OSI Model Explained: All 7 Layers with DevOps Context

What is the OSI Model?

The Open Systems Interconnection (OSI) model is a conceptual framework that standardizes the functions of a communication system into seven distinct layers. Created by the International Organization for Standardization (ISO), it helps you understand how data travels from one application to another across a network.

Layer 1: Physical Layer

The physical layer is the foundation — it deals with the raw transmission of bits over a physical medium. This includes electrical signals over copper cables, light pulses through fiber optic cables, and radio waves for Wi-Fi.

• What it does: Converts bits (0s and 1s) to signals and back
• Your concern: Usually none — but if you're connecting bare-metal servers, NIC issues, or diagnosing cable problems
• DevOps context: When AWS says "Enhanced Networking" — it's optimizing at this layer

Layer 2: Data Link Layer

The data link layer handles communication between devices on the same network using MAC addresses. It packages raw bits into frames and provides error detection.

# View MAC address
ip link show eth0
# output: link/ether 00:1a:2b:3c:4d:5e

# ARP — Maps IP to MAC (L2 to L3 bridge)
arp -a                         # View ARP cache
arp -n 192.168.1.1             # Look up specific IP

# Docker uses L2 for bridge networking
# When containers communicate on the same bridge network,
# they use virtual Ethernet (veth) pairs at Layer 2

Layer 3: Network Layer

This is where IP addressing and routing happen. The network layer determines the best path to get data from source to destination — even across multiple networks.

# IP commands (Layer 3)
ip addr show                   # View IP addresses
ip route show                  # View routing table
ip route add 10.0.0.0/8 via 192.168.1.1  # Add static route

# ICMP operates at Layer 3 (ping)
ping -c 3 8.8.8.8              # Test network connectivity
traceroute 8.8.8.8             # See L3 routing path

# AWS VPC operates at Layer 3
# - Route tables decide where packets go
# - Security Groups filter at L3/L4
# - Internet Gateway bridges VPC to internet

Layer 4: Transport Layer

The transport layer provides end-to-end communication between applications. This is where TCP and UDP live — along with port numbers.

# View open ports (Layer 4)
ss -tulnp                      # TCP/UDP listeners with process
ss -tnp                        # TCP connections

# Common ports you must know:
# 22  — SSH        80  — HTTP
# 443 — HTTPS      3306 — MySQL
# 5432 — PostgreSQL 6379 — Redis
# 8080 — HTTP alt   27017 — MongoDB

# Docker port mapping works at Layer 4
docker run -p 8080:80 nginx    # Map host:8080 → container:80

# Kubernetes Services also operate at L4
# ClusterIP, NodePort, LoadBalancer all use TCP/UDP ports

Layers 5-7: Session, Presentation, Application

In modern TCP/IP networking, these three layers are often merged into the "Application Layer". As a DevOps engineer, you'll interact with these most.

• Session (L5): Manages connections (relevant in stateful load balancing, session persistence)
• Presentation (L6): TLS/SSL encryption happens here — crucial for HTTPS
• Application (L7): HTTP, gRPC, WebSockets — your Nginx/Kubernetes Ingress operates here