Before you can configure a router, diagnose why "the Internet is down" or design a company's infrastructure, you need to understand what a network actually is, what pieces it is made of, and the vocabulary professionals use to talk about it. This first lesson lays that foundation: we will define what a computer network is, meet its components (hosts, servers, switches, routers, access points...), see which physical media data travels over, and learn the essential terms that will come up again and again throughout the course. We will also introduce Grupo Meridiano, the fictional company whose network we will design and troubleshoot lesson by lesson: by the end of this lesson we will have a complete inventory of its devices.

1. What is a computer network?
2. Benefits of working on a network
3. Network components
4. Transmission media
5. Essential vocabulary
6. Case study: meet Grupo Meridiano and its network inventory

A computer network is a set of two or more interconnected devices that can exchange data and share resources. Nothing more, nothing less. When you send a file to a colleague, print on the office printer or open a web page, you are using a network.

For a network to exist, you need at least three things:

• End devices that want to communicate (computers, phones, printers, servers...).
• A transmission medium that data travels over (a cable, an optical fiber or the air, in the case of wireless networks).
• Common rules that every device understands so they can all "speak the same language". These rules are called protocols, and we will study them in depth in module 2; for now it is enough to know they exist.

A useful analogy: a network is like the postal system. The devices are the people writing letters to each other, the transmission medium is the roads the mail truck drives along, and the protocols are the conventions (envelope format, where the stamp goes, how the address is written) that make it possible for any letter to reach any destination.

Why bother connecting the machines at all? Because a well-built network delivers direct benefits to the business:

• Resource sharing: a single printer, a single storage drive or a single Internet connection serves the whole office. Without a network, 20 employees would need 20 printers.
• Information sharing: files live in one central place everyone can reach, instead of being passed around on USB sticks.
• Communication: email, messaging, video calls... all of it depends on the network.
• Centralized administration: backups, updates and security policies are managed from one point, not machine by machine.
• Scalability: adding a new employee means plugging in one more computer, not redesigning everything.
• Cost savings: a direct consequence of all of the above.

The trade-off, which is worth knowing from day one: a network introduces dependency (if the network goes down, work stops) and security risks (whatever is connected can be attacked). A large part of a network professional's job is minimizing both.

The devices in a network fall into two big families: end devices (the ones that generate or consume data) and interconnection devices (the ones that move data from one place to another).

• Host: the generic name for any end device connected to the network. A laptop, a phone, a network printer or a server are all hosts. Memorize this term: we will use it constantly.
• Workstation / client: the machine a person uses (desktop computer, laptop, tablet, smartphone). It is called a client because it usually requests services: it requests a web page, requests a print job, requests a file.
• Server: a machine (usually more powerful and always on) whose job is to provide services to clients: storing shared files, serving web pages, handling email... A server is not necessarily a huge box in a cabinet: any machine that provides a service is acting as a server.
• Printers and other network peripherals: printers, scanners, IP cameras, IP phones... specialized end devices that connect directly to the network so everyone can use them.

• Switch: the "smart power strip" of the wired network. It has many ports (8, 24, 48...) and the machines in an office plug into it with cables. Its intelligence lies in learning which device sits on each port and delivering data only to the intended recipient, instead of shouting it at everyone.
• Router: the device that connects different networks to each other. The most everyday case: it connects your office network to the Internet. It decides which way data should go to reach its destination, like a signposted road junction. The "router" your ISP installs at home is actually several devices in one (router + switch + access point).
• Access Point (AP): the device that provides Wi-Fi coverage. It acts as a bridge between wireless devices (laptops, phones) and the wired network.
• Modem / ONT: the equipment that adapts the carrier's signal (telephone line, coaxial cable or optical fiber) so the router can use it. With fiber, the little box is called an ONT (Optical Network Terminal).
• Network Interface Card (NIC): not a separate appliance, but the component inside every host that lets it connect to the network, whether over cable (Ethernet port) or over radio (Wi-Fi). Without a NIC, no connection is possible.

A typical small-office layout, with all these elements in place:

Note the hierarchy: hosts connect to the switch, the switch to the router, and the router to the Internet. This pattern will repeat throughout the entire course.

Data has to physically travel through something. There are three big families of media:

This is the classic network cable, the one with the clear RJ45 connector. Inside it carries 8 copper wires twisted into 4 pairs (the twisting reduces interference). It is the most common medium inside offices thanks to its balance of price and performance.

Cables are classified into categories:

An important limitation: the maximum recommended length of a run is 100 meters. Plenty within an office floor, not enough between buildings.

It transmits data as pulses of light through a glass filament. Its advantages: extremely high speeds, distances of kilometers, and total immunity to electromagnetic interference. Its drawbacks: it is more expensive and delicate to install (splicing fiber requires specialized equipment). It is the medium your carrier uses to bring the Internet to the office, and the one used to link buildings or floors with heavy traffic.

Data travels as radio waves through the air. The ever-present example is Wi-Fi, but Bluetooth and mobile networks (4G/5G) belong here too. The obvious advantage: mobility and zero cabling. The drawbacks: speed and stability depend on distance, walls and interference, and since the signal travels through the air it can be picked up by anyone, which makes encryption mandatory.

The rule of thumb professionals apply: cable whenever the device doesn't move (desktops, servers, printers), Wi-Fi for whatever moves (laptops, phones), and fiber for long distances or heavy traffic.

Four terms you need to master right now. We present them intuitively here; in later modules we will study them rigorously.

It is the amount of data a connection can carry per unit of time. It is measured in bits per second: Mbps (megabits) or Gbps (gigabits). Analogy: the number of lanes on a highway. More bandwidth = more data at once.

Watch out for a classic mix-up: ISPs sell megabits (Mb), but files are measured in megabytes (MB), and 1 byte = 8 bits. A 600 Mbps connection downloads, at its theoretical best, 75 MB per second — not 600.

It is the time it takes a piece of data to travel from source to destination, measured in milliseconds (ms). Staying with the highway analogy: it is not how many lanes there are, but how long the trip takes. A connection can have huge bandwidth and still suffer from poor latency (and vice versa). Bandwidth matters for downloading large files; latency matters for video calls, remote desktops or online gaming, where immediacy is what counts.

For data to reach its destination, every host needs a unique address within its network, just as every house needs a postal address. That is the IP address. It looks like this: 192.168.1.20. It is a logical address: it is assigned through configuration and can change (take your laptop to another office and it will get a different IP). Module 5 is devoted entirely to IP addressing; for now, hold on to the idea of "the device's postal address on the network".

Every network card leaves the factory with a unique identifier burned in, the MAC address, which looks like this: A4:5E:60:B2:1F:C3. It is a physical address: it comes built in and doesn't change even if you move the machine to another network. Analogy: the IP is your postal address (it changes when you move house); the MAC is your national ID number (it stays with you for life).

You can see both on your own machine right now:

In the output you will see something like this (trimmed and annotated):

No need to understand the rest of the output yet: in module 6 we will learn to interpret all of it.

Grupo Meridiano is a 25-employee consulting firm with two sites:

• Main office in Valencia: 20 people (consultants, administration and management).
• Bilbao branch: 5 people (the northern sales team).

The company has just hired us to review and redesign its network, and will use the example domain grupomeridiano.example for its services. The first thing any professional does before touching anything: take inventory of what is there. Here is the result:

Valencia site (20 employees):

Bilbao branch (5 employees):

Notice how the inventory applies everything we have covered: every device is classified as host or interconnection, and the transmission medium follows the rule of thumb (cable for what stays put, Wi-Fi for what moves, fiber for Internet access). The questions this inventory leaves open — what type of network does each site form? and how do we connect the devices, and the two sites, to each other? — are exactly the next two lessons.

• Confusing megabits with megabytes. "I pay for 600 megs but I only download at 70" is not a fault: 600 Mbps ≈ 75 MB/s. Always divide by 8.
• Confusing switch and router. The switch connects machines within the same network; the router connects different networks. When in doubt, ask yourself: "does this box send data out to another network (Internet included)?" If yes, it's a router.
• Believing more bandwidth fixes latency. They are independent quantities: paying for more "megs" won't fix a video call that stutters because of latency.
• Thinking Wi-Fi "is the network". Wi-Fi is just an access medium; behind the access point there are almost always cables, switches and routers. When "the Wi-Fi is down", the problem may be in any of those pieces.
• Pro tip: faced with any new network, always start with an inventory like Grupo Meridiano's. You can't diagnose or design what you don't know.
• Pro tip: cable everything that doesn't move. Wi-Fi is convenience; cable is reliability.

Classify each of these items as host (client), host (server), host (peripheral), interconnection device or transmission medium: (a) the phone of a Bilbao sales rep; (b) the 24-port switch in Valencia; (c) the machine that stores the shared files; (d) the Cat 6 cable linking a PC to the switch; (e) the multifunction printer; (f) the Wi-Fi access point.

For each situation at Grupo Meridiano, pick the most suitable transmission medium (twisted pair, optical fiber or wireless) and justify it in one sentence: (a) connecting the file server to the switch; (b) providing connectivity for the 6 consultants' laptops, which move between meeting rooms; (c) the line bringing the Internet from the carrier to the Valencia router; (d) connecting the Bilbao network printer, which never moves from its spot.

A Valencia consultant complains: "We pay for 600 Mbps and my video calls still keep dropping — the ISP is ripping us off!". Meanwhile, he downloads large files without a hitch at about 70 MB/s. (a) Is the download speed consistent with the contract? Do the math. (b) Is the video call problem a bandwidth issue or a latency issue? Why?

Solution 1: (a) client host; (b) interconnection device; (c) server host (it provides a service: storing files); (d) transmission medium; (e) peripheral host; (f) interconnection device (it bridges wireless devices to the wired network).

Solution 2: (a) Twisted pair: the server is stationary and a few meters from the switch; cable gives maximum reliability. (b) Wireless (Wi-Fi): the laptops move around; mobility is the main requirement. (c) Optical fiber: it is the carrier's medium for covering the distance to the office with high bandwidth. (d) Twisted pair: it could run over Wi-Fi, but it is a stationary device, and the professional rule is to cable everything that doesn't move.

Solution 3: (a) Yes: 600 Mbps ÷ 8 = 75 MB/s theoretical maximum, and he downloads at 70 MB/s — practically the maximum. The ISP is delivering. (b) It is a latency problem (or an instability problem), not bandwidth: video calls consume relatively few Mbps but demand that data arrives fast and steadily. Buying more bandwidth wouldn't fix it. Common mistake this exercise illustrates: assuming "more megs" solves any network problem.

We now have the foundations: a network is a set of devices that share data and resources over a transmission medium using common rules. We can tell end devices (hosts, servers, printers) apart from interconnection devices (switches, routers, access points), choose between twisted pair, fiber and wireless as the case demands, and handle the basic vocabulary: bandwidth, latency, IP address and MAC address. Above all, we have inventoried the network of Grupo Meridiano, our working case for the whole course. The next question practically asks itself when you look at that inventory: the Valencia office network, the Bilbao branch network and the Internet are not "networks" of the same type or the same size. How are networks classified by reach and by organization? That is exactly what we will cover in the next lesson, Types of Networks.