Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 87 is the Cisco approved textbook to use alongside version of the Cisco Networking Academy Program CCNA 1 and CCNA 2 web-based courses. The topics covered provide you with the necessary knowledge to begin your preparation for the CCNA certification exam (640-801, or 640-821 and 640-811) and to enter the field of network administration. | Page 829 Tuesday May 20 2003 2 53 PM Overview of Transport Layer Ports 829 because both SMTP and the World Wide Web services use TCP as their transport layer protocol. A port number must be associated with the conversation between hosts to ensure that the packet reaches the appropriate service on the server. Without a way to distinguish between different conversations the client is unable to send an e-mail and browse a web page at the same time using one server. A method to separate transport layer conversations must be used. Hosts running TCP IP associate ports at the transport layer with certain applications. Port numbers are used to keep track of different conversations that cross the network at the same time. Port numbers are necessary when a host is communicating with a server running multiple services. Both TCP and UDP use port or socket numbers to pass information to the upper layers. Figure 19-13 shows an example of TCP and UDP port numbers. Figure 19-13 TCP and UDP Port Numbers F T S D T S T e M N F N P l T S T M Application n P P P Layer 21 e t 23 - 25 - 53 - - 69 - - w Port Transport Numbers Layer TCP UDP Application software developers have agreed to use the well-known port numbers that are defined in RFC 1700. For example any conversation bound for the FTP application uses the standard port number 21. Conversations that do not involve applications with well-known port numbers are assigned port numbers that have been randomly selected from within a specific range. These port numbers are used as source and destination addresses in the TCP segment. Table 19-1 lists the reserved TCP and UDP port numbers. Table 19-1 Reserved TCP and UDP Port Numbers Decimal Keyword Description 0 - Reserved 1-4 - Unassigned 5 rje Remote job entry RJE continues Page 830 Tuesday May 20 2003 2 53 PM 830 Chapter 19 Intermediate TCP Table 19-1 Reserved TCP and UDP Port Numbers Continued Decimal Keyword Description 7 echo Echo 9 discard Discard 11 users Active
