Cisco Systems CCNA 2 Overview of Transport Layer Ports, Multiple conversations between hosts

UDP is often used for applications and services such as real-time audio and video. These applications require less overhead. They also do not need to be resequenced since packets that arrive late or out of order have no value.

10.2 Overview of Transport Layer Ports

Essential Labs:	10.2.5
Optional Labs:	None
Core TIs:	10.2.5
Optional TIs:	10.2.1 – 10.2.4, and 10.2.6

Course-Level Claim: Students can describe the operation of the major transport layer protocols and the interaction and transportation of application layer data.

Certification-Level Claim: Students can evaluate TCP/IP communication process and the associated protocols.

Hands-on skills: none

10.2.1 Multiple conversations between hosts

At any given moment, thousands of packets destined for hundreds of different services travel through a network. Servers provide services for a multitude of different requests. This causes unique problems for addressing of packets. For example, if a server uses SMTP and WWW services, a client cannot construct a packet that is destined for just the IP address of the server with TCP because both SMTP and WWW use TCP as their transport layer protocol. A port number must be associated with the conversation to ensure that the packet reaches the appropriate service.

Port numbers are used to keep track of different conversations that cross the network at the same time. Port numbers are needed when a host communicates with a server that uses multiple services. Both TCP and UDP use port numbers to pass information to the upper layers. Software developers use the well-known port numbers defined in RFC1700. Conversations that do not involve applications with well-known port numbers are assigned port numbers that have been randomly selected from a specific range.

Port numbers have the following ranges:

•Numbers below 255 are used for public applications

•Numbers from 255-1023 are assigned to marketable applications

•Numbers above 1023 are unregulated

A good analogy to help students understand this process is a post office box number. Each piece of mail is sent to a zip code, city, and then a P. O. Box. Similarly, the IP address and transport layer send the packet to the correct server, but the port number guarantees that the packet will contact the correct application.

115 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 10 Copyright ♥ 2004, Cisco Systems, Inc.