CCNA v 1.1: OSI vs. TCP/IP and TCP/IP Transport Layer Protocols

OSI Model versus TCP/IP Protocol Suite


OSI Model Layer Number OSI Model Layer TCP/IP Protocol Suite Layer Protocol Data Unit Network Device
7 Application Application Data
6 Presentation
5 Session
4 Transport Transport Segment
3 Network Internet Packet (or Datagram) Multilayer Switch or Router
2 Data Link Data Link Frame Switch or Bridge
1 Physical Physical Bits Hub


TCP/IP Transport Layer Protocols

Transmission Control Protocol (TCP) is a reliable, connection-oriented, protocol that uses sequence and acknowledgement numbers to provide reliability. TCP verifies that the remote end is listening prior to sending data, using a 3-way handshake: SYN, SYN/ACK, ACK.

User Datagram Protocol (UDP) is a best-effort, connectionless, protocol that does not have sequence or acknowledgement numbers, and does not do far-end verification.

TCP Header


16-bit source port 16-bit destination port
32-bit sequence number
32-bit acknowledgement number
4-bit header length resv ns cwt ece urg ack psh rst syn fln 16-bit window size
16-bit source port 16-bit destination port


UDP Header


16-bit source port 16-bit destination port
16-bit UDP length 16-bit UDP checksum


TCP and UDP Port Numbers

Well-known port numbers are 1 – 1023 (typically used for well-known applications). Registered port numbers are 1024 to 49151. 49152 to 65535 are dynamically assigned port numbers (and are typically used as source port numbers).

Application Port Transport
File Transfer Protocol (FTP) 20/21 TCP
Telnet 23 TCP
Simple Mail Transfer Protocol (SMTP) 25 TCP
Domain Name Services (DNS) 53 UDP/TCP
Trivial Files Transfer Protocol(TFTP) 69 UDP
Simple Network Management Protocol 161/162 UDP
Routing Information Protocol (RIP) 520 UDP

Next week we’ll look at Internet Layer Protocols.

Excerpted and available for download from Global Knowledge White Paper: CCNA v1.1 Exam Review: Critical Concepts of the 640-802 CCNA Exam

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Related Courses:
ICND1 — Interconnecting Cisco Network Devices 1
ICND2 — Interconnecting Cisco Network Devices 2
CCNAX — CCNA Boot Camp v1.1

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