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Technical · Chapter 2 · English

Chapter 2 — Networks and protocols

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Chapter 2 — Networks and protocols

Lesson map

Lesson 02 published. Map, example, EX005.

flowchart TD
  N0["Learning outcomes"]
  N1["2.1 Addresses and subnets"]
  N2["2.2 Routing and gateways"]
  N3["2.3 Key protocols"]
  N4["2.4 Reading diagrams"]
  N5["Synthesis"]
  N0 --> N1 --> N2 --> N3 --> N4 --> N5
  N5 --> EX["Worked example"]
  EX --> Q["Exercise EX005"]

Logical drawing

flowchart LR
  N0["Learning outcomes"]
  N1["2.1 Addresses and subnets"]
  N2["2.2 Routing and gateways"]
  N3["2.3 Key protocols"]
  N4["2.4 Reading diagrams"]
  N5["Synthesis"]
  N0 --> N1 --> N2 --> N3 --> N4 --> N5 --> Q["EX005"]

[Learning outcomes]
    |
    v
[2.1 Addresses and subnets]
    |
    v
[2.2 Routing and gateways]
    |
    v
[2.3 Key protocols]
    |
    v
[2.4 Reading diagrams]
    |
    v
[Synthesis]
    |
    v
[Worked example] --> [Exercise EX005]

Learning outcomes

You will (1) explain addressing, subnets and routing; (2) describe key protocols; (3) read a small network diagram.

2.1 Addresses and subnets

Each device on a network has an IP address. Addresses are grouped into subnets, written in CIDR notation such as 10.8.10.0/24, where the number after the slash tells how many bits identify the network. A /24 holds 256 addresses, of which 254 can be used by devices. Castleford's main office uses one /24 for everything: staff, printers, servers and guests. Amina learns that putting different groups in different subnets is the first step towards controlling traffic between them, because routers and firewalls make decisions based on subnet addresses.

2.2 Routing and gateways

Devices send traffic for other subnets to their default gateway, which forwards it. At Castleford, the gateway is the office firewall. Traffic between two devices in the same subnet never reaches it, so it cannot be filtered there. Traffic between different subnets must pass through the gateway, where rules can allow or deny it. Amina draws both cases and sees why the guest wireless can reach the housing system: they share one subnet, so the firewall never sees their traffic and cannot block it.

2.3 Key protocols

Amina learns the protocols she will meet most often. DHCP gives devices addresses automatically. DNS turns names into addresses. TCP provides reliable connections for most applications, and UDP carries quick exchanges such as DNS queries. HTTPS protects web traffic with TLS. SSH provides encrypted remote administration. IPsec protects the VPN between offices. For each, she notes the usual port and whether it encrypts data. Knowing these lets her read firewall logs and rules confidently, since every entry refers to a protocol and port.

2.4 Reading diagrams

Network diagrams use simple symbols for routers, switches, firewalls and clouds, with lines for links and labels for subnets. Amina practises reading Castleford's diagram and redrawing it more clearly, adding subnet labels and marking the firewall at each office. She checks the diagram against device configurations Rosa provides and finds one switch missing from it. A diagram that does not match reality misleads everyone who uses it, so checking it is a basic technical task worth doing regularly. She schedules a diagram check every six months.

Worked example

Amina redraws the diagram with subnets labelled, adds a missing switch and marks where guest and housing traffic share a subnet.

Synthetic case. Diagram is synthetic.

Errors and limits

Assuming a firewall filters traffic inside one subnet leads to false confidence.

Exercises

Synthesis

Subnets, gateways and protocols determine what traffic controls can see and filter.

Continue studying — chapter 02 of 18

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