NET 121b: Essentials of Networking
Chapter 13: Troubleshooting Network Connectivity
This chapter discusses several utilities that may be used in diagnosing
network problems. The topics of this chapter are:
- Network troubleshooting utilities
- File Transfer Protocol (FTP)
The text discusses several utilities found on most Windows and UNIX workstations..
- ARP - ARP stands for Address Resolution Protocol. In standard
Ethernet networks, machines may communicate inside the network with
their MAC addresses. Communications across networks are more likely
to use IP addresses. An ARP cache is a table that lists the IP addresses
and MAC addresses of devices on a network. This table is consulted to
change from one kind of addressing to the other. For example, I have
just issued the command
to my workstation. It has responded with the contents of its ARP cache:
its own IP and MAC addresses, and those of my default router.
- hostname - This command will respond with the name of your
device in your domain.
- IPCONFIG - shows useful information on Windows NT and later
machines, like the IP address, default router, and subnet mask. More
information is shown if the command is entered as
- IPCONFIG /all
- IPCONFIG /release will release the currently held IP address
to the DHCP server that gave it
- IPCONFIG /renew will obtain a new lease from the DHCP server
for an IP address
- WINIPCFG - Like a light version of IPCONFIG, found on Windows
95, 98, and Me computers.
- IFCONFIG - a UNIX command that can be used to view or configure
the network interface settings for a workstation
- nbtstat - The name of this utility is NetBIOS over TCP/IP
Statistics. Not very enlightening. You need to know that your computer
will typically hold the names and IP addresses of several devices in
memory. Sometimes those devices go offline, and others come online.
This may make it desirable to check what is in memory:
or tell the computer to reload this information from standard
(Note that the case of the letter R in the command above is required
to be capital.)
nbtstat IP_address This version lets you check the tables
in memory of the device specified by the IP address
- NETSTAT - Can be used to view the status of current connections
using TCP, UDP, ICMP, and IP. The status messages are a bit cryptic,
so you will want to keep a reference for them handy when using this
- NSLOOKUP - This can be used to report the IP address of a DNS
name. It does not send a ping to the named device. The example
in the text explains that this command checks what is stored in your
DNS server about the name in question. A response to the command may
take this format:
Server: server name
Address: IP address
Name: DNS name
Address: IP address
The first pair of responses are about the DNS server on your network.
The second pair are about the DNS name you are looking up. When I tried
this with nslookup microsoft.com, I received two IP
addresses in the line about Microsoft's server. Not unexpected, since
a busy network will have more than one server responding to requests.
- used on UNIX and Windows platforms, but must be installed on a Windows
platform. The link provided goes to an IBM site that tells us DIG stands
for Domain Information Groper. It digs information out of a DNS server.
Think of it as a tool for troubleshooting DNS servers and services.
- PING - can be issued on a command line, and has an extensive
list of options. Usually, the options are unnecessary. You can ping
the address 127.0.0.1, which stands for the IP stack on the machine
you are using. You may also want to ping the IP address you think you
have, with your network cable unplugged. Pinging the local loopback
proves you have a working IP stack. Pinging your actual address, when
unplugged, proves you have that address.
Be aware that you can ping an IP address or a DNS name.
- TRACERT (Trace Route) - This command will show how long each
link in a route takes, as well as showing links that fail to pass packets
to the next link. Successful transfers of data will report the total
time to the destination. You can limit the trace to a specific number
of hops with the command
tracert -h hop_limit
where hop_limit is a number.
- ROUTE - all devices on an IP network have routing information
tables in their memory. ROUTE allows you to view that information, and
to modify that information. The reason you would want to modify it is
you need to do so when routing tables are static and they need to be
Telnet is utility that allow your computer to connect like a dumb terminal
to a host running a Telnet daemon. As we discussed in class, a daemon
is a program, typically on a server, that runs in the background until
it is needed or called by another device. It is typically used to connect
to a UNIX server or a mainframe. Windows and Novell servers do not support
Telnet connections to themselves.
File Transfer Protocol (FTP)
FTP is used to copy files to and from devices on an IP network. Numerous
versions of FTP utilities exist for Windows, UNIX, and other platforms.
The text states that it is important to correctly specify whether a file
being transferred is text or binary. In practice, treating all files as
binary files usually works.
The text presents a series of story problems about troubleshooting with
these utilities. We will discuss them in class.