Introduction Network Layer in the OSI Model
Welcome to Video 4 in our CompTIA Network+ training series, where we break down the OSI Model layer by layer.
In this lesson, we dive into Layer 3 – the Network Layer. This is where communication across different networks becomes possible through logical addressing and packet forwarding.
Devices like routers operate at this layer, using IP addresses to route data efficiently between networks.
Whether you’re studying for your Network+ certification or just getting started with networking, understanding the role of the network layer is essential. Watch the video below and explore the detailed notes to strengthen your foundational knowledge of Layer 3 of the OSI Model.
📺 Watch: Network Layer in OSI Model | Network+ Video 4
Understand how data moves between networks and how IP addressing works.
What Is the Network Layer?
The Network Layer (Layer 3) of the OSI Model is responsible for routing data between different networks. While Layer 2 (Data Link Layer) handles communication within the same local network using MAC addresses, Layer 3 uses logical addressing (IP addresses) to connect devices across different networks—this is what makes the Internet possible.
Data Units: Packets
At the network layer, data is encapsulated into packets. Each packet contains:
- Source IP address
- Destination IP address
These addresses help determine how and where data should be routed.
Key Functions of the Network Layer:
- Routing: Finding the best path for data to reach its destination.
- Logical Addressing: Assigning and recognizing IP addresses (IPv4/IPv6).
- Packet Forwarding: Moving packets from one network to another using routers.
Devices Operating at Layer 3:
- Routers: The Primary device used to connect different networks.
- Layer 3 Switches: Advanced switches that can route data like routers.
- Firewalls: Some firewalls operate at Layer 3 for packet filtering.
Real-World Example:
Imagine three networks:
- Network 1: 1.1.1.0/24
- Network 2: 2.2.2.0/24
- Network 9: 9.9.9.0/24 (used as a transit network)
When a device in Network 1 wants to communicate with Network 2, data is sent to a router, which uses a routing table to determine the best path. The packet may pass through Network 9 before reaching Network 2. Each interface on a router has its own IP address and belongs to a specific network.
Datagram vs. Packet:
- When using UDP, the term datagram is often used.
- When using TCP, it’s commonly referred to as a packet.
Both serve the same purpose at Layer 3—to transport encapsulated data from one host to another using IP-based addressing.