Last Updated: July 15, 2025
The 7 Layers
| # | Layer | PDU | Key Protocols | Devices |
|---|---|---|---|---|
| 7 | Application | Data | HTTP, DNS, SMTP, FTP, SSH | Load balancer (L7) |
| 6 | Presentation | Data | TLS/SSL, ASCII, JPEG, gzip | — |
| 5 | Session | Data | NetBIOS, RPC, SOCKS | — |
| 4 | Transport | Segment | TCP, UDP, QUIC | Firewall (L4) |
| 3 | Network | Packet | IP, ICMP, IPSec, OSPF | Router |
| 2 | Data Link | Frame | Ethernet, ARP, VLAN (802.1Q) | Switch, Bridge |
| 1 | Physical | Bit | Ethernet (cable), Wi-Fi, fiber | Hub, Repeater |
Encapsulation Flow
L7 → L4: SegmentApplication data + TCP/UDP header (port numbers)
L4 → L3: PacketSegment + IP header (source/dest IP)
L3 → L2: FramePacket + Ethernet header (MAC addresses) + FCS trailer
L2 → L1: BitsFrame encoded as electrical/optical/radio signal
Receiving: ReverseL1 bits → L2 frame → L3 packet → L4 segment → L7 data
TCP/IP vs OSI
| TCP/IP Layer | OSI Layers | Examples |
|---|---|---|
| Application | 7 + 6 + 5 | HTTP, DNS, TLS, JPEG |
| Transport | 4 | TCP, UDP, QUIC |
| Internet | 3 | IPv4, IPv6, ICMP, ARP |
| Network Access | 2 + 1 | Ethernet, Wi-Fi, fiber |
Troubleshooting by Layer
| Layer | Problem | Tool |
|---|---|---|
| 1 — Physical | Cable unplugged, signal loss | Cable tester, link lights |
| 2 — Data Link | MAC misconfig, VLAN wrong | arp -a, switch MAC table |
| 3 — Network | Wrong IP, routing loop | ping, traceroute |
| 4 — Transport | Port blocked, TCP reset | nc -vz host port, tcpdump |
| 7 — Application | DNS failure, TLS cert | dig, curl -v, openssl |
Pro Tip: The OSI model is a teaching tool, not a blueprint. Real networks use TCP/IP (4 layers). Map OSI Layer 3-4 to TCP/IP Internet/Transport — that's where 90% of troubleshooting lives.