A2.3 · Book pp. 136–143
Data transmissions
A video you stream arrives as thousands of tiny packets, which may each take a different route. This section follows them from address to destination.
What you need to know
A2.3.1 Types of IP addressing
IPv4 and IPv6
| IPv4 | IPv6 | |
|---|---|---|
| Size | 32 bits (4 bytes) | 128 bits (16 bytes) |
| Written as | Four decimal numbers, 0–255 | Eight groups of four hex digits |
| Example | 192.168.0.1 | 2001:0db8:85a3:0000:0000:8a2e:0370:7334 |
| Possible addresses | 2³² ≈ 4.3 billion | 2¹²⁸ ≈ 3.4 × 10³⁸ |
✏️ Book correction
The book describes IPv6 as “eight groups of four bytes”. Each group is four hexadecimal digits, which is two bytes. Eight groups of two bytes makes the 16 bytes (128 bits) in total.
Public and private
Public addresses are unique worldwide, for websites and routers that face the internet. Private addresses are for use inside a network and cannot be routed on the internet. The IPv4 ranges are 10.0.0.0–10.255.255.255, 172.16.0.0–172.31.255.255 and 192.168.0.0–192.168.255.255.
Static and dynamic
A static address never changes, which servers need so that clients can find them. A dynamic address is given out by DHCP from a pool, which uses a limited supply of addresses efficiently.
Network address translation (NAT)
IPv4 addresses have all but run out. NAT lets many private devices share one public address. It also hides internal addresses from the outside.
Check yourself
- How many bytes is 2001:0db8:85a3:0000:0000:8a2e:0370:7334? 16. Each hex digit is 4 bits, and there are 32 digits.
- Does your phone’s Wi-Fi address appear on websites you visit? No, they see your router’s public address.
A2.3.2 Types of media
| Factor | Fibre optic | Twisted pair | Wireless |
|---|---|---|---|
| Bandwidth | Highest, up to terabits per second | Typically gigabit, up to about 10 Gb/s | Several Gb/s shared between devices |
| Interference | Immune to electromagnetic interference | Susceptible, especially unshielded | Very susceptible, and walls weaken it |
| Range | Tens or hundreds of km | About 100 m | About 90 m at best |
| Security | Very hard to tap undetected | Needs physical access | Can be intercepted from nearby |
| Cost | Highest | Low | Low, with no cabling |
| Installation | Specialist skills, and the cable is fragile | Simple and flexible | Simplest |
| Typical use | Undersea links, backbones, data centres | Offices and homes | Homes, campuses, mobile devices |
Attenuation is signal loss with distance. Fibre has very little, which is why it runs across ocean floors.
✏️ Book correction
The book says microwave ovens use 2.4 GHz because it is water’s resonant frequency. That is a popular myth. Water absorbs across a wide range of frequencies, and 2.4 GHz was chosen largely because that band was set aside for such uses and penetrates food well. The practical point stands: a leaking oven can disturb 2.4 GHz Wi-Fi.
A2.3.3 Packet switching
Older circuit switching held a dedicated channel for a whole session. Packet switching breaks data into packets that share the links.
- Segmentation: data is split into packets, typically up to 1500 bytes each (the MTU) including headers.
- Headers: each has the source and destination addresses, a sequence number and a checksum.
- Routing: each packet travels independently. Routers choose the next hop using routing tables, and switches forward within a LAN using MAC addresses.
- Reassembly: the destination puts the packets back in order and asks for any that are missing or corrupted.
⚠️ Common mistake
Do not mix up routers and switches. Routers join networks using IP addresses. Switches work inside one network using MAC addresses.
HL A2.3.4 Static and dynamic routing
| Static | Dynamic | |
|---|---|---|
| Routes set by | An administrator, by hand | An algorithm, as routers share information |
| Failed link | Stays broken until someone updates it | Traffic is rerouted automatically |
| Overhead | No extra processing or bandwidth | Extra processing and bandwidth |
| Scale | Small, stable networks | Large, changing networks |
| Behaviour | Predictable | Brief inconsistencies during convergence |
Quick quiz
HL Extra questions
Exam-style questions
These are written for this site in the style of Paper 1. Answer on paper first, then open the mark points.
-
Describe the main purpose of network address translation. [2]
Mark points
- It converts private IP addresses to a public address, and back [1].
- So many devices can share one public address, which conserves IPv4 addresses [1].
-
Outline why a home laptop usually has a dynamic IP address. [2]
Mark points
- A DHCP server assigns it from a pool when it connects [1].
- Addresses can be reused, which suits devices that join and leave [1].
-
Describe one advantage and one disadvantage of fibre-optic cable. [2]
Mark points
- Advantage: very high bandwidth, or immunity to interference [1].
- Disadvantage: high cost, or fragile and specialist to install [1].
-
A school links two buildings 400 m apart, and needs Wi-Fi inside each. Recommend media and justify your answer. [4]
Mark points
- Fibre optic between buildings [1], as twisted pair reaches only about 100 m [1].
- Wireless access points inside [1], which give mobility for portable devices without new cabling [1].
-
Explain how a large file is sent using packet switching. [4]
Mark points
- It is split into packets with headers that include addresses and sequence numbers [1].
- Routers forward each packet independently, by the best available route [1].
- Packets can arrive out of order, so the destination reorders them [1].
- Missing or corrupted packets are requested again [1].
HL Exam-style questions
-
Explain one advantage of static routing compared with dynamic routing. [3]
Mark points
- Routes are set manually, so traffic follows a predictable path [1].
- Routers need not exchange updates [1].
- So no extra processing or bandwidth is used [1].
-
Explain how dynamic routing deals with a failed link. [4]
Mark points
- Routers regularly share information about their connections [1].
- The failure is noticed by neighbouring routers and passed on [1].
- Routing tables are updated automatically with a new path [1].
- Traffic continues along that path, with possible brief inconsistency during convergence [1].