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ICT Officer calculations with worked answers

Updated 16 September 2026 · 2 min read

An ICT officer runs many small systems at once, and a few calculations keep them working properly. You may need to address a new office network, estimate how long a data transfer will take, justify a password policy, or work out how long a UPS will keep a server running through a power cut. Each example below shows the full working, and every figure was verified by separate code before it was published.

How this guide was made

Written for this site with AI assistance, then checked automatically before publishing. The topic table, sample questions, worked examples and study plan are generated from this site’s question bank: each worked example is recalculated by separate code, and each sample answer says where it came from.

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Worked examples

Try each one on paper first. Every figure is generated and then recalculated by separate code before it is published.

  1. Subnet details for a /28 address

    A host is configured as 10.244.13.145/28. Find the subnet mask, network address, broadcast address, usable host range and number of usable hosts.

    Show the working
    1. /28 means 28 network bits, so the mask is 255.255.255.240.
    2. The interesting octet is the fourth, where the mask is 240. Block size = 256 − 240 = 16.
    3. The network address is the multiple of 16 at or below the address in that octet: 10.244.13.144.
    4. The broadcast is one below the next network: 10.244.13.159.
    5. Usable hosts run from 10.244.13.145 to 10.244.13.158: 2^4 − 2 = 14 addresses.

    Answer: Mask 255.255.255.240, network 10.244.13.144, broadcast 10.244.13.159, hosts 10.244.13.145–10.244.13.158 (14 usable)

  2. How long a file takes to transfer

    How long does a 12 GB file take to copy over a 100 Mbps link? Use decimal units (1 GB = 1,000 MB) and ignore protocol overhead.

    Show the working
    1. Storage is sized in bytes, link speed in bits. Multiply megabytes by 8 to get megabits.
    2. 12000 MB × 8 = 96000 Mb
    3. Time = 96000 Mb ÷ 100 Mbps = 960 s ≈ 16 minutes
    4. Real transfers take longer: TCP/IP headers, retransmissions and disk speed all eat into the link rate.

    Answer: 16 minutes

  3. Keyspace and entropy of a password policy

    A password of 9 characters is chosen at random from lowercase letters (26 possible symbols). How many bits of entropy does it have, and how long would trying every combination take at 10 billion guesses a second?

    Show the working
    1. Entropy = length × log₂(symbols) = 9 × log₂(26) = 9 × 4.7
    2. = 42.3 bits
    3. Combinations = 26^9 ≈ 5.43e+12
    4. Time to try them all = combinations ÷ 10¹⁰ ≈ 9.05 minutes. On average an attacker finds it in half that.
    5. This only holds for truly random passwords. A human-chosen word with a digit on the end falls to a dictionary attack far sooner.

    Answer: 42.3 bits; about 9.05 minutes to exhaust

  4. How long a battery bank carries a load

    A battery bank stores 720 Wh, of which 90 % can be used after inverter losses and depth-of-discharge limits. It carries a steady 450 W load. Roughly how long will it run?

    Show the working
    1. Usable energy = 720 Wh × 90 ÷ 100 = 648 Wh
    2. Runtime = usable energy ÷ load = 648 Wh ÷ 450 W = 1.44 h
    3. That is about 86 minutes. Real batteries deliver less at high discharge rates and as they age, so treat this as an upper estimate.

    Answer: 1.44 hours (about 86 minutes)

  5. Subnet details for a /22 address

    A host is configured as 172.25.214.88/22. Find the subnet mask, network address, broadcast address, usable host range and number of usable hosts.

    Show the working
    1. /22 means 22 network bits, so the mask is 255.255.252.0.
    2. The interesting octet is the third, where the mask is 252. Block size = 256 − 252 = 4.
    3. The network address is the multiple of 4 at or below the address in that octet: 172.25.212.0.
    4. The broadcast is one below the next network: 172.25.215.255.
    5. Usable hosts run from 172.25.212.1 to 172.25.215.254: 2^10 − 2 = 1022 addresses.

    Answer: Mask 255.255.252.0, network 172.25.212.0, broadcast 172.25.215.255, hosts 172.25.212.1–172.25.215.254 (1022 usable)

  6. Keyspace and entropy of a password policy

    A password of 14 characters is chosen at random from lowercase letters (26 possible symbols). How many bits of entropy does it have, and how long would trying every combination take at 10 billion guesses a second?

    Show the working
    1. Entropy = length × log₂(symbols) = 14 × log₂(26) = 14 × 4.7
    2. = 65.8 bits
    3. Combinations = 26^14 ≈ 6.45e+19
    4. Time to try them all = combinations ÷ 10¹⁰ ≈ 204.4 years. On average an attacker finds it in half that.
    5. This only holds for truly random passwords. A human-chosen word with a digit on the end falls to a dictionary attack far sooner.

    Answer: 65.8 bits; about 204.4 years to exhaust

  7. How long a battery bank carries a load

    A battery bank stores 7200 Wh, of which 90 % can be used after inverter losses and depth-of-discharge limits. It carries a steady 600 W load. Roughly how long will it run?

    Show the working
    1. Usable energy = 7200 Wh × 90 ÷ 100 = 6480 Wh
    2. Runtime = usable energy ÷ load = 6480 Wh ÷ 600 W = 10.8 h
    3. That is about 648 minutes. Real batteries deliver less at high discharge rates and as they age, so treat this as an upper estimate.

    Answer: 10.8 hours (about 648 minutes)

  8. Subnet details for a /30 address

    A host is configured as 10.181.67.74/30. Find the subnet mask, network address, broadcast address, usable host range and number of usable hosts.

    Show the working
    1. /30 means 30 network bits, so the mask is 255.255.255.252.
    2. The interesting octet is the fourth, where the mask is 252. Block size = 256 − 252 = 4.
    3. The network address is the multiple of 4 at or below the address in that octet: 10.181.67.72.
    4. The broadcast is one below the next network: 10.181.67.75.
    5. Usable hosts run from 10.181.67.73 to 10.181.67.74: 2^2 − 2 = 2 addresses.

    Answer: Mask 255.255.255.252, network 10.181.67.72, broadcast 10.181.67.75, hosts 10.181.67.73–10.181.67.74 (2 usable)

When an ICT officer needs these numbers

Subnet details come up when you set static addresses for printers and servers, or add a new office to the network. Transfer times help you schedule large uploads, such as moving files to a shared system, outside busy hours. Password keyspace gives you a clear way to explain why a policy asks for longer passwords. UPS runtime tells you whether critical equipment will survive a typical power cut, or whether it needs a larger battery or a generator.

Using the results in reports

Numbers make recommendations stronger. A request for a larger UPS is more persuasive when it states that the current unit carries the server for about ten minutes, while recorded outages last longer. Keep calculations simple, state your assumptions, and round results to a sensible level for the reader.

Common questions

Why does a UPS last less time than its rating suggests?

Batteries lose capacity as they age and deliver less under heavy load. Inverter losses and depth-of-discharge limits also reduce usable energy, so runtime estimates should be treated as an upper limit.

Should an office password policy focus on length or complexity?

Length contributes most to strength, so a longer passphrase is usually better than a short password with forced symbols. Combine length with multi-factor authentication for important accounts.

More ICT Officer guides

The same guide for related positions