What Are Network Layers?

In computer technologies, network layers are a conceptual framework that provides a common understanding of how a computer network is architected and how different technologies interrelate to enable network communications. Two network layer models are widely referenced today: The Open Systems Interconnection (OSI) model and the TCP/IP model are the most common.

There are other network layers like UDP and ICMP that all run on IP networking, as well as the Recursive InterNetwork Architecture (RINA) model. The TCP/IP model used to be referenced as the Department of Defense (DoD) model, DARPA model, or ARPANET model, but was changed due to the perception that these are government services and not standards for wider use.

Computer networks are highly complex technologies that involve multiple types of software, operating systems, computers, routers, switches, firewalls, connections, and more. A framework that divides all this technology into different categories or layers helps to organize system communications and troubleshoot issues more efficiently. The network layer framework also enhances vendor interoperability by creating standards for devices to talk with each other. Layers help vendors adopt standards and protocols that ensure consistency in the way technology in one layer interacts with technology in layers above and below it.

The OSI model is a working product of the Open Systems Integration group at the International Organization for Standardization (ISO) and was initially published in 1984. The OSI reference model breaks down complex network systems into seven discrete layers based on the type of tasks performed and the role of each system within the networking stack. The OSI network layers include:

• Layer 1 — the Physical Layer. The physical layer sends and receives raw bit streams over a physical medium.
• Layer 2 — the Data Link Layer. This layer defines the format of data to be transmitted, breaking into frames for transition at the physical layer. It also manages connections between two different nodes, handles error control, and terminates connections when a session is complete.
• Layer 3 — the Network Layer. The network layer is responsible for forwarding and routing data packets efficiently across multiple networks.
• Layer 4 — the Transport Layer. This layer coordinates end-to-end reliable and transparent data transfer between network hosts.
• Layer 5 — the Session Layer. The session layer manages the communication sessions and synchronization between applications on different devices. It also handles setup, authentication, termination, and reconnections.
• Layer 6 — the Presentation Layer. This layer translates data for the application layer, handling data formatting, compression, encryption, and decryption.
• Layer 7 — the Application Layer. This layer provides network services directly to applications, including protocols for email, web browsing, and file transfer.

The TCP/IP model is another widely used reference for understanding network layers. The four layers of the TCP/IP model include:

• The application layer. This layer combines the application, presentation, and session layers of the OSI model. The application layer provides application-specific protocols for services like web browsing (HTTP), email (SMTP), and file transfers (FTP). DNS services also operate at the application layer.
• The transport layer. This layer maps to the transport layer of the OSI model and manages communication services, including reliable transmission through the transmission control protocol (TCP) or connectionless datagram delivery through the User Datagram Protocol (UDP).
• The internet layer. This layer is equivalent to the network layer in the OSI model and is responsible for addressing, routing, and managing the fragmentation of IP packets. In addition to the internet protocol (IP), ARP, IGMP, and ICMP also operate at this layer.
• The network access layer. This layer correlates to the physical and data link layers of the OSI model and handles the transmission of network datagrams over specific physical networks.

In general, the OSI model provides a more comprehensive and modular framework, while TCP/IP is a simpler model with practical relevance and widespread adoption.

• OSI is a generic, protocol-agnostic model designed to encompass and describe all forms of network communication. In contrast, TCP/IP is a functional model that’s based on specific, standard protocols to address specific communication issues.
• In the modular framework of the OSI model, each OSI layer is distinct and has specific functions, allowing for easier troubleshooting and interoperability of protocols between layers. OSI also provides a standardized framework for network communications, enhancing compatibility and interoperability between different vendor products.
• The TCP/IP model aligns closely with the actual implementation of network protocols used on the internet today. Its flexibility and simplicity make it suitable for a wide range of network sizes and topologies.