OSI vs TCP/IP Model

Networking is the backbone of modern distributed systems.
To understand how data travels across the internet, we need a clear picture of the OSI model and the TCP/IP model.

Mnemonics to remember OSI model

Please Do Not Throw Sausage Pizza Away
(Physical, Data link, Network, Transport, Session, Presentation, Application)

1. OSI Model (Open Systems Interconnection)

The OSI model is a conceptual framework created by ISO to standardize network communication.
It divides communication into 7 layers, each with a specific role.

Layers of OSI Model

1. Physical Layer

   • Deals with raw data transmission (bits).
   • Cables, switches, radio frequencies.

2. Data Link Layer

   • Provides node-to-node delivery.
   • Error detection & correction.
   • Examples: Ethernet, Wi-Fi (802.11).

3. Network Layer

   • Responsible for addressing & routing.
   • Example: IP (IPv4, IPv6).

4. Transport Layer

   • End-to-end communication, reliability.
   • TCP (reliable), UDP (fast, unreliable).

5. Session Layer

   • Manages sessions (open/close connections).
   • Example: NetBIOS, RPC.

6. Presentation Layer

   • Data translation, encryption, compression.
   • Example: SSL/TLS, JPEG, ASCII.

7. Application Layer

   • User-facing services.
   • Example: HTTP, FTP, SMTP, DNS.

2. TCP/IP Model (Internet Protocol Suite)

The TCP/IP model is the practical framework used in the real internet.
It has 4 layers (sometimes 5, depending on variant).

Layers of TCP/IP Model

1. Network Access Layer

   • Combines OSI Physical + Data Link.
   • Deals with MAC addresses, Ethernet, Wi-Fi.

2. Internet Layer

   • Equivalent to OSI Network Layer.
   • Provides addressing & routing (IP).

3. Transport Layer

   • Same as OSI Transport Layer.
   • TCP, UDP.

4. Application Layer

   • Combines OSI Application + Presentation + Session.
   • Example: HTTP, HTTPS, FTP, DNS, SMTP.

3. OSI vs TCP/IP: Side-by-Side

OSI Model (7 Layers)	TCP/IP Model (4 Layers)	Example Protocols
7. Application	Application	HTTP, FTP, SMTP, DNS
6. Presentation	Application	SSL/TLS, JPEG, ASCII
5. Session	Application	RPC, NetBIOS
4. Transport	Transport	TCP, UDP
3. Network	Internet	IP, ICMP, ARP
2. Data Link	Network Access	Ethernet, Wi-Fi
1. Physical	Network Access	Cables, radio signals

4. Key Differences

• OSI is a theoretical model, rarely used directly in implementations.
• TCP/IP is a practical model, used in the real internet.
• OSI separates Presentation and Session layers, TCP/IP merges them into Application.
• TCP/IP is simpler, OSI is more descriptive.

Connect: LinkedIn

© 2025 Official CTO. All rights reserved.