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Chapter 6 — Network and communications security: system model

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Chapter 6 — Network and communications security: system model

Lesson map

Lesson 06 published. Map, example, EX021.

flowchart TD
  N0["Learning outcomes"]
  N1["6.1 Zones"]
  N2["6.2 Needed flows"]
  N3["6.3 Excess access"]
  N4["6.4 Using the model"]
  N5["Synthesis"]
  N0 --> N1 --> N2 --> N3 --> N4 --> N5
  N5 --> EX["Worked example"]
  EX --> Q["Exercise EX021"]

Logical drawing

flowchart LR
  N0["Learning outcomes"]
  N1["6.1 Zones"]
  N2["6.2 Needed flows"]
  N3["6.3 Excess access"]
  N4["6.4 Using the model"]
  N5["Synthesis"]
  N0 --> N1 --> N2 --> N3 --> N4 --> N5 --> Q["EX021"]

[Learning outcomes]
    |
    v
[6.1 Zones]
    |
    v
[6.2 Needed flows]
    |
    v
[6.3 Excess access]
    |
    v
[6.4 Using the model]
    |
    v
[Synthesis]
    |
    v
[Worked example] --> [Exercise EX021]

Learning outcomes

You will (1) model Castleford's network as zones and flows; (2) identify needed flows; (3) spot excess access.

6.1 Zones

Amina proposes zones for each office: staff, servers, printers, guest and management. Field officers connecting by VPN form a remote zone. Each zone gets its own subnet. The servers zone exists only at the main office; the branch office reaches it over the site-to-site VPN. Zones group devices by trust and purpose, so rules can treat them differently. The guest zone gets internet access only, the printers zone accepts printing from staff, and the management zone holds switches, access points and firewall management interfaces.

6.2 Needed flows

A needed flow is a connection required for work. Amina lists them with Rosa: staff to servers for the housing system and files, staff to printers, remote to servers for the housing system and files, servers to the internet for updates, management to all network devices for administration, and all zones to the DNS servers. Each flow names a source zone, destination, service and reason. The list is short, which is typical; most traffic between zones is not needed. Rosa signs off the final list.

6.3 Excess access

Today, everything at the main office shares one subnet, so every device can reach every other. Comparing the current reachability with the needed flows shows the excess: guests can reach servers, printers can reach staff laptops, and staff can reach switch management pages. Amina counts pairs of zones that can reach each other now and pairs that need to; the difference measures excess access. This simple measure helps managers understand why separation matters. She presents the count in a simple chart for the managers, showing current and needed pairs side by side.

6.4 Using the model

The model lets Amina reason before changing anything. If a new printer arrives, it goes in the printers zone and inherits printing rules. If the housing system moves to the cloud, the staff-to-servers flow changes to staff-to-internet for that service, and remote users can reach it directly. The model also exposes dependencies: everything depends on DNS, so DNS servers need protection and redundancy. Keeping the model up to date makes later changes faster and safer. Rosa keeps the model with the network documentation and updates it after every change.

Worked example

The model has six zones and seven needed flows; twenty of thirty zone pairs can reach each other today, versus seven needed.

Synthetic case. The model is synthetic.

Errors and limits

Listing flows from memory alone misses rarely used but important services.

Exercises

Synthesis

A zone-and-flow model makes excess access visible and guides safe change.

Continue studying — chapter 06 of 18

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