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Data communications and networks: board exam questions with solutions

On the ECE exam under Electronic Systems and Technologies (EST). Try each question before you open its solution, then practise with fresh numbers below: 3 computed question types on this topic.

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Questions

1. Which OSI layer is responsible for logical addressing and routing packets between networks?

  1. Data link layer (Layer 2)
  2. Network layer (Layer 3)
  3. Transport layer (Layer 4)
  4. Session layer (Layer 5)
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Answer: B. Network layer (Layer 3)

IP addressing and routing are Network-layer (Layer 3) functions; Layer 2 handles MAC framing within a link.

2. How long is an Ethernet MAC address?

  1. 48 bits
  2. 32 bits
  3. 64 bits
  4. 128 bits
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Answer: A. 48 bits

A MAC address is 48 bits (6 octets), usually written as six hex pairs. 32 bits is IPv4 and 128 bits is IPv6.

3. How many usable host addresses are available in an IPv4 /26 subnet?

  1. 64
  2. 30
  3. 126
  4. 62
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Answer: D. 62

Host bits = 32 − 26 = 6; 2⁶ − 2 = 62 (network and broadcast addresses excluded).

4. What is the network address of host 192.168.10.77/27?

  1. 192.168.10.32
  2. 192.168.10.95
  3. 192.168.10.64
  4. 192.168.10.72
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Answer: C. 192.168.10.64

/27 has a block size of 32 in the last octet: 0, 32, 64, 96… 77 falls in 64–95, so the network is 192.168.10.64 (broadcast 192.168.10.95).

5. What is the dotted-decimal subnet mask for a /20 prefix?

  1. 255.255.255.240
  2. 255.255.240.0
  3. 255.255.224.0
  4. 255.240.0.0
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Answer: B. 255.255.240.0

20 ones: 255.255 (16 bits) + 11110000 (4 bits) = 255.255.240.0.

6. The network 172.16.0.0/16 must be split into at least 6 equal subnets, each as large as possible. What prefix and how many usable hosts per subnet result?

  1. /19 with 8,190 hosts
  2. /18 with 16,382 hosts
  3. /22 with 1,022 hosts
  4. /19 with 8,192 hosts
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Answer: A. /19 with 8,190 hosts

2² = 4 < 6 ≤ 8 = 2³, so borrow 3 bits → /19. Host bits = 13 → 2¹³ − 2 = 8,190 usable hosts.

7. Which transport-layer protocol is connection-oriented and provides reliable, ordered delivery with acknowledgements?

  1. UDP
  2. IP
  3. ARP
  4. TCP
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Answer: D. TCP

TCP sets up a connection (three-way handshake) and uses sequence numbers, ACKs and retransmission; UDP is connectionless and unreliable.

8. How long does it take to transmit a 1500-byte frame onto a 100 Mbps link (transmission delay only)?

  1. 15 µs
  2. 12 µs
  3. 120 µs
  4. 1.2 ms
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Answer: C. 120 µs

t = bits/rate = 1500 × 8/(100 × 10⁶) = 120 µs.

Worked examples with computed numbers

These numbers are fixed on this page; in practice they change every time.

9. How many usable host addresses does an IPv4 /21 subnet have?

  1. 1,024
  2. 2,048
  3. 2,047
  4. 2,046
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Answer: D. 2,046

Host bits = 32 − 21 = 11; usable hosts = 2^11 − 2 = 2046 (network and broadcast addresses excluded).

10. What is the dotted-decimal subnet mask for /21?

  1. 255.255.248.0
  2. 0.0.7.255
  3. 255.255.240.0
  4. 255.255.252.0
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Answer: A. 255.255.248.0

/21 means 21 one-bits: 255.255.248.0.

11. How long does a 5 MB file take over a 50 Mb/s link, ignoring overhead (1 MB = 10⁶ bytes)?

  1. 102 s
  2. 0.8 s
  3. 20 s
  4. 0.1 s
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Answer: B. 0.8 s

Bytes to bits ×8: 5 MB = 40 Mb; at 50 Mb/s, t = 0.8 s (ignoring overhead).

Other topics in the same subjects

Communications fundamentals (dB, noise) · AM, FM and PM · Digital communications · Transmission lines · Antennas and wave propagation · Microwave, fiber optics and satellite · Broadcasting and acoustics

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