Demystifying Network Layers: From Physical to Application

Network Layering Overview

A network connects hosts via physical links (cable, fiber, radio) to enable data exchange. Communication follows a layered model where each layer provides specific services to the layer above and uses protocols to ensure interoperability.

Physical Layer

Transmits raw binary signals (0/1) over media such as twisted‑pair, fiber optics, or wireless. It defines voltage levels, timing, and bit synchronization but carries no meaning for the data.

Data Link Layer

Groups bits into frames and adds a header that contains source MAC (6 bytes), destination MAC (6 bytes), and EtherType (2 bytes). Frames range from 64 bytes to 1518 bytes. MAC addresses are 48‑bit (12‑hex‑digit) identifiers unique to each network interface.

Broadcast frames are sent to the MAC address FF:FF:FF:FF:FF:FF; all devices on the LAN receive them, but only the device whose MAC matches the destination processes the payload.

Network Layer

Introduces logical IP addressing to route packets across subnets, avoiding broadcast flooding. An IP packet consists of a header (20‑60 bytes) and payload (up to 65 535 bytes).

Subnet masks (e.g., 255.255.255.0) separate the network and host portions of an IPv4 address using a binary AND operation. Devices in the same subnet share the same network prefix.

ARP resolves an IP address to a MAC address by broadcasting an ARP request; the target host replies with its MAC. ARP packet format includes sender MAC/IP, target MAC/IP, and optional data.

Transport Layer

Provides end‑to‑end port‑to‑port communication. Ports are 16‑bit numbers (0‑65535); 0‑1023 are well‑known system ports.

TCP – connection‑oriented, reliable, uses a three‑handshake for connection establishment and a four‑handshake for termination. TCP segments are limited to the size of the underlying IP packet (< 65 535 bytes).

UDP – connectionless, no delivery guarantee, header is 8 bytes (source port, destination port, length, checksum).

Application Layer

Defines data formats and protocols used by end‑user services. Common protocols include:

HTTP (port 80)

FTP (port 21)

DNS (port 53)

SMTP (port 25)

POP3 (port 110)

Encapsulation Example (HTTP Request)

An HTTP request traverses the layers as follows:

Application : HTTP header and body.

Transport : TCP segment with source/destination ports.

Network : IP packet with source/destination IP addresses.

Data Link : Ethernet frame with source/destination MAC addresses and EtherType.

Physical : Binary signal on the chosen medium.

Each layer adds its own header, creating a “Russian nesting doll” structure that is stripped off in reverse order at the receiver.

Key Concepts Summary

Layered architecture promotes high cohesion, low coupling, reusability, and extensibility.

Physical layer handles raw bits; data link adds MAC addressing and framing; network adds IP routing; transport adds ports and reliability; application defines service semantics.

Understanding the header fields and size limits at each layer is essential for network troubleshooting and protocol implementation.