A protocol is a set of rules that defines how devices on a network communicate with each other.
On this page we're going to look at how data is actually sent across a network.
A protocol is a set of rules that defines how devices on a network communicate with each other.
The protocols involved in sending data across networks can be grouped into "layers".
Table 1 shows the protocol layers.
Table 1
| Layer | Description | Protocols |
|---|---|---|
| Application | Protocols in the application layer create the data that is being sent and interpret the data that is being received. | HTTP, HTTPS, FTP, SMTP, IMAP |
| Transport | Protocols in the transport layer set up communication between hosts. They are also responsible for splitting the data into packets, and reassembling the packets on receipt. | TCP and UDP |
| Internet | Protocols in the internet layer communicate with routers to make sure the packets reach the correct recipient. | IP |
| Link | Protocols in the link layer physically transmit the data packets from one device to another. | WLAN (Wi-Fi) and Ethernet |
When data is being sent, it starts at the application layer and makes its way down to the link layer. When data is being received, it starts at the link layer and makes its way up to the application layer.
Now, let's examine each protocol in detail.
The Transmission Control Protocol (TCP) operates at the transport layer.
Firstly, it establishes a connection between the sending and receiving devices.
Then, it splits the data into numbered "packets" (smaller bits of data).
It communicates with the receiving device to make sure all the packets have arrived. If not, they are re-sent.
It then reassembles the packets into the right order on the receiving device.
The sending device gets confirmation when the transfer is complete.
The User Datagram Protocol (UDP) also operates at the transport layer.
Just like TCP, it establishes a connection between the two devices and splits the data up into packets.
However, unlike TCP, it does not check if the packets have been received.
This makes UDP a lot faster than TCP, which is good for uses such as live streaming, however it is not very
good for anything else because some data might be missing.
The Internet Protocol (IP) operates at the internet layer.
You have probably heard of IP addresses before. These are unique numbers assigned to every device on a network.
The IP addresses of the sender and recipient are attached by the Internet Protocol to each data packet before it is sent.
Then, IP directs directs the data packets across networks in a process known as packet switching.
Each packet is sent between a series of routers: each router reads the packet's destination IP address and
decides where to send it next.
The HyperText Transfer Protocol (HTTP) operates at the application layer.
It manages the transmission of data between a web server and a client (browser), providing a way for users to interact with webpages.
HyperText Transfer Protocol Secure (HTTPS) is an extension to HTTP that encrypts communications between the web server and the client, meaning that it is not possible for data to be stolen.
Your browser used HTTPS to load this page onto your device.
The File Transfer Protocol (FTP) also operates at the application layer.
It allows users to transfer files between client and server computers. Servers can accept connections from
multiple clients simultaneously and limit file access to authenticated users or provide anonymous access to
any connected user.
The Simple Mail Transfer Protocol (SMTP) operates at the application layer.
It controls the transmission of email messages from a user's email client to their mail server, and from one
user's mail server to another, using a process called "store and forward". Messages are then stored on the
recipient's mail server, waiting to be downloaded via IMAP.
The Internet Messaging Access Protocol (IMAP) also operates at the application layer.
It allows email client applications to interact with mail servers, providing users with the option to
download and read their messages, delete messages, and organise messages within folders. Changes (including
whether a message is read or not) are synchronised with the server, providing consistency across all of a
user's devices.
Wi-Fi is a family of related protocols which operate at the link layer.
It controls the physical transmission of data as radio waves.
It uses two radio frequency bands: 2.4GHz (which has a greater range and is better at getting through walls)
and 5GHz (which is faster over shorter distances).
Ethernet is also a family of related protocols which operate at the link layer.
It controls the physical transmission of data as electrical currents.
While Wi-Fi is specifically for wireless connections, Ethernet is specifically for wired connections.
IPv4 is the most common type of IP address. It is essentially a 32-bit number. How many unique IPv4 addresses are there?
Tap/click to reveal
232 ≈ 4.3 billion unique IPv4 addresses. This might sound like a lot, but
we're actually running out of them. To solve this, many devices now use IPv6, which are 128-bit numbers.
There are over 300 undecillion (3 followed by 38 zeros) unique IPv6 addresses.