TCP/IP, protocols, IP addressing, subnetting and client–server – A-level Computer Science exam questions with mark scheme

1Core3 marks

The TCP/IP stack has four layers: application, transport, network and link.

State the layer responsible for each of the following. [3]

(i) Routing packets between networks using IP addresses.
(ii) Splitting data into packets and using port numbers to identify the receiving program.
(iii) Defining the format of an HTTP request.

(i) Network layer (allow internet layer)(1)
(ii) Transport layer(1)
(iii) Application layer(1)
Answer: (i) network (internet) (ii) transport (iii) application
Examiner tip: Ports belong to the transport layer; IP addresses belong to the network (internet) layer.

2Standard4 marks

A computer has the IP address 192.168.14.77 and the subnet mask 255.255.255.224.

(a) Calculate the network identifier (network address) of the subnet that this computer is on. Show your working. [2]

(b) The computer needs to send a packet to the IP address 192.168.14.101. Explain how the computer uses the subnet mask to decide whether it can send the packet directly to the destination or must send it to the router. [2]

(a) Working: last octet 77 = 01001101 ANDed with 224 = 11100000 (bitwise AND of the address with the mask) (allow equivalent working, e.g. block size 256 − 224 = 32 and 77 lies in the block 64–95)(1)
(a) 192.168.14.64 (ignore /27 suffix)(1)
(b) The subnet mask is ANDed with the destination IP address (and with its own address) to find the network identifier of each (allow: the mask is applied to / used to pick out the network part of the addresses)(1)
(b) If the network identifiers match (both on the same subnet) the packet is sent directly; otherwise it is sent to the router / default gateway. (For this destination: 101 AND 224 = 96, so 192.168.14.96 ≠ 192.168.14.64; it must go via the router. Award the mark for the general rule or for the correct conclusion with reason.)(1)
Answer: (a) 192.168.14.64 (b) AND the destination address with the mask to get its network ID (192.168.14.96); it differs from 192.168.14.64, so the packet is sent to the router
Examiner tip: Show the AND in binary for the working mark; just writing the answer risks losing a mark.

3Hard6 marks

A router connects four subnets of a company network (see diagram). Every host on the network uses the subnet mask 255.255.255.192.

Four hosts have these IP addresses:

Host A: 192.168.10.37
Host B: 192.168.10.70
Host C: 192.168.10.120
Host D: 192.168.10.200

(a) Write the last octet of the subnet mask in binary and state how many bits of the IP address form the network identifier. [2]

(b) Calculate the network identifier of Host B. Show your working. [2]

(a) 11000000(1)
(a) 26 bits (8 + 8 + 8 + 2)(1)
(b) Working: 70 = 01000110 ANDed with 11000000 (allow equivalent block working: blocks of 64, 70 lies in 64–127)(1)
(b) 192.168.10.64(1)
(c) Host B and Host C(1)
(c) Justification: both B and C have network identifier 192.168.10.64 (120 AND 192 = 64) (allow: both lie in the block 64–127). Stating that A is on 192.168.10.0 and D is on 192.168.10.192 is not required. Do not award the justification mark if the hosts identified are wrong.(1)
Answer: (a) 11000000; 26 bits (b) 192.168.10.64 (c) B and C, both have network ID 192.168.10.64
Examiner tip: Hosts are on the same subnet only if ANDing each address with the mask gives the same network identifier; addresses that 'look close' are not enough.

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