Networks and Cyber Security

1 State two differences between a LAN and a WAN. 2

One mark for each of the following, to a maximum of 2.

• A LAN covers a small area and a WAN covers a large geographical area
• LAN equipment is owned by the organisation, WAN infrastructure is not

Model answer. A LAN covers a small geographical area such as a single school or house, whereas a WAN covers a large area and may span whole countries, with the internet being the largest example. In addition, the organisation using a LAN owns all of the cabling and hardware itself, while a WAN uses infrastructure belonging to other companies.

2 Describe what happens when a user types a web address into a browser and presses enter. 4

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

• The browser asks a DNS server to look up the address
• DNS returns the IP address of the web server
• A request is sent to that IP address and the server responds
• The page is sent as packets which are reassembled by the browser

Model answer. When the user presses enter, the browser first needs to turn the human readable address into a number, because networks route data by IP address rather than by name. It therefore sends a request to a DNS server, which looks the name up and returns the IP address of the web server. The browser then sends a request to that IP address asking for the page. The web server responds by sending the page back, but the data is split into small packets, each carrying a sequence number and the destination address. These packets may travel by different routes and arrive out of order, so the receiving computer uses the sequence numbers to reassemble them correctly before the browser displays the finished page.

3 Explain why data is split into packets when sent across a network. 3

One mark for each of the following, to a maximum of 3.

• Large files would otherwise block the network for other users
• Packets can take different routes, using the network efficiently
• If a packet is lost only that packet needs resending, not the whole file

Model answer. Splitting data into packets means that a single very large file cannot occupy a connection completely and block everyone else, because packets from different transfers can be interleaved and the network is shared fairly. Packets can also travel by different routes depending on which parts of the network are congested, which uses the available capacity efficiently and means a fault on one route does not stop the transfer. Finally, if a packet is damaged or lost in transit, only that individual packet has to be sent again rather than the entire file, which is far quicker and far less wasteful.

4 A school is worried about malware on its network. Describe three measures it could take. 6

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

• Install anti malware software
• Keep it updated so new threats are recognised
• Use a firewall to filter network traffic
• Restrict user access levels so students cannot install software
• Train staff and students to recognise suspicious emails and links
• Keep backups so data can be restored if it is encrypted or lost

Model answer. The first measure is to install anti malware software on every computer and set it to update automatically. This scans files against a database of known malware and removes or quarantines anything it finds, but because new malware appears constantly the software is only effective if that database is kept current. The second measure is to use a firewall between the school network and the internet. A firewall examines all the traffic going in and out and blocks anything that does not match its rules, which stops many threats reaching the network at all and prevents malware that does get in from communicating with outside servers. The third measure is to set user access levels so that students, and most staff, cannot install software or change system files. That means that even if someone is tricked into running something harmful, it only has the limited permissions of that account and cannot spread across the whole system. Alongside these, training people to recognise suspicious emails addresses the route most infections actually take, and keeping regular backups means that if ransomware does encrypt the school's files they can be restored rather than paid for.

5 Explain the purpose of encryption and state whether it prevents data from being intercepted. 3

One mark for each of the following, to a maximum of 3.

• Encryption scrambles data using a key
• Without the key the data cannot be understood
• It does not prevent interception, it makes intercepted data useless

Model answer. Encryption scrambles data using a key so that what is actually transmitted is a meaningless sequence of characters. Only someone who has the correct key can convert it back into the original information, so an attacker who captures the traffic cannot understand any of it. Encryption does not prevent data from being intercepted: anyone in a position to capture the traffic can still capture it. What encryption does is make that interception pointless, because the attacker ends up with data they cannot read.