OSI Model vs TCP/IP Model: All 7 Layers, Protocols and What Happens When You Open a Website
Each layer’s job and protocols, how the two models map, and the request journey interviewers ask you to explain.
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Networking is built in layers. Each layer solves one problem and relies on the layer below it. The OSI model has seven layers and is mostly a teaching model; the TCP/IP model has four and describes how the internet actually works. You are expected to know both and how they line up.
1. The seven OSI layers#
From top (closest to the user) to bottom (the wire):
| # | Layer | Job | Examples | Data unit |
|---|---|---|---|---|
| 7 | Application | Network services for applications | HTTP, DNS, SMTP, FTP | Data |
| 6 | Presentation | Encoding, encryption, compression | TLS, JPEG, UTF-8 | Data |
| 5 | Session | Open, manage and close sessions | RPC, session checkpoints | Data |
| 4 | Transport | End-to-end delivery between processes (ports) | TCP, UDP | Segment / datagram |
| 3 | Network | Logical addressing and routing between networks | IP, ICMP, routers | Packet |
| 2 | Data link | Delivery on one link, MAC addresses, error detection | Ethernet, Wi-Fi, switches, ARP | Frame |
| 1 | Physical | Bits as signals on a medium | Cables, radio, hubs | Bit |
"Please Do Not Throw Sausage Pizza Away", from layer 1 up: Physical, Data link, Network, Transport, Session, Presentation, Application.
2. How OSI maps to TCP/IP#
The TCP/IP model merges OSI layers 5–7 into one Application layer and layers 1–2 into a Link (network access) layer.
| TCP/IP layer | OSI layers | Protocols |
|---|---|---|
| Application | 7, 6, 5 | HTTP, HTTPS, DNS, SMTP, SSH |
| Transport | 4 | TCP, UDP |
| Internet | 3 | IP (v4, v6), ICMP |
| Link | 2, 1 | Ethernet, Wi-Fi, ARP |
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3. TCP vs UDP#
| TCP | UDP | |
|---|---|---|
| Connection | Connection-oriented (three-way handshake: SYN, SYN-ACK, ACK) | Connectionless |
| Reliability | Retransmits lost data, delivers in order | No guarantee of delivery or order |
| Flow and congestion control | Yes | No |
| Header size | 20–60 bytes | 8 bytes |
| Used for | Web pages, email, file transfer | DNS lookups, video calls, online games |
4. What happens when you type a URL and press Enter#
This is one of the most common networking interview questions. A good answer walks down the layers:
- DNS: the browser checks its cache, then asks a DNS resolver to turn the domain name into an IP address (usually over UDP port 53).
- TCP: the browser opens a TCP connection to that IP on port 443 with the three-way handshake.
- TLS: client and server agree on keys and the server proves its identity with a certificate, so the rest is encrypted.
- HTTP: the browser sends GET / with headers; the server responds with status 200 and the HTML.
- Below that, IP routes the packets hop by hop through routers, and on each link Ethernet or Wi-Fi delivers frames using MAC addresses found with ARP.
- Rendering: the browser parses the HTML, requests CSS, JavaScript and images (often over the same connection) and paints the page.
Mentioning DNS caching, the TLS handshake, or that HTTP/3 runs over QUIC on UDP shows you understand the stack rather than a memorised list.
Test Your Concept Mastery
Verify your retention on: OSI Model vs TCP/IP Model: All 7 Layers, Protocols and What Happens When You Open a Website
At which OSI layer do routers mainly operate?
Which protocol does a typical DNS lookup use?
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