CCENT (Cisco Certified Entry Network Technician) Certification Exam Notes

B. TCP/UDP concepts

1. FTP (File Transfer Protocol) is used for transferring files from and to a client computer to a host. FTP is a connection oriented protocol. It uses TCP/IP for file transfer. Compare this with UDP protocol used by TFTP.

2. Telnet:
· Telnet command works at Layer 7 of your TCP/IP stack. If you are able to Telnet to your remote router, you can be sure that the TCP/IP stack is properly installed.
Other commands, such as Ping, trace, and ipconfig work at lower layers of the TCP/IP stack.
· Telnet is used for terminal emulation that runs programs remotely.
FTP is used to transfer files.
· TCP/IP is the protocol used when you are Telnetting to a remote host. HTTP is used for accessing the World Wide Web services.

3. The default port numbers used for some important services are:
· FTP:21
SSL:443 (Secure socket layer)
When you are accessing a Web page, the browser sends the request over the default port number (For WWW, the default port number is 80). For any reason, If you change the default port number that a Web server listens to, users need to specify the port number in the URL.

4. The distinguishing characteristics of a TCP frame are Sequence Number, Acknowledgment Number, and Window.

5. TCP and UDP Protocol Characteristics:
· TCP is a full-duplex, connection-oriented protocol. It incorporates error checking as well.
· UDP (User Datagram Protocol): UDP is a thin protocol. UDP is a connectionless protocol. It doesn't contact the destination before sending the packet and doesn't care whether the packet is reached at the destination. UDP uses port number 6.

6. The port numbers used by different programs are as below:
· FTP : Port #21
Telnet: Port #23
SMTP: Port #25
SNMP: Port #161
· It is also worthwhile to know that FTP, Telnet, SMTP use TCP; whereas TFTP, SNMP use UDP.

7. UDP is known as unreliable protocol because there is no way for the source to know if any packets are lost during transit to the destination. Important fields that are available with TCP header, but not available with UDP header include sequence number, acknowledgement number, and window size.

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