and compare it to its own MAC Address Table. We'll go more into detail with examples on how a switch works in this article.

The main purpose of a router is to facilitate communication between networks. That usually means that each interface is part of a different network.

Routers operate at Layer 3 of the OSI Model, which means it looks at the Layer 3 header. What makes end-to-end delivery possible is the layer 3 header which contains the source and destination IP address.

But how does the router know where the destination IP address is? For that the router creates its own table that is conveniently named the "Routing Table". The router has its own table, just as the switch has its own MAC address table.

The difference between the two is that switches use the table for local traffic within a network, whereas the routers table is utilized for traffic between networks. We'll go more into detail with examples on how a router works in this article.

Regular people usually think of a firewall as software running on their PC. The software is either installed manually or it’s integrated in the operating system itself. From a networking perspective it’s always seen as a physical firewall. Yes, you can run a virtual firewall in a hypervisor, but that is out of scope of this article. Here we will focus on a physical machine running firewall software.

Firewalls are typically deployed at the edge of a network, meaning it’s the last barrier between the Local Area Network (LAN) and the Wide Area Network (WAN). It’s the first line of defense between the secure and trusted LAN and the untrusted outside WAN.

The main purpose is to provide network security by monitoring incoming and outgoing network traffic and decide whether to allow or block specific traffic based on a defined set of rules. There are many different types of firewalls and how they approach network filtering. More advanced firewalls today inspect all the layers of the OSI Model.