Communication

IP addresses and subnetting

Networks including the internet AS Level 15 min read

On this page

What you need to be able to do
IPv4
IPv6
Public and private, static and dynamic
Subnetting
Mark-scheme focus
Worked examples
Summary
Can I do this?
Quiz

What you need to be able to do

Syllabus 2.1 Networks including the internet

2.1Describe the format of IPv4 and IPv6 addresses and decide whether an address is valid.
2.1Explain subnetting: network ID and host ID, subnet masks, and why networks are subnetted.
2.1Explain how an IP address is associated with a device on a network.
2.1Explain public vs private (and the security implications) and static vs dynamic IP addresses.

You need: binary, denary and hex from Chapter 1, and routers. Next: URLs and DNS.

An IP address identifies a device (strictly, its network connection) so that packets can be routed to it across networks. It is a logical address that can change; the MAC address of the network card is a fixed hardware address used inside one LAN.

IPv4

Format

32 bits written as 4 groups (octets) of 8 bits, each shown in denary 0 to 255, separated by full stops: 192.168.3.2

How many?

\(2^{32}\approx4.3\) billion addresses: not enough for every device now online. That shortage is why IPv6 was introduced (and why LANs use private addresses behind one public one).

Address	Valid IPv4?	Why
192.168.3.2	yes	4 groups, each 0-255, full stops
256.0.0.1	no	256 is more than 255 (8 bits hold 0-255)
10.4.7	no	only 3 groups
172.16.300.1	no	300 > 255
256.0.0.A	no (and not IPv6 either)	256 > 255; A is not a denary digit; not IPv6 because it uses full stops not colons and has too few groups

IPv6

Format

128 bits written as 8 groups of 4 hexadecimal digits (each group 16 bits, 0000-FFFF), separated by colons: 2001:0db8:0000:0000:0000:ff00:0042:8329

Shortening

Leading zeros in a group can be dropped (0db8 → db8). One run of consecutive all-zero groups can be replaced by a double colon “::”, only once in an address: 2001:db8::ff00:42:8329

How many?

\(2^{128}\approx3.4\times10^{38}\) addresses: enough for every device for the foreseeable future, so no need to share public addresses.

	IPv4	IPv6
Size	32 bits	128 bits
Groups	4 groups of 8 bits	8 groups of 16 bits
Written in	denary (0-255)	hexadecimal (0-FFFF)
Separator	full stops	colons; “::” for consecutive zero groups

“::” is a double colon. A single colon was the commonest wrong answer in June 2023. And it can appear only once: 2001:db8::1::5 is invalid because the reader couldn't tell how many zero groups each one stands for.

Public and private, static and dynamic

Type	Meaning (credited wording)	Notes
Public	assigned to a device to allow it to be visible on / directly accessed from the internet	unique across the whole internet; e.g. the router's internet-facing address, a web server
Private	can only be accessed by other devices in the same LAN; assigned by the router within the LAN	reused in every LAN (e.g. 192.168.x.x, 10.x.x.x); never seen on the internet
Static	does not change each time the device connects to the network	set manually or reserved; needed for servers, printers, anything others must always find
Dynamic	may change each time the device connects (re-joins) the network	handed out automatically (DHCP) from a pool; most phones and laptops

A router has both: a public address so it is visible to the internet / WAN, and a private address so the computers within the LAN can identify it. Through Network Address Translation it lets every device on the LAN share the one public address.

Security implication of private addresses

Devices with private addresses cannot be reached directly from the internet: an outsider sees only the router's public address, which (with its firewall) filters what gets in. A public address is directly visible, so it is more exposed to attack.

How an IP address gets onto a device

Dynamic: when a device joins the LAN, the router (DHCP server) allocates it an IP address from its pool, for a limited time (a lease). Static: the address is configured manually or reserved in the router for that device's MAC address.

Subnetting

Subnetting divides a network into smaller networks (subnetworks). Each IP address is then split into two parts: the network ID (which subnetwork) and the host ID (which device in it).

Every device in a subnetwork has the same network ID; each subnetwork has a different network ID.
The host ID uniquely identifies the device within its subnetwork.
A subnet mask shows where the split is: the octets that are 255 (all 1s in binary) mark the network ID; the octets that are 0 mark the host ID.
The device / router finds the network ID by ANDing the IP address with the mask, bit by bit: 1 keeps the bit, 0 clears it.

IP address	Subnet mask	Network ID	Host ID
192.168.5.77	255.255.255.0	192.168.5	77
172.16.40.9	255.255.0.0	172.16	40.9
10.10.12.1	255.0.0.0	10	10.12.1

In the last octet the mask is 0, so ANDing clears it: that octet belongs to the host ID, not the network ID.
IP 77		1	1	1	1
mask 0	AND	0	0	0	0
→ host part		0	0	0	0

Why subnet?

Reason	Because…
Reduces network traffic / congestion, improves performance	data sent between devices in the same subnet stays in that subnet, so it doesn't flood the whole network
Improves security	not all devices can access all segments; a compromised device does not expose the whole network; devices don't receive data not meant for them
Easier maintenance / management	faults can be isolated: one subnetwork can be taken down or changed while the rest continues
Easier to expand / control growth	new subnetworks can be added in an organised way

The June 2023 report: many described the general benefits of any network instead of the benefits of subnetting, and wrote “Network IP / Host IP” instead of network ID / host ID.

Mark-scheme focus

Complete the description of IPv4 and IPv6 (5 marks)

(d) The CCTV cameras transfer data wirelessly. (i) Complete the description: An IPv4 address contains … groups of digits. Each group is represented in … bits and the groups are separated by full stops. An IPv6 address contains … groups of digits. Each group is represented in … bits. Multiple groups that only contain zeros can be replaced with a … . [5] — 9618/11/M/J/23 Q4Paper 1 · Theory fundamentalsPart (d)(i)5 marksFull size

Credited

IPv4: 4 groups, 8 bits each B2
IPv6: 8 groups, 16 bits each B2
zero groups replaced with :: (a double colon) B1

Not credited

IPv6 groups of 4 bits (it is 4 hex digits = 16 bits)
a single colon “:”

From: 9618/11/M/J/23 Q4(d)(i)

Identify and describe the two parts of an IP address in a subnetwork (3 marks)

(e) One site has split the network into several subnetworks. An IP address in a subnetwork is divided into two parts. Identify and describe the two parts of an IP address in a subnetwork. [3] — 9618/13/M/J/23 Q2Paper 1 · Theory fundamentalsPart (e)3 marksFull size

Credited

Network ID and host ID B1
Each device in a subnetwork has the same network ID // each subnetwork has a different network ID B1
Every device in a subnetwork has a different host ID // the host ID uniquely identifies the device within the subnetwork B1

Not credited

“Network IP and host IP”
Public and private (a different idea)

From: 9618/13/M/J/23 Q2(e)

Complete the table of IP address types (4 marks)

(d) Complete the table of types of IP addresses and descriptions: (type missing) an IP address assigned to a device to allow direct access on the internet; static IP address (description missing); (type missing) an IP address used for internal LAN communication only; dynamic IP address (description missing). [4] — 9618/12/O/N/23 Q7Paper 1 · Theory fundamentalsPart (d)4 marksFull size

Credited

public IP address B1
static: fixed / doesn't change each time a device re-joins a network B1
private IP address B1
dynamic: may be refreshed / changed each time a device re-joins a network B1

Not credited

static = “stays still”; dynamic = “moves”
mixing public/private with static/dynamic

From: 9618/12/O/N/23 Q7(d)

IP addresses in almost every paper: fill-in descriptions (5-6 marks), IPv4 vs IPv6 differences (2), valid / invalid with justification (2), types (2-4).
Subnetting: purpose / benefits (2-4), network ID and host ID (3), reading a subnet mask (2).

Worked examples

Example 1 · Static and public; is 256.0.0.A valid? (Paper 1, 4 marks) ★ Easy

Web browsers use IP addresses. (a) Complete the table by describing these types of IP address: Static, Public. [2] (b) Consider the IP address 256.0.0.A. Circle whether it is IPv4, IPv6 or an invalid IP address. Justify your choice. [2] — 9618/11/O/N/25 Q7Paper 1 · Theory fundamentalsParts (a) and (b)4 marksFull size

1 Read it with me what is actually happening?

(b) test it against both formats: is every IPv4 group 0-255? Does it look like IPv6 (8 hex groups with colons)?

justifyno mark for circling; 2 for reasons

2 What is it asking for? the target, part by part

(a)two descriptions2
(b)justification2

3 What do we know? everything the question gives us

256.0.0.A

4 Plan the route work backwards from what we want

FindKey ideaAlready have itWork out firstTrap

(b)Invalid
- Not IPv4256 > 255
- Not IPv6full stops, too few groups

5 Now solve it have a go on paper first, then open the worked solution

(a) Static: an IP address that does not change each time a device connects to the network B1; Public: assigned to a device to allow it to be visible on the internet B1
(b) Invalid (no mark): not IPv4 because the maximum denary value is 255 and it uses 256 B1; not IPv6 because it uses full stops not colons // there are not enough groups B1

6 Check it does the answer make sense?

“A” could be a hex digit, but hex belongs to IPv6, which this can't be ✓.

Example 2 · IP address fill-in (Paper 1, 6 marks) ★ Easy

(c) Devices that connect to the WAN have an IP address. Complete the description: IPv4 is displayed as four groups of 8-bit numbers separated by … . Each IPv4 address is 32 bits. IPv6 is made of eight groups of 4 … numbers separated by colons. Multiple consecutive groups of … can be replaced with a double colon. Each IPv6 address is … bits. A … IP address can change each time the computer connects to a network. A … IP address can only be accessed by other devices in the same LAN and is assigned by the router within the LAN. [6] — 9618/12/M/J/25 Q6Paper 1 · Theory fundamentalsPart (c)6 marksFull size

1 Read it with me what is actually happening?

Six gaps: four about format, two about types. Read the clue after each gap.

2 What is it asking for? the target, part by part

(c)six words6

3 What do we know? everything the question gives us

4 Plan the route work backwards from what we want

FindKey ideaAlready have itWork out firstTrap

5 Now solve it have a go on paper first, then open the worked solution

full stops
hexadecimal
zeros
128
dynamic
private B6

6 Check it does the answer make sense?

128 = 8 groups × 16 bits ✓.

Example 3 · Subnetting benefits and reading subnet masks (Paper 1, 4 marks) ★★★ Hard

(c) LANs can be made up of several subnetworks. (i) Give two benefits of dividing a network into subnetworks by subnetting the LAN. [2] (ii) Two devices are in different subnetworks: 10.10.12.1 with subnet mask 255.0.0.0, and 192.168.12.4 with subnet mask 255.255.255.0. Identify the network ID for the device with IP address 10.10.12.1, and the host ID for the device with IP address 192.168.12.4. [2] — 9618/13/O/N/22 Q7Paper 1 · Theory fundamentalsPart (c)4 marksFull size

Why it is hard: most candidates had never applied a mask; the octet-by-octet line-up is all it takes.

1 Read it with me what is actually happening?

(ii): line the mask up with the address, octet by octet. 255 → that octet is network ID; 0 → host ID.

255.0.0.0first octet network, last three host
255.255.255.0first three network, last host

2 What is it asking for? the target, part by part

(c)(i)two benefits of subnetting2
(c)(ii)network ID, host ID2

3 What do we know? everything the question gives us

4 Plan the route work backwards from what we want

FindKey ideaAlready have itWork out firstTrap

(ii)
- Device 1mask 255 only in octet 1 → network ID 10
- Device 2mask 0 only in octet 4 → host ID 4

Why is the network ID of 10.10.12.1 not 10.10?

Only the first octet of its mask is 255.

What did the report say?

“A small number of correct answers … many need to improve their understanding of subnet masks.”

5 Now solve it have a go on paper first, then open the worked solution

(i) improves security B1; reduces congestion // allows extension of the network // aids day-to-day management // improves performance B1
(ii) network ID of 10.10.12.1 = 10 B1
host ID of 192.168.12.4 = 4 B1

6 Check it does the answer make sense?

10.10.12.1 AND 255.0.0.0 = 10.0.0.0 ✓; 192.168.12.4 has host part .4 under the 0 octet ✓.

Example 4 · A router's public and private address; why subnet (Paper 1, 4 marks) ★★ Medium

(d) The LAN has a router with a public IP address and a private IP address. (i) State the purpose of a public IP address and a private IP address. [2] (ii) The LAN uses subnetting. Describe the purpose of subnetting in a network. [2] — 9618/13/O/N/24 Q9Paper 1 · Theory fundamentalsPart (d)4 marksFull size

1 Read it with me what is actually happening?

(i): who each address is for. (ii): what subnetting does + one consequence.

2 What is it asking for? the target, part by part

(d)(i)two purposes2
(d)(ii)purpose2

3 What do we know? everything the question gives us

4 Plan the route work backwards from what we want

FindKey ideaAlready have itWork out firstTrap

5 Now solve it have a go on paper first, then open the worked solution

(i) Public: so that the router is visible to the internet / external network / WAN B1; private: so that the router is identified to the computers within the LAN B1
(ii) Divides the network into smaller networks B1 … which reduces traffic / congestion because traffic only travels through the parts necessary // easier maintenance B1

6 Check it does the answer make sense?

✓

Summary

IPv4: 32 bits, 4 × 8-bit denary groups (0-255), full stops. IPv6: 128 bits, 8 × 16-bit hex groups, colons, “::” once for zero groups.
Public: visible on the internet. Private: within the LAN only, assigned by the router (more secure). Static: never changes. Dynamic: may change at each connection (DHCP).
Subnetting: network ID + host ID; mask 255 = network part, 0 = host part; less traffic, more security, easier management.

If you see…	do this
“is this IPv4 / IPv6 / invalid?”	check group count, value range, separator, digits
a subnet mask	255 octets → network ID; 0 octets → host ID
“purpose of subnetting”	divides network + consequence (traffic, security, management)
public / private vs static / dynamic	visibility vs permanence: two separate ideas

Can I do this?

I can describe IPv4 and decide whether an address is valid.
I can describe IPv6, shorten it correctly and compare it with IPv4.
I can explain public, private, static and dynamic addresses and how a device gets one.
I can explain network ID / host ID and read a subnet mask.
I can give the purposes / benefits of subnetting.

Anything unticked: revise that section first. All ticked: take the quiz.

Quiz

The multiple-choice and guided questions need JavaScript. The concept checks and exam practice below work without it.

Concept checks

Answer each one in your head first, then open it.

1Why can two different homes both use 192.168.1.5?Show answer

It is a private address: it only has to be unique inside each LAN. On the internet both homes appear as their routers' different public addresses.

2Why does IPv6 use hexadecimal?Show answer

128 bits in denary groups would be very long; each hex digit is exactly 4 bits, so 8 groups of 4 hex digits write it compactly.

3Why must “::” appear only once?Show answer

It stands for “as many zero groups as needed to make 8”; with two of them you couldn't tell how many zeros each one replaces.

Exam-style practice

New questions, not used above. Do them on paper, then mark your work against the official mark scheme.

(d) Draw one line from each term (public, private, dynamic, static IP address) to its most appropriate description: only visible to devices within the LAN; increments by 1 each time the device connects; a new one is reallocated each time a device connects to the internet; can only be allocated to a router; visible to any device on the internet; does not change each time a device connects to the internet. [4] — Q19618/12/M/J/21 Q5Paper 1 · Theory fundamentalsPart (d)4 marksFull size

Show the mark scheme

Part	Answer / working	Marks
(d)	Public → it is visible to any device on the internet	B1
	Private → it is only visible to devices within the LAN	B1
	Dynamic → a new one is reallocated each time a device connects to the internet	B1
	Static → it does not change each time a device connects to the internet	B1
Total: 4 marks

(d) The devices in the office have static private IP addresses. State what is meant by a static private IP address. [1] — Q29618/13/M/J/24 Q5Paper 1 · Theory fundamentalsPart (d)1 markFull size

Show the mark scheme

Part	Answer / working	Marks	Guidance
(d)	Static: the IP for that device does not change; private: it can only be accessed / seen / used within the LAN	B1	both ideas needed
Total: 1 mark

Q3Past-paper style3 marks

A device has IP address 192.168.20.33 and subnet mask 255.255.255.0.

(a)
State the network ID and the host ID.
[2]
(b)
State whether 2001:db8::1::7 is a valid IPv6 address. Justify your answer.
[1]

Show the mark scheme

Part	Answer / working	Marks
(a)	Network ID: 192.168.20	B1
	Host ID: 33	B1
(b)	Invalid: “::” is used twice (it may only appear once)	B1
Total: 3 marks

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