How devices talk: addresses and protocols

For two devices to talk, they need to find each other and agree how to talk. Addresses do the first job; protocols do the second.

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Addresses

An IP address identifies a device on a network so data can be sent to it.
An IPv4 address is 32 bits, written as four numbers from 0 to 255 separated by dots, for example 192.168.1.25. Each number is 8 bits.
IPv4 allows 232 = 4,294,967,296 addresses, which is not enough for every device in the world, so the longer IPv6 (128 bits) was created.
A MAC address identifies a device's network hardware. It is 48 bits, usually written in hexadecimal, and is normally set by the manufacturer. A device's IP address can change when it joins a different network.

Protocols

A protocol is a set of rules for how data is sent, so different devices and programs can understand each other.
ProtocolUsed for
HTTPsending web pages to a browser
HTTPSsending web pages securely, encrypted
SMTPsending email
IMAP and POPfetching email from a mail server
FTPtransferring files
TCP/IPsplitting data into packets and getting them to the right address

Wired and wireless

Bluetooth is designed for short distances, such as connecting headphones to a phone.
Wired (for example Ethernet cable)Wireless (for example Wi-Fi, Bluetooth)
Speed and reliabilityusually faster and more reliablecan slow down with distance, walls and interference
Moving arounddevices must stay near a socketdevices can move freely
Securitysomeone needs physical access to the cablesignals travel through the air, so they should be encrypted
Worked example

Is 192.168.300.4 a valid IPv4 address? Explain.

  1. An IPv4 address has four numbers, each stored in 8 bits.
  2. 8 bits can hold 0 to 255.
  3. 300 is bigger than 255, so it cannot be stored.

Answer: No, because 300 is more than 255.

Key idea

IP addresses (IPv4: four numbers from 0 to 255) and MAC addresses identify devices. Protocols are agreed rules, such as HTTPS for secure web pages and SMTP for sending email. Wired is usually faster and more reliable; wireless lets devices move.

Key words
AND
A Boolean operator that gives True (1) only when both inputs are True.
IP address
A unique number that identifies a device on a network so data can be sent to it.
NOT
A Boolean operator that flips its input: True becomes False and False becomes True.
Hardware
The physical parts of a computer system that you can touch, such as the CPU, keyboard and screen.

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Extra practice

2 more questions

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  1. Why do devices need protocols?

    1. To make cables longer
    2. To speed up the CPU
    3. To store files
    4. So devices from different makers can communicate using the same rules
    Show the answer

    Answer: So devices from different makers can communicate using the same rules

    Shared rules let devices communicate.

  2. Which protocol is used to FETCH email from a server and keep it there?

    1. SMTP
    2. HTTP
    3. IMAP
    4. FTP
    Show the answer

    Answer: IMAP

    IMAP keeps email on the server.

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Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.