A2 Networks – IB Computer Science exam questions with mark scheme

1Core4 marks

Distinguish between the function of a router and a switch in a network. [4]

A switch connects devices within the same network/LAN(1)
It uses MAC addresses to forward frames only to the intended device (allow: rather than broadcasting to all devices like a hub; network interface/link layer)(1)
A router connects different networks together (e.g. a LAN to the internet / a WAN)(1)
It uses IP addresses to forward packets along the best route to the destination network (allow: internet layer)(1)
Answer: Switch: MAC-based forwarding within a LAN. Router: IP-based routing between networks.
Examiner tip: Pair each device with its address type (MAC vs IP) and scope (within vs between networks).

2Standard4 marks

Describe how a large file is sent across the internet using packet switching. [4]

The file is broken into (small) packets(1)
Each packet has a header containing the destination IP address (allow: source address, sequence/packet number)(1)
Packets are sent independently and may take different routes, chosen by routers according to current traffic/availability(1)
At the destination the packets are reassembled in the correct order (using sequence numbers)(1)
Answer: Split into numbered packets, routed independently, reassembled at destination.
Examiner tip: Mention sequence numbers; they are what makes reassembly possible when packets arrive out of order.

3Standard4 marks

A school's network connects 600 student devices, 80 staff computers and 40 CCTV cameras as one single, flat LAN. The technician proposes dividing it into three segments (separate subnets or VLANs) for students, staff and CCTV.

Explain how this segmentation would improve:

(i) the network's performance [2]

(ii) the network's security. [2]

(i) Broadcast traffic is confined to each segment / each segment is a smaller broadcast domain(1)
(i) so there is less unnecessary traffic/congestion on each segment and more bandwidth available to each group (e.g. broadcasts from the 600 student devices no longer reach the staff computers or cameras; allow: CCTV traffic can be kept separate/prioritised)(1)
(ii) Traffic between segments must pass through a router/firewall/layer-3 switch where rules can restrict access (e.g. students cannot reach staff computers or the CCTV system)(1)
(ii) A breach or malware on one segment is contained / cannot easily spread to the others (allow: easier to monitor each segment)(1)
Answer: Smaller broadcast domains reduce congestion; traffic between segments can be filtered, so access is restricted and attacks are contained.
Examiner tip: Give a mechanism and its effect for each part, and apply it to the scenario (students, staff, CCTV); 'it makes it faster and safer' earns nothing.

4Standard4 marks

Compare and contrast the client–server model with the peer-to-peer (P2P) model of networking. [4]

Similarity: both models let devices share resources/data/services over a network (allow: both can operate over a LAN or the internet; allow: in both, a device can request a service from another device)(1)
Difference in roles: in client–server, dedicated central server(s) provide services and clients only request them, whereas in P2P every peer can act as both client and server, sharing resources directly(1)
Difference in management: client–server allows central administration, security, backup and updates, whereas in P2P each peer manages its own resources, so it is harder to secure/back up consistently(1)
Difference in resilience/cost: a client–server network depends on the server (single point of failure/bottleneck) and servers are costly to buy and run, whereas P2P has no single point of failure and is cheaper/scales as peers join (accept either half if clearly contrasted with the other model)(1)
Answer: Similarity: both share resources over a network. Differences: central server serving clients vs peers acting as both; central management/security vs each peer managing itself; single point of failure and higher cost vs no single point of failure and lower cost.
Examiner tip: 'Compare and contrast' needs at least one similarity as well as differences, and each difference should say what happens in both models (use 'whereas'); describing only one model scores very little.

5Standard4 marks

A 450 MB file is downloaded over a link with a bandwidth of 80 Mbit/s. (Use 1 MB = 8 Mbit.)

(a) Calculate the minimum time to download the file. [2]

(b) Protocol overhead means 12% more data than the file itself must be transmitted. Calculate the new download time. [2]

450 × 8 = 3600 Mbit(1)
3600 ÷ 80 = 45 s (unit required)(1)
3600 × 1.12 = 4032 Mbit (allow 45 × 1.12, or 504 MB)(1)
4032 ÷ 80 = 50.4 s (follow through from (a); unit required)(1)
Answer: (a) 45 s (b) 50.4 s
Examiner tip: Convert bytes to bits before dividing by a rate given in bits per second.

6Hard4 marks

Explain the difference between symmetric and asymmetric encryption, and state why HTTPS uses both. [4]

Symmetric encryption uses the same (shared secret) key to encrypt and decrypt(1)
Asymmetric encryption uses a key pair: a public key to encrypt and the matching private key to decrypt (allow: data encrypted with one key of the pair can only be decrypted with the other)(1)
Symmetric is faster for large amounts of data, but the key must be shared securely (allow: asymmetric is slower / computationally expensive)(1)
HTTPS (TLS) uses asymmetric encryption to exchange/agree the symmetric session key securely (allow: and to authenticate the server with its certificate), then symmetric encryption for the data for speed(1)
Answer: Same key vs key pair; asymmetric exchanges the session key, symmetric encrypts the bulk data.
Examiner tip: Say why both are used: asymmetric solves key sharing, symmetric gives speed.

7StandardHL only4 marks

A user types www.example.org into a browser. Describe how the Domain Name System (DNS) is used to load the web page. [4]

The browser/OS sends a request containing the domain name to a DNS server (resolver) (allow: first checks its local DNS cache)(1)
The DNS server looks up (or queries other DNS servers for) the matching IP address(1)
The IP address is returned to the browser (and may be cached)(1)
The browser uses the IP address to send an HTTP(S) request to the web server(1)
Answer: Domain name sent to DNS server, IP address returned, browser contacts server at that IP.
Examiner tip: Say what is sent and what comes back: name goes out, IP address returns.

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