← IGCSE Computer Science Paper 1

Shortening an IPv6 address

Replaces one run of all-zero groups in an IPv6 address, and can be used only once. Leading zeros in each group can also be dropped, e.g. 0db8 becomes db8.

Part of: IGCSE Computer Science 0478 — 3.4 Network Hardware — Module 18, WA Computer Science ATAR Year 11 — Network Communications — Module 42

Understanding Shortening an IPv6 address in IGCSE Computer Science Paper 1

What it does: The six all-zero groups in a row are replaced by a single ::, giving fe80::1. ::1 has only one written group, so :: stands for seven all-zero groups to reach eight. Only leading zeros can be dropped. The zeros at the end of the group are part of the value, so 0A00 becomes A00. The second address uses :: twice, which is not allowed. 0d80 should become d80, because only the leading 0 is removed.

Common mistakes: Both leading zeros must be dropped, so 00a4 becomes a4. Dropping only one zero leaves 0a4. A group of all zeros shortens to a single 0, not to nothing. Only :: can stand in for whole zero groups.

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This module's key terms are also in a printable crossword (Modules 342–354) and word search (Modules 325–354).

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Learning

Duo

An IPv6 group is 0A00. Which is its shortened form with leading zeros dropped?

AA00

Matches

2001::db8::1

↓

The :: is used twice. It can only appear once, otherwise nobody can tell how many zero groups each one replaces.

Applying

Meanings

Term

Meaning

Replaces one run of all-zero groups in an IPv6 address, and can be used only once. Leading zeros in each group can also be dropped, e.g. 0db8 becomes db8.

Practice

2001:0db8:0000:0000:0000:0000:0000:0001

Testing

Gaps

Short: 2001:db81

Check Answer

Spots

3drop all leading zeros

4short = 0a4

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Quick Questions

1. An IPv6 group is 0A00. Which is its shortened form with leading zeros dropped?

A. A00 ✓

B. A

Only leading zeros can be dropped. The zeros at the end of the group are part of the value, so 0A00 becomes A00.

2. How many times can :: appear in one IPv6 address?

A. Once for every run of zero groups

B. Only once ✓

Using :: twice would make it impossible to tell how many zero groups each one replaced, so it is allowed only once per address.

3. Which is a valid shortened IPv6 address?

A. 2001::db8::1

B. 2001:db8::1 ✓

The second address uses :: twice, which is not allowed.

4. The address 2001:db8::1 is expanded back to eight groups. What does :: stand for?

A. Five groups of 0000 ✓

B. Three groups of 0000

The address shows 3 groups (2001, db8 and 1). Eight groups are needed, so :: replaces 8 - 3 = 5 all-zero groups.

5. What can be replaced with :: in an IPv6 address?

A. A run of groups that are all zeros ✓

B. Any group that contains a zero

Only groups made entirely of zeros (0000) can be replaced. A group such as 0db8 still holds data, so it can only lose its leading zero.

Match the Pairs

0db8 -> db8 → Drops the leading zero from a group, so the group is written with fewer digits

2001:db8::1 → Expands to 2001:0db8:0000:0000:0000:0000:0000:0001, with :: standing for five all-zero groups

Replace one run of all-zero groups with :: to shorten the address → fe80:0:0:0:0:0:0:1 -> fe80::1

Expand a shortened address back to eight groups by filling :: with zero groups → ::1 = 0000:0000:0000:0000:0000:0000:0000:0001

2001::db8::1 → The :: is used twice. It can only appear once, otherwise nobody can tell how many zero groups each one replaces.

0d80 -> d8 → Only leading zeros can be dropped. The zero at the end of the group is part of the value, so d8 is a different number.

Key Term

::

Replaces one run of all-zero groups in an IPv6 address, and can be used only once. Leading zeros in each group can also be dropped, e.g. 0db8 becomes db8.

Fill in the Blanks

1. Full: 2001:0db8:0000:0000:0000:0000:0000:0001 Short: 2001:db8::1

Hint: Type the symbol that replaces one run of all-zero groups.

2. Full: fe80:0000:0000:0000:0202:b3ff:fe1e:8329 Short: fe80::202:b3ff:fe1e:8329

Hint: Type the group that is left after the leading zero is dropped from the fourth-from-last group.

3. Short: 2001:db8::ff00:42:8329 Full: 2001:0db8:0000:0000:0000:ff00:0042:8329

Hint: Type the group that has had its leading zeros put back, so it has four digits.

4. Rule: :: can be used only once per address

Hint: Type how many times :: may appear in one IPv6 address.

5. Address: fe80::1 Groups written out: 2 of 8 :: replaces 6 groups of 0000

Hint: Type the number of groups missing from the full eight.

Spot the Error

Question 1

A group = 00a4

B leading zeros = 2

C drop all leading zeros

D short = 0a4

Line D — Both leading zeros must be dropped, so 00a4 becomes a4. Dropping only one zero leaves 0a4.

Question 2

A group = 0000

B short = nothing

C a group is never left empty

Line B — A group of all zeros shortens to a single 0, not to nothing. Only :: can stand in for whole zero groups.

Question 3

A group = 0d00

B short = d

C trailing zeros must stay

Line B — Only leading zeros can be dropped. The two zeros after the d are trailing zeros, so 0d00 becomes d00.

Question 4

A addr = 2001:db8:0:0:1:0:0:2

B zero runs: 0:0 and 0:0

C use :: on both runs

D short = 2001:db8::1:0:0:2

Line C — The :: can be used only once in an address. If it were used twice, nobody could tell how many zero groups each one hides. It should say to use :: on only one run.

Question 5

A short = fe80::1

B groups shown = 2

C zero groups hidden = 5

D full has 8 groups

Line C — A full address has 8 groups and 2 are shown (fe80 and 1), so :: hides 8 - 2 = 6 zero groups, not 5.

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