A protocol is a standard set of rules that allow electronic devices to communicate with each other. These rules include what type of data may be transmitted, what commands are used to send and receive data, and how data transfers are confirmed.
You can think of a protocol as a spoken language. Each language has its own rules and vocabulary. If two people share the same language, they can communicate effectively. Similarly, if two hardware devices support the same protocol, they can communicate with each other, regardless of the manufacturer or type of device. For example, an Apple iPhone can send an email to an Android device using a standard mail protocol. A Windows-based PC can load a webpage from a Unix-based web server using a standard web protocol.
Protocols exist for several different applications. Examples include wired networking (e.g., Ethernet), wireless networking (e.g., 802.11ac), and Internet communication (e.g., IP). The Internet protocol suite, which is used for transmitting data over the Internet, contains dozens of protocols. These protocols may be broken up into four categories:
- Link layer - PPP, DSL, Wi-Fi, etc.
- Internet layer - IPv4, IPv6, etc.
- Transport layer - TCP, UDP, etc.
- Application layer - HTTP, IMAP, FTP, etc.
Link layer protocols establish communication between devices at a hardware level. In order to transmit data from one device to another, each device's hardware must support the same link layer protocol. Internet layer protocols are used to initiate data transfers and route them over the Internet. Transport layer protocols define how packets are sent, received, and confirmed. Application layer protocols contain commands for specific applications. For example, a web browser uses HTTPS to securely download the contents of a webpage from a web server. An email client uses SMTP to send email messages through a mail server.
Protocols are a fundamental aspect of digital communication. In most cases, protocols operate in the background, so it is not necessary for typical users to know how each protocol works. Still, it may be helpful to familiarize yourself with some common protocols so you can better understand settings in software programs, such as web browsers and email clients.