Protocols and Models

Network communication works because devices follow agreed rules. These rules define how messages are formatted, addressed, transmitted, received, checked, and interpreted. In networking, these rules are called protocols.

Human communication and network communication both require rules. For a message to succeed, the sender and receiver must understand the message format, language, timing, and expected response.

Network rules commonly define:

• Message encoding
• Message formatting and encapsulation
• Message size
• Message timing
• Delivery options
• Error handling

A message may be sent to one destination, many destinations, or all devices in a local network segment.

A protocol is a set of rules that controls one part of communication. Network communication requires many protocols working together. For example, one protocol may handle addressing, another may handle routing, another may handle reliability, and another may handle application data.

Protocols may define:

• How data is formatted
• How addresses are used
• How errors are reported
• How data is divided and reassembled
• How devices know when to send
• How sessions begin and end

A protocol suite is a group of protocols designed to work together. The most important suite in modern networking is the TCP/IP protocol suite, which supports communication on the internet and most private networks.

Common TCP/IP protocols include:

Networking depends on open standards so devices from different vendors can interoperate. Standards organizations develop and maintain technical rules for internet technologies, Ethernet, wireless networks, addressing, and many other systems.

Examples of standards-related groups include organizations focused on internet protocols, addressing coordination, electrical and electronic standards, and telecommunications.

A reference model divides network communication into layers. Layered models make complex systems easier to study because each layer has a defined role.

OSI Model

TCP/IP Model

As data moves down the protocol stack, each layer adds control information. This process is called encapsulation. The unit of data at each stage is often called a Protocol Data Unit (PDU).

When data reaches the destination, the receiving device removes the added information step by step. This reverse process is called de-encapsulation.

Diagram description: An encapsulation diagram usually shows data moving downward through layers. Each layer adds a header, and the data becomes a segment, packet, frame, and finally bits on the medium.

To deliver data, a device needs both logical and physical addressing. The IP address identifies the source and destination across networks, while the MAC address is used for delivery on the local link.

When a message stays within the same network, the sender uses the destination device's MAC address. When a message must reach a remote network, the sender uses the MAC address of the default gateway for the next local hop.