Lesson 1: What a Network Is

Why start here

Every support ticket that says "the internet is down" ends up with someone answering three questions: is this computer connected, what address does it have, and can it reach anything? By the end of this level you'll be able to answer all three from a terminal in under a minute. First, though, you need a picture of what a network actually is.

What a network is

A network is two or more devices connected so they can exchange data. Three things make one up:

• Devices (also called hosts or nodes): laptops, phones, servers, printers, smart TVs. Anything that sends or receives data.
• Links: what carries the signal between devices. That can be a copper Ethernet cable, a fibre, or radio waves (Wi-Fi).
• Network gear that joins the links together. The two kinds you'll meet first are switches and routers.

To picture it, think of a postal system. Devices are houses, links are roads, and network gear does the job of sorting offices.

• A switch connects devices within one local network, such as the desks in an office. It's like the sorting office for one town: it delivers between houses on the same local roads.
• A router connects different networks together and picks a path between them. It's the office that sends mail out of town. Your home "router" is usually a router, a switch and a Wi-Fi access point in one plastic box.

This level only needs the rough roles. How switches and routers do their jobs comes later in the course.

Packets: data in small parcels

Networks don't send a 5 MB photo as one continuous stream. They split it into packets, small chunks of usually no more than about 1,500 bytes, and each packet carries a label saying where it came from and where it's going. That gives you three practical benefits:

1. Sharing: packets from many conversations take turns on the same link, so one big download doesn't freeze everyone else.
2. Resilience: if one packet is lost, only that piece has to be sent again, not the whole file.
3. Flexible paths: routers can send each packet the best way available at that moment.

The receiver puts the packets back together in order. You'll see this for real when you use ping, which sends one tiny packet at a time and times each reply.

Addresses

That label on each packet only works if every device has an address. On today's networks that's an IP address, for example 192.168.1.42 (IPv4) or a longer one like fe80::1c2b:3a4d:5e6f:7a8b (IPv6). Later levels explain how these numbers are built. For now, all you need is that a device's IP address is how other devices reach it.

A device doesn't connect to the network as a whole. It connects through network interfaces: a Wi-Fi adapter, an Ethernet port, and often virtual interfaces created by VPNs or virtual machines. Each interface can have its own address.

Hands-on: find your interfaces and IP address

Windows (open Command Prompt or PowerShell):

C:\> ipconfig                      # basic view: IPv4/IPv6 addresses, mask, gateway per adapter

Wireless LAN adapter Wi-Fi:
   Connection-specific DNS Suffix  . : home
   IPv6 Address. . . . . . . . . . . : fe80::1c2b:3a4d:5e6f:7a8b%12
   IPv4 Address. . . . . . . . . . . : 192.168.1.42     <-- your address
   Subnet Mask . . . . . . . . . . . : 255.255.255.0
   Default Gateway . . . . . . . . . : 192.168.1.1      <-- your router

C:\> ipconfig /all                 # full detail for every adapter, including disconnected ones

Linux (use ip. The older ifconfig is missing from many modern distributions):

$ ip addr show          # short form: ip a
1: lo: <LOOPBACK,UP> ...
    inet 127.0.0.1/8 scope host lo                 # loopback: the machine talking to itself
2: wlp2s0: <BROADCAST,MULTICAST,UP> ...
    inet 192.168.1.57/24 ... scope global wlp2s0   # your real address on the network

macOS (Terminal):

$ ifconfig              # lists every interface: lo0, en0, plus several virtual ones
en0: flags=8863<UP,BROADCAST,RUNNING,...>
        inet 192.168.1.63 netmask 0xffffff00 broadcast 192.168.1.255
$ ipconfig getifaddr en0   # prints just the IPv4 address of en0
192.168.1.63

Gotcha: On macOS, en0 is often Wi-Fi on laptops, but not always. Look for the interface that is active and has an inet line. On every OS, ignore the loopback (lo, lo0, 127.0.0.1) when you want your real network address.

Hands-on: is it reachable? Use ping

ping sends small "echo request" packets and reports each reply with its round-trip time. Run these from the inside out:

ping 127.0.0.1        # 1. is networking working on this machine at all?
ping 192.168.1.1      # 2. can I reach my router? (use YOUR default gateway)
ping 1.1.1.1          # 3. can I reach a server out on the internet?

Typical output on Windows:

Reply from 1.1.1.1: bytes=32 time=14ms TTL=57
Reply from 1.1.1.1: bytes=32 time=13ms TTL=57
...
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss)

Two platform differences catch everyone out:

• Windows sends 4 pings and stops (-n 10 sends 10, -t keeps going).
• Linux and macOS keep pinging until you press Ctrl+C. Use -c 4 to send four and stop.

$ ping -c 4 1.1.1.1    # Linux/macOS: send 4, then print a summary

What each result tells you: if step 1 fails, the problem is on your own machine. If step 2 fails, it's your local connection. If only step 3 fails, it's something beyond your router. Working outward like that is the start of a real troubleshooting method.

Pitfall from production: "Request timed out" does not prove a device is down. Many devices and firewalls deliberately ignore ping. Windows' built-in firewall, for example, blocks incoming echo requests on public and private networks by default. A server can be serving web pages perfectly well while refusing to answer ping. Treat a ping failure as a clue, not a verdict.

Key takeaways

• A network is devices, links between them, and gear that joins them: switches connect devices within a local network, and routers connect separate networks.
• Data travels as packets, small labelled chunks, so links can be shared and lost pieces can be resent.
• Every interface that's on a network gets an IP address. Find yours with ipconfig (Windows), ip addr (Linux) or ifconfig (macOS).
• ping tests reachability. Windows sends 4 by default, while Linux and macOS run until Ctrl+C unless you pass -c.
• Ping from the inside out: loopback, then router, then the internet. A missing reply might just mean a firewall is ignoring ping.