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