Network Models

OSI Model

The OSI (Open Systems Interconnection) model is a conceptual framework that standardizes network communication into 7 layers. Each layer serves the layer above it and is served by the layer below it.

┌─────────────────────────────────────────────────────────────────┐
│  Layer 7 - Application    │ HTTP, FTP, DNS, SMTP, gRPC          │
├─────────────────────────────────────────────────────────────────┤
│  Layer 6 - Presentation   │ Encoding, Serialization, Encryption  │
├─────────────────────────────────────────────────────────────────┤
│  Layer 5 - Session        │ TLS handshake, Connection mgmt       │
├─────────────────────────────────────────────────────────────────┤
│  Layer 4 - Transport      │ TCP, UDP (ports, segments)           │
├─────────────────────────────────────────────────────────────────┤
│  Layer 3 - Network        │ IP, Routing, ICMP                    │
├─────────────────────────────────────────────────────────────────┤
│  Layer 2 - Data Link      │ Ethernet, MAC address, ARP, Frames   │
├─────────────────────────────────────────────────────────────────┤
│  Layer 1 - Physical       │ Cables, fiber optics, radio waves    │
└─────────────────────────────────────────────────────────────────┘

How Data Flows (Encapsulation)

When you send an HTTP request, it travels down the stack on the sender and up the stack on the receiver, with each layer adding/removing its own header:

Sender side:
Application  →  HTTP data
Transport    →  TCP segment (adds port numbers)
Network      →  IP packet (adds IP addresses)
Data Link    →  Ethernet frame (adds MAC addresses)
Physical     →  Bits over the wire

Receiver side (reverse):
Physical     →  Receive bits
Data Link    →  Strip Ethernet header
Network      →  Strip IP header
Transport    →  Strip TCP header, reorder packets
Application  →  Read HTTP data

TCP/IP Model

The TCP/IP model is the practical model that the internet actually uses. It collapses OSI’s 7 layers into 4:

Modern world’s most used architecture: Star topology using the TCP/IP model.