Syngress - Eleventh Hour Network+ Exam N10-004 Study Guide (11 ...
Syngress - Eleventh Hour Network+ Exam N10-004 Study Guide (11 ...
Syngress - Eleventh Hour Network+ Exam N10-004 Study Guide (11 ...
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168 CHAPTER 10 Network Troubleshooting<br />
TROUBLESHOOTING THE PRESENTATION LAYER<br />
When troubleshooting the presentation layer, the functions will often actually<br />
be performed by a protocol that you would normally think of as functioning at<br />
the application layer, so that troubleshooting will often take place with the same<br />
steps described in the application layer section. When troubleshooting gateways,<br />
an effective approach is to restart or reinstall the gateway service.<br />
Some examples of problems you may find at the presentation layer include the<br />
following:<br />
■<br />
■<br />
■<br />
An image file becomes garbled or corrupted when it’s sent through e-mail<br />
from one person to another.<br />
E-mail messages between two different server types (for example, exchange<br />
and groupwise) become scrambled or unreadable.<br />
You are unable to copy or move files between two different network types,<br />
usually Microsoft and Novell, or you are unable to open a file once it’s been<br />
copied.<br />
TROUBLESHOOTING THE APPLICATION LAYER<br />
The application layer doesn’t provide services to any other OSI layer. Instead, it<br />
provides network services to user applications.<br />
■ Troubleshooting the application layer.<br />
■ Utilize Telnet to connect to an application layer services by specifying the<br />
port that’s used by the service.<br />
■ Verify name resolution services, such as DNS and WINS are functioning<br />
properly.<br />
Some of the common protocols that operate at the application layer are as<br />
follows:<br />
■<br />
■<br />
■<br />
■<br />
FTP FTP is used for copying files from one computer to another.<br />
SNMP SNMP provides a way to gather statistical and troubleshooting information<br />
about devices such as PCs, routers, switches, and hubs. An SNMP<br />
management system sends requests to an SNMP agent, and the information<br />
is stored in an MIB, which is a database that holds information about<br />
a networked computer.<br />
Telnet Telnet is a TCP/IP-based service that allows users to log onto a computer<br />
from a remote location, run character-mode or command-line utilities<br />
on the remote computer or device, and view files on a remote device. The<br />
Telnet server service uses TCP port 23 to listen for Telnet requests.<br />
Simple Mail Transfer Protocol (SMTP) SMTP is used for sending e-mail<br />
messages, typically across the Internet. Most e-mail client programs use<br />
SMTP for sending e-mail only, and either POP3 (Post Office Protocol version<br />
3) or Internet Message Access Protocol (IMAP) for storing any messages that<br />
are received by an e-mail server. The SMTP service uses TCP port 25 to send<br />
messages using SMTP.