← IGCSE Computer Science Paper 1

IPv4 address format

32 bits written as four denary numbers from 0 to 255 separated by full stops, such as 190.0.5.20. Each number is a group of 8 bits.

Part of: IGCSE Computer Science 0478 — 3.4 Network Hardware — Module 14, ACARA Digital Technologies Years 7–8 — Wired and Wireless Networks — Module 33, ACARA Digital Technologies Years 9–10 — Networked Systems and Access Control — Module 17, WA Computer Science ATAR Year 11 — Network Communications — Module 38

Understanding IPv4 address format in IGCSE Computer Science Paper 1

What it does: Four groups with 8 bits in each gives 32 bits, the length of an IPv4 address. The full stops between the four denary numbers are the separators used in IPv4 notation. 4 groups x 8 bits = 32 bits. Eight groups of 16 bits describes IPv6 (128 bits). 8 bits give 256 values, and counting from 0 the highest is 255. The correct separator is a full stop, so the address should be written 192.168.1.10.

Common mistakes: An IPv4 address has four groups (numbers), not six. It should be 'Number of groups: 4'. The total number of bits is found by multiplying 4 groups by 8 bits (4 * 8 = 32), not adding. It should be total ← groups * bitsPerGroup.

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Learning
Duo
Which is a correctly formatted IPv4 address?
190.0.5.20190:0:5:20
Matches
192.168.1.300
↓
The last group is 300, which is above 255. A group of 8 bits cannot go higher than 255.
Applying
Meanings
Term
IPv4 address
Meaning
32 bits written as four denary numbers from 0 to 255 separated by full stops, such as 190.0.5.20. Each number is a group of 8 bits.
Practice
IPv4 address: 190.0.5.20
Testing
Gaps
An IPv4 address is bits long in total.
Check Answer
Spots
1IPv4 address: 190.0.5.20
2Number of groups: 6

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Quick Questions

1. Which is a correctly formatted IPv4 address?
A. 190.0.5.20 ✓
B. 190:0:5:20
IPv4 addresses use full stops to separate the four numbers, not colons.
2. How many bits are in an IPv4 address?
A. 128 bits
B. 32 bits ✓
IPv4 uses 32 bits. 128 bits is the size of an IPv6 address.
3. An IPv4 address is made up of...
A. eight groups of 16 bits
B. four groups of 8 bits ✓
4 groups x 8 bits = 32 bits. Eight groups of 16 bits describes IPv6 (128 bits).
4. What is the range of each number in an IPv4 address?
A. 0 to 255 ✓
B. 0 to 256
8 bits give 256 values, and counting from 0 the highest is 255.
5. Which character separates the numbers in an IPv4 address?
A. A full stop (.) ✓
B. A comma (,)
The four numbers are separated by full stops, as in 190.0.5.20.

Match the Pairs

190.0.5.20 → A valid IPv4 address: four denary numbers, each from 0 to 255, separated by full stops
255 → The largest denary value one group of 8 bits can hold in an IPv4 address
An IPv4 address is four groups of 8 bits, which makes 32 bits in total → 4 x 8 = 32
The four numbers in an IPv4 address are separated by full stops → 192.168.1.10
192.168.1.300 → The last group is 300, which is above 255. A group of 8 bits cannot go higher than 255.
192,168,1,10 → Commas are used instead of full stops. IPv4 groups must be separated by full stops.

Key Term

IPv4 address
32 bits written as four denary numbers from 0 to 255 separated by full stops, such as 190.0.5.20. Each number is a group of 8 bits.

Fill in the Blanks

1. An IPv4 address is 32 bits long in total.
Hint: Type the total number of bits in an IPv4 address.
2. Each number in an IPv4 address is 8 bits.
Hint: Type the number of bits in each group.
3. An IPv4 address has 4 numbers in it.
Hint: Type how many groups of numbers make up the address.
4. Each IPv4 number is from 0 to 255.
Hint: Type the highest value one 8-bit group can have in denary.
5. Example IPv4 address: 190.0.5.20
Hint: Type the separator character placed between the numbers.

Spot the Error

Question 1
A IPv4 address: 190.0.5.20
B Number of groups: 6
C Bits in each group: 8
D Total bits: 32
Line B — An IPv4 address has four groups (numbers), not six. It should be 'Number of groups: 4'.
Question 2
A address ← 172.16.254.1
B groups ← 4
C bitsPerGroup ← 8
D total ← groups + bitsPerGroup
Line D — The total number of bits is found by multiplying 4 groups by 8 bits (4 * 8 = 32), not adding. It should be total ← groups * bitsPerGroup.
Question 3
A ip ← 192.168.0.256
B highest ← 255
C lowest ← 0
Line A — Each group in an IPv4 address can only be 0 to 255, so 256 is not allowed. The last group should be 255 or less.
Question 4
A ip ← 10.20.30.40
B groups ← 4
C maxGroup ← 255
D bitsPerGroup ← 16
Line D — Each group is 8 bits, which is why its largest value is 255. It should be bitsPerGroup ← 8.
Question 5
A ip ← 203,0,113,9
B separator ← full stop
C groups ← 4
Line A — The four numbers in an IPv4 address are separated by full stops, not commas. It should be 203.0.113.9.

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