TCP/IP & OSI Layer Reference
The seven OSI layers side by side with the four-layer TCP/IP model — protocol data units, functions, and example protocols.
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OSI model (ISO/IEC 7498-1)
| Layer | OSI name | PDU | Function | TCP/IP layer | Example protocols |
|---|---|---|---|---|---|
| 7 | Application | Data | Process-to-process protocols that applications use directly | Application | HTTP, DNS, SMTP, IMAP, SSH, FTP, DHCP, SNMP, NTP |
| 6 | Presentation | Data | Data representation, serialization, encryption, compression | Application | TLS*, MIME, ASCII, Unicode, JPEG, ASN.1 |
| 5 | Session | Data | Establish, manage, and tear down sessions between applications | Application | RPC, NetBIOS, PPTP, SIP |
| 4 | Transport | Segment (TCP) / Datagram (UDP) | End-to-end delivery, ports, multiplexing; reliability in TCP | Transport | TCP, UDP, SCTP, DCCP, QUIC* |
| 3 | Network | Packet | Host-to-host addressing and routing across networks | Internet | IPv4, IPv6, ICMP, ICMPv6, IPsec, OSPF |
| 2 | Data Link | Frame | Node-to-node delivery on one link: framing, MAC addressing, error detection | Link | Ethernet (802.3), Wi-Fi (802.11), PPP, 802.1Q VLAN, ARP*, MPLS* |
| 1 | Physical | Bit / Symbol | Transmit raw bits or symbols over a physical medium | Link | 1000BASE-T, SONET/SDH, DSL, Wi-Fi radio (PHY), RS-232 |
TCP/IP model (RFC 1122)
| Name | OSI layers | Reference |
|---|---|---|
| Application | 5–7 | RFC 1122 |
| Transport | 4 | RFC 1122 |
| Internet | 3 | RFC 1122 |
| Link | 1–2 | RFC 1122 |
Layer assignments follow common teaching practice. Real protocols often span layers; an asterisk marks the frequently-cited examples.
How it works
The OSI reference model (ISO/IEC 7498-1) defines seven layers; the internet protocol suite (RFC 1122) uses four. This table lines them up: each OSI layer with its protocol data unit, its short function, the TCP/IP layer it corresponds to, and representative protocols. A trailing asterisk marks a protocol usually described as spanning layers — TLS between session/presentation and application, QUIC over UDP, ARP between the link and internet layers. The layer boundaries are a teaching model, not a strict classification every protocol obeys.