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ES 101. Module 7 Basic Network Troubleshooting


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Title: ES 101. Module 7 Basic Network Troubleshooting

ES 101. Module 7Basic Network Troubleshooting
Useful Utilities
Last Lecture
  • Uniform Resource Locators (URL)
  • Hypertext Transfer Protocol (HTTP)
  • Common Gateway Interface (CGI)

This Lecture
  • Internet Control Message Protocol (ICMP)
  • Ping
  • Traceroute
  • File Transfer Protocol (FTP)
  • Web Development Utilities

Internet Protocol Architecture
Internet Control Message Protocol
  • ICMP provides a means for hosts and routers to
    communicate about problems that occur in the
  • Congestion notification
  • Destination not reachable
  • Packet TTL expiring
  • Timestamp
  • ICMP utilizes IP as the underlying transport
  • So, ICMP messages are routeable
  • These capabilities are used by utility programs
    that are very useful in network troubleshooting
  • PING
  • Traceroute

ICMP (contd)
  • Before discussing the PING and Traceroute
    utilities, lets examine some of the details of
    certain ICMP packets

Destination Unreachable, Time Exceeded
Echo Request, and Echo Reply
Timestamp Reply
ICMP Field Descriptions
  • The Type and Code fields are utilized by
    routers and hosts to determine the function of
    the packet
  • Destination unreachable
  • Echo Request
  • The remaining fields (Identifiers, sequence
    numbers, and data) are used to achieve the
    function of the utility

Packet Internet Groper (PING)
  • Ping is a low-level utility that allows trouble
    shooting of networking problems
  • Utilizes the Echo Request and Reply packet of
  • Ping packets are the smallest packets that are
    routed on an IP network
  • Ping provides a means of testing connectivity to
    a remote device on an IP network
  • It is the first thing that you do to troubleshoot
    networking problems
  • If you can successfully ping a remote server,
    then the IP connectivity exists to that system
  • This implies that the problem is with higher
    layers than the IP layer

PING Operation
  • An ECHO Request packet is sent to a destination
    IP address
  • Packet sequence numbers are used to uniquely
    identify the packet
  • The current time is stored in the data field of
    the Echo Request packet
  • The destination then encapsulates the ECHO
    Request packet in an ECHO Reply packet, and
    returns it to the sender
  • The sender can then determine the round trip
    travel time of the original packet
  • Round trip time is measured in milli-seconds (ms)

Useful PING Features
  • If you run the Ping command without specifying an
    IP address, you can change some of the parameters
  • Number of packets
  • Packet size
  • Timeout values
  • Change the source IP address
  • Transmitted data pattern
  • These features are useful in troubleshooting
    problems that are sensitive to packet size,
    and/or data patterns

Ping Error Messages
  • Destination unreachable
  • Network unreachable
  • Port unreachable
  • Percentage of packets that timed out
  • TTL expired

Continuous Ping
  • Sometimes it is necessary to turn-on Ping, and
    let it run for an extended period of time
  • This allows you to look for intermittent problems
    with excessive propagation delay across the
  • At the command prompt, this command is invoked
  • ping t ltIP addressgt
  • To breakout of this command, enter CTRL C
  • Look for large periodic changes in round trip
    travel time and the number of failed replies
  • Are all packets returning?
  • Are some packets being lost?

PING by Domain Name
  • If you can ping a remote device using the IP
    address, but cannot access it via the web
    browser, try pinging using the domain name
  • Ping t lttarget domain namegt
  • If this does not work, then you know something is
    wrong with DNS
  • Try pinging the DNS server IP address on your
  • If this does not work, then you know that your
    machine does not have connectivity to the DNS
    server, or that the DNS server is down

Default Gateway Problems
  • If you can ping devices on your local area
    network, but cannot ping remote devices, there is
    something wrong with your default gateway setup
  • If your computer has no default gateway (router)
    defined, it does not know where to send packets
    that need to be routed to foreign networks

What Is My IP Address?
  • In some networks, IP addresses are assigned
  • This means that if you go into the control panel,
    you cant determine your IP address
  • This is useful for other troubleshooting
  • This assumes that you have been assigned a public

Note You cannot do this if you are behind a
firewall, and use private IP addresses.
What Is My IP Address? (contd)
  • If you have a public IP address you can find out
    what the IP address of your computer is by doing
    the following
  • Open the command prompt
  • Start Run cmd
  • Go to the root directory
  • Enter the command ipconfig

Private IP Addresses
  • When an organization uses firewalls, a technique
    called Network Address Translation is used to
    mask the network details from machines outside of
    the firewall
  • NAT utilizes private IP addressing
  • Private IP addresses are not routed on the
  • This is a requirement to connect a router to the
  • Private IP addresses have the following format
  • Class A
  • Class B through
  • Class C through

Trace Route
  • Traceroute, or trace is another utility that
    uses ICMP messages to troubleshoot network
  • Time-to-live Exceeded, and Port Unreachable
  • This utility is used to report all routers that
    are in the path between a source and destination
    IP address
  • It works by sending a UDP/IP packet from the
    source to the destination
  • The TTL field is incremented from 1 to 30, which
    reports the trace findings as the packet TTLs

Trace Operation
  • The TTL field is set to 1, and the source sends
    the packet to the destination
  • When the packet makes it one hop, the first
    router decrements the TTL field to 0
  • The router then discards the packet, since the
    life of the packet has expired, and sends an
    ICMP message back to the source indicating
    packet death
  • The router IP address is included as the Source
  • The source then knows the IP address of the first
    router in the path
  • The source does this three times, recording the
    round-trip time for each packet

Trace Operation (contd)
  • At the end of this sequence, the TTL is
    incremented by one hop
  • The previously described procedure is then
  • This continues until the packet reaches the final
  • The UDP packet is addressed to a high-numbered
    (unused) port
  • There is no process to receive and process the
    data, so the destination returns an ICMP port
    unreachable message to the source
  • When you invoke the trace command without any
    arguments, it interactively starts a dialogue
    similar to the Ping utility

Trace Asterisk Indication
  • Sometimes when you perform the traceroute
    operation, you will see an asterisk () instead
    of the round-trip time
  • This tells you that there was no reply received
    before trace timed out
  • This is done to limit the vulnerability of the
    router to a denial-of-service attack

Trace Route Output
Trace Route Output
  • The previous IP address ( was
    determined by pinging
  • The output obtained shows that Vanderbilts edge
    router to the Internet is hit in 3 hops
  • The route then propagates to the edge of the
    Qwest network
  • At this point, the path peers with another
    service provider, and ultimately hits Vallnets
    DNS server

11 router hops from Vanderbilt to Fayetteville,
  • Just because you are near the destination from
    the standpoint of geography, this does not mean
    that you are close from the standpoint of the
    network topology
  • Each router hop takes a finite amount of time to
  • The round-trip time to the Vallnet DNS is around
    32 ms, which is pretty good
  • If there are any points of congestion between you
    and the destination, performance degrades rapidly

Looking Glass
  • Most big ISPs peer at a number of points with
    other large providers
  • http//
  • It is sometimes useful to diagnose connectivity
    problems by pinging, or tracerouteing, from
    these peering points
  • This service is typically called Looking Glass
  • The service allows you to use the ISPs router to
    ping back and/or trace back to your PC

Sprint Looking Glass
Sprint Looking Glass Ping Output
Sprint Looking Glass Trace Output
Other Useful Utilities
  • Telnet
  • File Transfer Protocol (FTP)

TCP/IP Architecture
  • Before networking, users were restricted to
    working on computers to which they had physical
  • In a networked environment, transport layer
    protocols can provide virtual connections across
    the network
  • This replaces the requirement that a user be
    physically attached to the computer
  • Remote login across the network
  • A very simple, terminal-based interface is
    frequently used by network engineers to
    communicate with remote devices
  • This interface has both a server-side application
    and a client application

Telnet (contd)
  • The user executes a client program that
    establishes a TCP connection to the server
  • The client sends keystrokes to the server, and
    the server generates responses that are then sent
    back to the client
  • The most common Internet remote terminal protocol
    is TELNET
  • Although not as sophisticated as a windowing
    interface, it is universally implemented and used
    on all Unix and Windows-based machines
  • Telnet is accessed from the command prompt
  • telnet hostname port
  • The port number is optional

File Transfer Protocol (FTP)
  • When you develop your web site, you will
    eventually need to migrate your locally developed
    files to the web server
  • This is most easily done using the File Transfer
  • The file transfer protocol consists of a
    server-side application and a windows-based
    client application, which is installed on your
  • Another alternative is to logon to an FTP server
    using the Telnet protocol
  • This would typically be a Unix server, which is
    beyond the scope of this course

Windows-based FTP is easier for beginners
Windows-based FTP
  • There are a variety of free, windows-based FTP
    clients available on the web
  • One of the more common ones is Smart FTP
  • http//
  • It would be advisable to install this utility on
    your computer
  • It allows you to drag and drop from Windows
    Explorer to the FTP directory, and vice versa

Smart FTP Download Screen
Smart FTP Directory Example
Web Server Options
  • Web (HTTP) server software is the application
    responsible for accepting browser requests,
    retrieving the specified file, and returning its
  • Most web servers on the Internet are Unix-based
  • The most common web server application is Apache,
    which is an open source application
  • http//
  • Of course, as in all other matters associated
    with computing, Microsoft has other ideas about
    who builds the best web server software
  • Microsofts web server application is the
    Internet Information Services application, or

Web Server Options (contd)
  • In EECS we provide a web server for use on
    department projects
  • http//
  • This server is an MS IIS server
  • Supports MS Frontpage Extensions
  • It also supports FTP
  • It is administered by Hampton Albert
  • The VSC-IT server is also available
  • LINUX/APACHE based
  • Requires the use of FTP to upload your code
  • Does not support MS IIS extensions for Front Page

Development of Web Content
  • Our next lectures will begin to delve into the
    art of developing your web content
  • Web pages are essentially folders and directories
    of Hypertext Markup Language (HTML) files
  • When you develop in HTML, you can use a variety
    of applications
  • Notepad, or some other text editor
  • Microsoft Word
  • Microsoft Front Page
  • Dreamweaver
  • etc.

Web Development Approach
  • MS FrontPage is installed on all EECS ICL
    computers, and is available for your use
  • This allows you to view, manage, move, copy,
    save, and rename files and folders on the Web
    server vs. doing this on your PC and uploading
    to the server via FTP
  • This will allow you to, just as you would perform
    the same actions in Windows Explorer
  • In addition, you get a kind of WYSIWYG interface
  • With some significant differences

You may use any web development software for your
Alternative Development Environments
  • However, use of FrontPage is not required
  • Students can utilize any development environment
    which they choose to use
  • I will attempt to get you the support required to
    assist you in this project
  • VSC-IT students

Next Lecture(s)
  • The next topics will be on HTML, and other web
    development utilities
  • At the conclusion of these lectures, we will
    start using these tools to build a web site
  • This will be your class project
  • Your project will result in your grade for the