Networks and Network Security

1 State two items of network hardware and describe the purpose of each. 4

One mark for each of the following, to a maximum of 4.

• Names a switch
• Describes it as joining devices on a LAN and sending frames to their destination
• Names a router
• Describes it as joining networks and directing packets between them

Model answer. A switch joins the devices on a local area network together and forwards each frame only to the device it is addressed to, which keeps traffic off the rest of the network. A router joins separate networks together and directs packets between them, deciding which route each packet should take, which is what connects a local network to the internet.

2 Explain why network protocols are organised into layers. 4

One mark for each of the following, to a maximum of 4.

• Each layer handles one part of the communication task
• So a very complex problem is broken into manageable pieces
• A layer can be changed without affecting the layers above and below it
• Gives an example such as swapping Ethernet for Wi-Fi

Model answer. Layering divides the problem of network communication into parts, each of which handles one aspect and interacts only with the layer directly above and below it. That means nobody has to understand the whole of networking at once, and different specialists can work on different layers. It also gives independence: because each layer only depends on the interface the next presents, one can be replaced entirely without disturbing the others. Moving a laptop from an Ethernet cable to Wi-Fi replaces the link layer completely, yet IP, TCP and HTTP carry on exactly as before and no application has to be rewritten.

3 An online shop finds that an attacker has read its entire customer table by typing text into the site's search box. Name the attack and explain how it could be prevented. 4

One mark for each of the following, to a maximum of 4.

• Names SQL injection
• Explains that text entered by the user is being executed as part of a database query
• Prevention by validating or sanitising input
• Or by using parameterised queries, and limiting the database account's permissions

Model answer. This is SQL injection. It happens because whatever the user types into the search box is being joined directly into an SQL query and sent to the database, so an attacker can type SQL of their own and have the database execute it, returning data the page was never meant to show. The primary prevention is to stop user input ever being treated as code: use parameterised queries, so the database is told in advance which part of the statement is a value and which is an instruction. Alongside that, input should be validated against what a search term is actually allowed to contain, and the account the website uses to reach the database should have only the permissions the site genuinely needs, so that even a successful injection cannot read the whole customer table.

4 A school is choosing between a star and a mesh topology for a new building. Discuss the advantages and disadvantages of each, and make a recommendation. 6

One mark for each of the following, to a maximum of 6.

• A star gives each device a dedicated connection to a central switch
• So it is fast and one cable failure affects only one device
• But the switch is a single point of failure for the whole network
• A mesh provides multiple routes between devices
• So it is very resilient, but expensive and complex to cable in full
• Reaches a justified recommendation for this situation

Model answer. A star topology gives every device its own cable running to a central switch. Nothing is shared, so devices do not compete for the medium and performance stays consistent as machines are added, and a damaged cable disconnects only the one device on it. Extending the network means running one more cable. Its weakness is concentration of risk: everything passes through the switch, so if the switch fails the entire network stops until it is replaced. A mesh topology connects devices to several or all of the others, so there are multiple routes between any two points and traffic can be rerouted automatically around a broken link or a failed device. That resilience is genuine, but a full wired mesh needs an enormous amount of cable, which becomes impractical very quickly: ten devices would need forty five separate links. For a school building the recommendation is a star, and for two reasons specific to the situation. The cost and complexity of a wired mesh cannot be justified for classroom machines where a short outage is inconvenient rather than dangerous, and the failure the school is most likely to experience is a damaged patch lead, which a star already handles. The single point of failure should be managed rather than designed out: keep a spare switch on site, and if the school later adds wireless coverage across the building, a wireless mesh of access points is a sensible way to get resilience where cabling would be difficult.