The Network and Transport Layers

Above the local-only data link layer sit two layers that make wider communication possible. Layer 3 is the network layer and Layer 4 is the transport layer. The network layer moves data between separate networks using logical addresses, while the transport layer manages the conversation between applications, including reliability. Together they carry data across the internet and deliver it to the right program.

The network layer handles logical addressing and routing, choosing paths so data can travel across multiple networks. Its star is the IP address (Internet Protocol), a logical address that identifies a device anywhere on the internet, unlike a local MAC address. The network layer is where the journey from one network to a distant one is planned and directed.

An IP address is a logical, hierarchical address that can be routed across the internet. Because it is structured, routers can use parts of it to decide direction, like reading a postal code. IP addresses can be assigned and changed, unlike fixed MAC addresses. This logical addressing is what lets a packet find a server on the other side of the world.

Routers are the key devices at Layer 3. A router reads the destination IP address of each packet and forwards it toward the correct network, hop by hop. Routers connect different networks together, which is why your home router joins your LAN to the internet. Whenever data must cross from one network to another, a Layer 3 router makes the decision.

The network layer unit is the packet. A packet wraps the transport-layer data with an IP header containing the source and destination IP addresses. Routers read this header to route the packet. Unlike a frame, a packet is built to travel across many networks, keeping the same IP addresses end to end even as the local frames around it change at each hop.

The transport layer manages end-to-end communication between applications. It breaks data into segments, can ensure reliable delivery, and uses port numbers to direct data to the right program. While Layer 3 gets packets to the right device, Layer 4 makes sure they reach the right application on that device and arrive correctly.

TCP (Transmission Control Protocol) is the reliable transport protocol. It sets up a connection, numbers segments, confirms delivery with acknowledgments, and resends anything lost. TCP guarantees data arrives complete and in order, which is ideal for web pages, email, and file transfers. The trade-off is some extra overhead from all that careful tracking.

UDP (User Datagram Protocol) is the fast, lightweight transport protocol. It sends data without connections, acknowledgments, or guaranteed order. This makes UDP quick and efficient, ideal for live video, voice calls, and gaming, where speed matters more than resending a lost packet. UDP trades reliability for low overhead and minimal delay.

Port numbers let one device run many network applications at once. Each service listens on a port, such as 80 for web traffic or 443 for secure web traffic. The transport layer uses ports to deliver incoming data to the correct application. Without ports, a device could not tell whether arriving data was for the browser, email, or a game.

Together these layers form the core of internet delivery. The network layer uses IP addresses and routers to deliver a packet to the right device across many networks. The transport layer then uses ports and TCP or UDP to deliver the data to the right application reliably or quickly. Layer 3 finds the device; Layer 4 finds the program.

Layer 3, the network layer, handles logical IP addressing and routing through routers, moving packets between networks. Layer 4, the transport layer, manages end-to-end delivery with ports and chooses between reliable TCP and fast UDP. These middle layers are why data can cross the global internet and still arrive at exactly the right application.

Which layer uses IP addresses and routers to move data between separate networks?

The network layer (Layer 3) uses logical IP addresses and routers to route packets between separate networks. The transport layer (Layer 4) manages end-to-end delivery, using port numbers to reach the right application and choosing TCP for reliable, ordered delivery or UDP for fast, lightweight transfer. Layer 3 finds the device; Layer 4 finds the program. Together they carry data across the internet.