Module 1: Network Foundations

What a Network Is and How Data Travels

Every time you send an M-Pesa confirmation by SMS, open a website or print to a shared office printer, data moves across a network. If you support computers for a living, or you build software that talks to other machines, you need a clear mental picture of what is happening between the click and the result. Without that picture, troubleshooting becomes guesswork. With it, you can reason step by step about where a problem is likely to be.

What a network actually is

A network is two or more devices connected so that they can exchange data. The devices are called hosts or nodes: laptops, phones, servers, printers, CCTV recorders and even card machines. They are joined by a medium (copper cable, fibre-optic cable or radio waves) and by network devices such as switches, routers and wireless access points that pass the data along.

Networks are usually described by size and reach:

• LAN (Local Area Network): devices in one building or site, such as a shop in Westlands or a school computer lab. You normally own and manage the equipment.
• WAN (Wide Area Network): links between sites over long distances, for example a head office in Nairobi connected to a branch in Mombasa. The links are usually rented from a service provider.
• The Internet: a global network of networks, run by thousands of independent organisations that agree to use the same protocols.

A protocol is simply an agreed set of rules for communicating. Just as two people need a common language, two computers need common protocols, such as IP for addressing and TCP for reliable delivery.

How data is broken up and delivered

Data does not travel as one large block. A photo, an email or a web page is broken into small pieces called packets. Each packet carries a small part of the data plus a header, which is like the address label on a parcel. The header includes the source address, the destination address and information that lets the receiver put the pieces back in order.

Breaking data into packets has real advantages. Many conversations can share the same cable at the same time, because packets from different users are interleaved. If one packet is lost or damaged, only that packet needs to be sent again, not the whole file. Packets can also take different routes if one link is busy or broken.

Along the way, each device makes a simple decision. A switch looks at the hardware (MAC) address and forwards the frame to the right port inside the local network. A router looks at the IP address and decides which network to send the packet to next. Every router along the path repeats this decision until the packet reaches the destination network, where the final switch delivers it to the right device.

A worked example: opening a website from an office

Imagine Achieng, an accountant at a small firm in Upper Hill, types a supplier's web address into her browser. Here is a simplified version of what happens:

1. Her laptop needs the server's IP address, so it asks a DNS server to translate the name into a number.
2. The laptop sees that the destination is not on the office LAN, so it sends the packets to its default gateway, the office router.
3. The office switch carries the packets from her desk cable to the router's port.
4. The router passes them to the Internet service provider (ISP), whose routers forward them across several networks towards the server.
5. The server replies, and the response packets travel back, possibly by a different path, to the office router and then to her laptop.
6. The browser reassembles the data and displays the page.

All of this usually takes a fraction of a second. When it fails, each numbered step is a place you can test. If the laptop cannot reach the router, the problem is local. If it reaches the router but not the Internet, look at the ISP link. If it can reach an IP address but not a name, suspect DNS.

Bandwidth, latency and reliability

Three words come up constantly when people describe network performance:

• Bandwidth is how much data a link can carry per second, measured in bits per second (for example Mbps, megabits per second). Note bits, not bytes: 8 bits make 1 byte.
• Latency is how long a packet takes to travel from source to destination, usually measured in milliseconds. A link can have high bandwidth and still feel slow if latency is high, such as with some satellite connections.
• Packet loss is the share of packets that never arrive. Even small amounts of loss can make voice and video calls choppy.

Common mistakes to avoid

• Confusing megabits with megabytes. A 20 Mbps line downloads at most about 2.5 megabytes per second, before overheads.
• Assuming "the Internet is down" when only one device or one service is affected. Always check whether other devices and other websites work.
• Thinking a router and a switch are the same thing. Many home boxes combine both, but the jobs are different, and knowing which job is failing speeds up repairs.
• Ignoring the physical layer. A loose cable or a device far from the Wi-Fi access point causes a large share of everyday problems.