Networking Basics

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26 Οκτ 2013 (πριν από 3 χρόνια και 7 μήνες)

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Networking Basics

B. Konkoth

Lesson
2
8



TCP/IP


Utilities

Troubleshooting with TCP/IP Utilities

The two most common TCP/IP problems are network connectivity problems and name
resolution problems. In this section, you will learn how to troubleshoot these
problems
and how to determine where the problems truly reside.

Given the following scenario, how do you know how to troubleshoot the problem?

You are trying to access a remote computer via TCP/IP. You are unable to connect to the
remote server at all.

To p
roperly troubleshoot this problem, you must know where to begin. A scenario like
this is very common in the TCP/IP world and could be categorized by one of the
following problems:




Basic network connectivity problem




Name Resolution problem

I
t is very easy to determine which problem is occurring in a given situation. Start by
trying to access the resource via the IP address rather than the host name. For example, if
the problem is related to name resolution,
PING

<hostname>

may not work but
PI
NG

<ipaddress>

will. This indicates that because the name cannot be resolved, the
application does not know what the IP address is and therefore cannot access the remote
host. If you cannot access the local resource via the IP address, this indicates a
con
nectivity problem.

1.

Connectivity Problems

Connectivity problems can be tough to isolate and resolve quickly, especially in complex
networks. Let’s use some of the tools you’ve learned about to troubleshoot the earlier
problem of using a third
-
party applicat
ion to access a remote computer via TCP/IP and
being unable to connect to the remote server. You cannot ping the remote host by its IP
address.

Check Your TCP/IP Configuration

Start by checking your TCP/IP configuration. TCP/IP requires several setting to
be
complete and accurate.

When you use TCP/IP as your network protocol, an incorrect
setting such as a mistyped subnet mask can keep your computer from talking with other
hosts on the network. For example, if you have an incorrect default gateway setup, yo
u
may not be able to communicate with anyone on a remote network.

Use the IPCONFIG utility to determine your computer’s basic TCP/IP settings.

Verify
that the IP address and subnet

mask displayed by the IPCONFIG

command are the
correct values for your comp
uter. Verify that your default gateway is set up with the
correct address.

Ping the Loopback Address

Try pinging the loopback address. You can use the ping command to verify that TCP/IP
is working properly. By pinging the loopback address, which is 127.0.0
.1, you are
actually verifying that the protocol stack is functioning properly. You should receive a
reply like the one shown in Figure 5
-
18.


An error while pinging the loopback address u
sually indicates a problem with the TCP/IP
protocol installed locally. If you do receive an error at this point, you should try
uninstalling and reinstalling TCP/IP. You can remove and install TCP/IP from within the
Control Panel.

Ping the Local IP Address

If you can successfully ping the loopback address, try pinging your local computer’s IP
address. If you do not know what you IP address is, remember that IPCONFIG will
display this information for you. By typing the following at a command prompt, you
shou
ld receive a response similar to the one shown in figure 5
-
18.

PING
local IP address

If an error occurs at this point, there may be a problem communicating with the Network
Interface Card. You can first try reinstalling the adapter driver for the card. If
that doesn’t
work, trying removing and reseating the card. This error may only be resolved by
completely replacing the NIC.

Clear the ARP Cache Table

If the local IP address responds correctly, try clearing the ARP cache. If an IP address
was errantly stor
ed here, it could cause the client to attempt to contact the wrong
computer.

Start by displaying the ARP cache. You can then see if there is an entry located for the
remote IP address. If an entry exists, try deleting it with the

d option.

Verify the Def
ault Gateway

When you have removed any errant entries, the next step is to ping the default gateway.
This will only be involved if the host is on a remote subnet. When trying to ping a host, if
it is not located on the local subnet, the request is automati
cally forwarded to the
appropriate route. If a route does not exist, then the packet is forwarded to the default
gateway. If the gateway does not respond, the packets will not be able to get to the
remote host.

You can use IPCONFIG to display your default
gateway. Once you have that address, try
pinging that address or host name.

Trace the Route to the Remote Host

After a packet leaves the default gateway, any route can be taken to reach a remote
computer. The next step is to try to trace the route to the r
emote computer. The following
example shows a tracert in action:


A wide array of problems could show up here. You may notice that when the utility gets
to a certain point, it responds with “R
equest timed out.” If this occurs, it could indicate a
route problem or a device failure. It could also indicate bandwidth issues. Try raising the
timeout value. If it responds, but with high values, your data transfers could be failing
because the applica
tion does not wait long enough. Try reconfiguring your application or
adding more bandwidth to your network.

Another error you may receive is “Destination Net Unreachable.” This usually indicates a
network routing problem. Contact the network administrator

responsible for that network
segment.

Check IP Security on the Server

The next thing to try is to verify the security and settings on the remote computer. Port
settings for services on the other computer may be different from the port settings you are
try
ing to use to connect.


2.

Name Resolution Problems

Suppose you are able to connect to a remote host but are unable to connect via its host
name. This indicates a name resolution problem. In the Microsoft world, there are two
types of computer names: TCP/IP
-
b
ased host names and NetBIOS names. These names
can be resolved in several ways, including Domain Name System (DNS), Windows
Internet Naming Service (WINS), a HOSTS file, or an LMHOSTS file. Each method has
its advantages and disadvantages.

Check Your Domai
n Name System (DNS) Configuration

A Domain Name System (DNS) provides TCP/IP name resolution services. This is a
central server that computers can use to query for name resolution. The advantage over
the host file here is that you only have to make the cha
nge on your server; all clients
querying it will receive the update. This is much easier to administer than 150 or more
workstations.

If you use DNS for name resolution, first verify that you have the DNS client set up
correctly on the workstation. From a
command prompt, type
IPCONFIG /ALL

to list the
DNS servers. If they exist and are correct, try pinging the DNS server to see if it is
online. If it responds, try changing your DNS server to another server. It is possible that
one DNS server may have differ
ent information than another on does. You also may need
to contact your DNS administrator to verify that the name exists in DNS and has the
correct information.

Check Your Windows Internet Naming Service (WINS)Configuration

A WINS (Windows Internet Naming
Service) server provides NetBIOS name resolution
much like DNS servers provide TCP/IP host name resolution. If you use WINS for
NetBIOS name resolution and you cannot connect to the other machine with its NetBIOS
name, there may be a problem with your comp
uter’s WINS configuration.

Start by verifying your WINS configuration. From a command prompt, type IPCONFIG
/ALL This will display the current WINS servers configured for your computer. If the
correct servers are listed, try pinging the primary WINS serve
r. This is the first server
that your requests will go to. If the host name is not located here, your computer will not
try to get to the secondary WINS server. This second WINS server is used only in the
event the primary cannot be reached.

If you cannot
ping the primary WINS server address, try switching your primary with the
secondary in the Control Panel. If you are able to resolve the name now, contact your
WINS administrator to correct the problem. You may also need to verify that the remote
host is r
egistered with WINS correctly.


Review

1.

What are the two

most common TCP/IP problems?

2.

How can you determine the type of problem?

3.

How do you check your TCP/IP configuration?

4.

What is the local loopback address?

5.

How do you ping the local IP address?

6.

How can yo
u determine the default gateway?

7.

How do you trace the route to a remote server?

8.

How do you determine a name resolution problem?

9.

How do you check the DNS settings on your computer?