SNHU - Network Troubleshooting

Introduction

Objective

CompTIA Network + Domain:

Domain 4: Troubleshooting

CompTIA Network + Objective:

Objective 4.1: Given a scenario, implement the following network troubleshooting

methodology.

Overview

This lab is part of a series of lab exercises designed to supplement coursework and provide

students with a hands-on training experience based on real-world applications. This series of

lab exercises is intended to support courseware for CompTIA Network+

certiﬁcation.

Networks are important to business processes, and when they are not fully operational, it is

costly and frustrating to the users. Network administrators need to understand not just how

to keep the network functional but also how to approach troubleshooting problems when a

network is not fully operational. This lab will review troubleshooting and a methodology that

will provide ideas on where to start in the problem-solving effort. This methodology will be

used as a guide in troubleshooting two protocols widely used in networks and it is important

to be able to diagnose issues with them. DNS and DHCP will be the two protocols focused

on in this exercise.

This lab includes the following tasks:

Using the problem-solving process to troubleshoot a suspected DNS issue using CLI

utilities and resolve the issue

Domain

Name Service

(DNS)

the protocol used to resolve and map hostnames and domain names into

IP addresses on the Internet. DNS uses UDP port 53 for initiating

requests. Name servers, or DNS servers, are servers that contain databases

of associated names and IP addresses and provide this information to

resolvers (hosts) on request.

nslookup

a utility used to perform query testing of DNS servers and obtain detailed

responses at the command prompt. This information can be useful for

diagnosing and solving name resolution problems.

Dynamic

Host

Conﬁguration

Protocol

(DHCP)

protocol used to automatically assign network conﬁguration parameters

to devices on a network. Parameters include IP address, subnet mask,

default gateway, server addresses such as DNS, and lease time. DHCP

uses port number 67 to communicate from client to server and port 68

from server to client.

DHCP Scope

the consecutive range of possible IP addresses that the DHCP server can

lease to clients on a network or subnet. Scopes typically deﬁne a single

physical subnet on your network to which DHCP services are offered.

Scopes are the primary way for the DHCP server to manage distribution

and assignment of IP addresses and any related conﬁguration parameters

to DHCP clients on the network.

Command

Line Interface

(CLI)

a text-based method of accessing the shell of an operating system.

Usually, CLI provides a more powerful, direct way of executing programs

and utilities.

Universal

Resource

Locator

(URL)

the named address of a resource on the Internet

Fully

Qualiﬁed

Domain

Name

(FQDN)

the complete domain name for a speciﬁc computer, or host, on the

Internet. The FQDN consists of two parts: the hostname and the domain

name.

ipconﬁg

The ipconﬁg command is used to view or modify a computer’s IP

addresses, to release and then renew the IP address, and to ﬂush the DNS

resolver cache.

Conﬁgure an operational DHCP scope of addresses

Observe the effects of a deactivated DHCP scope and resolve the problem

Key Term Description

Ping used to verify basic TCP/IP connectivity to a network host.

APIPA

(Automatic

Private IP

Addressing)

A Microsoft Windows feature used when there is a failure in DHCP

servers, allowing DHCP clients to obtain IP addresses. APIPA allocates IP

addresses in the private range 169.254.0.1 to 169.254.255.254 and are

displayed in ipconﬁg /all as autoconﬁguration IPv4 addresses. When the

DHCP server is operational, clients correctly update their addresses

automatically.

Troubleshoot a Suspected DNS Issue Using CLI Utilities

Scenario: Users are complaining that the new company website at http://www.ips.com

cannot be accessed. They were informed of the new website in a company memo. As a

member of the IT support group, you are tasked with troubleshooting and resolving the

issue.

Using the Problem-Solving Process Troubleshoot a Suspected DNS

Issue

Through your training, you know the ﬁrst thing you need to do is isolate the problem. Based

on your experience, this appears to be a DNS problem, but to be sure, you will use a logical

and methodical process of problem solving to resolve the issue.

You begin gathering information by speaking with one of the users experiencing the access

problem, asking the user questions, and based on the information received, you consider

possible causes. Problems with network connectivity and DNS are two possibilities that you

suspect as causes.

You remember that DNS is a protocol that is part of the TCP/IP suite of protocols. It is used

to resolve the URL or website addresses that are readable to people into numerical IP

addresses that are readable by computers. The DNS server keeps records of the addresses

that are readable to people to IP address resolutions. The local computer will also build a

cache of addresses that have been previously resolved by the DNS server. The user’s

computer accesses the correct DNS server by its conﬁgured TCP/IP settings. DNS is

implemented in two software components; the DNS server and the DNS client. Your user’s

machine is the DNS client. Using utilities such as ipconﬁg /all, ping, and nslookup, you will

troubleshoot to ﬁnd a solution to the problem. Once solved, you will implement the solution,

document it, and explain the situation to the user.

You start the investigation by checking to see if there are any other network connectivity

issues.

Key Term Description

Click on the Internal 192.168.12.10 Windows Server icon on the topology diagram.

Click the Send Ctrl+Alt+Delete link in the upper-right corner of the viewer window.

In the password text box, type

P@ssw0rd

and press Enter to log in.

Go to Start > Search box and type

cmd

, and then press Enter.

Issue the ping command to the loopback address 127.0.0.1 to verify the TCP socket

on the local machine. It is successful.

Issue the ping command to the host IP address to test the IP conﬁgurations of the

local host (the user’s computer). The IP address of the Internal 192.168.12.10 Windows

Server is 192.168.12.10. It is successful.

The previous tests have helped to determine that the computer has its TCP/IP setting

conﬁgured and the stack is functioning properly.

Next, the tests will determine that the TCP/IP setting on the host computer is correct in

relation to the network it is conﬁgured for, particularly those settings that are used for DNS

name resolution since that is one of the potential causes of the existing problem.

At the command prompt, type

ipconfig /all

. This will display the full TCP/IP

conﬁguration for all NICs on the system, including DNS, DHCP, and WINS settings.

Locate the default gateway IP address from the ipconﬁg /all output. Ping the default

gateway to verify other hosts on the LAN are accessible. It is successful. This will help

you narrow down what could be causing the original problem of not being able to

access the new website at http://www.ips.com by eliminating lack of network

connectivity.

You have eliminated loss of network connectivity within the LAN as the cause of the problem

by proving connectivity to local hosts. Now it is time to test connectivity to the DNS server to

be sure that the user’s computer can access it.

Locate the DNS server IP address from the ipconﬁg /all output. At the command

prompt, ping the DNS server to make sure it is accessible from the user’s computer. It

is successful. This proves that the DNS server is accessible, but that does not mean

that DNS is functioning properly because you are still using an IP address in your

testing.

At the command prompt, type

nslookup ?

Nslookup is a command line utility for

testing and troubleshooting DNS servers. Nslookup ? shows available commands in

the utility.

Nslookup can be used to test DNS failures. At the command prompt, type

nslookup

192.168.12.11

for the IP address of the user’s computer.

Nslookup is useful in this situation because it has allowed you to verify that name resolution

is working. The output has returned the name and IP address of the DNS server that resolved

the name. It also shows you the fully qualiﬁed domain name and the IP address of the host

you speciﬁed in the command which is that of the user’s computer.

You have now determined that network connectivity is functional and DNS is working

properly, and it is time to reevaluate possible causes of the user not being able to

access the new website at http://www.ips.com. Based on your ﬁndings, it can be

concluded that the URL provided may be incorrect.

By contacting the department that distributed the memo and checking the URL of

the new website, you learn that the correct URL is http://www.isp.com.

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CONCLUSION:

There are many useful CLI utilities that can be used for troubleshooting and problem solving

on the network. Even with all the tools available, it is still important to work through solving

problems using a logical and methodical process. A good starting point for troubleshooting

is to identify the problem and gather information about what is wrong and who is affected.

This makes it easier to come up with possible causes and work toward solutions of the

problem.

Conﬁgure an Operational DHCP Scope of Addresses

A DHCP server is used to automatically assign TCP/IP conﬁguration parameters to devices

on a network such as IP address, subnet mask, default gateway, and server addresses such

as DNS. The DHCP Scope is the consecutive range of IP addresses that the DHCP server can

lease to clients on a network or subnet. In this task, you will conﬁgure DHCP on a server and

test it to be sure the client is receiving the correct DHCP conﬁgurations.

Open a web browser and try to access the new website using the corrected URL,

http://www.isp.com

. The website opens in the browser.

Conﬁgure a DHCP Server Scope

On the Internal 192.168.12.10 Windows Server, click Start -> Administrative Tools ->

DHCP.

Click on the + next to the w2k8r2internal1.netplus.com machine to expand.

Expand the IPv4 by clicking + then right-click and select New Scope from the

shortcut menu.

The Welcome to the New Scope Wizard screen appears. Click Next >.

On the Scope Name Wizard screen, type in the Name,

Internal1

, and a Description,

DHCP

. These are just variables decided by the network administrator so a name or

description would be selected according to the company conventions. Click Next >.

On the IP Address Range Wizard screen, type the Start and End IP address. Use the

start address,

192.168.12.3

, and the end address,

192.168.12.30

. These

addresses need to be usable IP addresses within the same network/subnet as the

host to which they will be assigned. You may also need to conﬁgure a length or

subnet mask. The length and subnet mask are used to indicate the number of bits

being used to identify the network portion of the host IP address. In this case, you

will accept the default settings because that is the correct subnet for this scenario.

Click Next.

On the Add Exclusions and Delay Wizard screen, enter a valid Start and End IP

address. For this scope, the start IP address for excluding IP addresses from being

dynamically assigned to hosts is

192.168.12.3

and the end address in the range is

192.168.12.14

. This range of addresses is selected to be left out of the scope

because some of them are statically assigned to other devices on the network, and

since IP addresses need to be unique within a network, excluding these from possibly

being assigned again will prevent duplicate IP address problems on the network.

Type the addresses into the wizard and be sure to click Add to save the settings.

Leave the Subnet delay at the default setting. Click Next.

On the Lease Duration Wizard screen, accept defaults and click Next >. DHCP

allocates the IP address for speciﬁed periods of time known as a lease. The Lease

Duration speciﬁes the time, in seconds, from address assignment until the client’s

lease on the address expires.

A host IP address, subnet mask, and default gateway are the main options that need

to be conﬁgured through DHCP. If a system is part of a domain or needs to connect

to the Internet, the domain name and DNS server options need to be conﬁgured. For

Windows systems, WINS can also be conﬁgured. On the Conﬁgure DHCP Options

Wizard screen, click the Yes, I want to conﬁgure these options now button and click

Next >.

On the Router (Default Gateway) Wizard screen, type in

192.168.12.1

, click Add,

and then click Next >.

There is no domain to conﬁgure for this network. Notice the DNS server IP address

has already been added to the options to be conﬁgured on the DHCP clients. On the

Domain Name and DNS Servers Wizard screen, click Next >.

You will not be conﬁguring a WINS setting on this scope. On the WINS Servers

Wizard Screen, click Next >.

On the Activate Scope Wizard screen, click the Yes, I want to activate this scope now

button and click Next >.

Click Finish to complete the New Scope Wizard.

Close the DHCP Dialog Box and all other open windows.

To test out the scope conﬁguration, we will use the Internal 192.168.12.11 Windows

Server, enabling it as the DHCP client. Click on the Internal 192.168.12.11 Windows

Server icon on the topology diagram.

Click the Send Ctrl+Alt+Delete link in the upper-right corner of the viewer window.

In the password text box, type

P@ssw0rd

and press Enter to log in.

Open the Network and Sharing Center by right-clicking the network access icon on

the bottom-right of the taskbar and click Open Network and Sharing Center.

Click Change adapter settings in the left panel.

Double-click on the network card named Local Area Connection.

Click Properties.

Click Internet Protocol Version 4 (TCP/IPv4) to highlight and then click Properties.

Notice the radio button next to Use the following IP address is selected. This is how

you indicate that static IP conﬁguration settings are to be used; these are manually

set by the network administrator.

In the Internet Protocol Version 4 (TCP/IPv4) Properties dialog box, set this machine

to a DHCP client by clicking the buttons Obtain an IP address automatically and

Obtain DNS server address automatically.

Click OK and then click X to all open windows.

Test DHCP on the client (Internal 192.168.12.11 Windows Server). Open the command

prompt by clicking Start, type

cmd

in the search box, and press Enter.

The Command prompt dialog window opens. Type

ipconfig/all

to verify that this

machine is a DHCP client. Maximize the window to see all the information. You

should see something similar to the following:

The Internal 192.168.12.11 Windows Server client obtained IP conﬁguration information such as

IPv4 Address, Subnet Mask, Default Gateway, Lease information, DHCP, and DNS Server

information dynamically, and this is indicated by looking at the DHCP Enabled row and

seeing Yes as the setting. If the client was conﬁgured to use static IP addressing, No would

replace the Yes in that row.

Release and renew the IP conﬁguration by typing

ipconfig/release

, then

ipconfig/renew

Verify that you still have LAN network connectivity by pinging both the default

gateway and DHCP server addresses. Type

ping 192.168.12.1

and press Enter,

then type

ping 192.168.12.10

and press Enter.

CONCLUSION:

Setting up a DHCP server is a step-by-step process requiring planning in order to provide

accurate IP conﬁguration parameters to client devices. Settings such as the size of the DHCP

pool, addresses of key devices to exclude, DNS server, and default gateway parameters are

paramount to ensure proper Scope conﬁguration. Testing and veriﬁcation commands such as

ipconﬁg, ipconﬁg/all, ipconﬁg/release, and ipconﬁg/renew are signiﬁcant in ensuring DHCP

works as planned.

Observe the Effects of a Deactivated Scope and Resolve the

Conﬁguration

To complete this lab, you will now misconﬁgure the DHCP server scope and observe the

effects of incorrect DHCP settings on a network.

Deactivate the DHCP Scope, Observe the Effects, and Resolve the

Problem

On the Internal 192.168.12.10 Windows Server, click Start > Administrative Tools >

DHCP.

Maximize and resize the windows to suit your viewing preference. Click the + next to

the w2k8r2internal1.netplus.com and IPv4 to expand them. Highlight Scope

[192.168.12.0] Internal1, right-click, and select Deactivate. When you are asked if you

are sure you want to deactivate the scope, click Yes.

Go back to the Internal 192.168.12.11 Windows Server, and at the command prompt,

type

ipconfig /release

and press Enter. Then, type

ipconfig /all

and press

Enter. Notice that DHCP Enabled is still set to Yes, but the TCP/IP conﬁgurations are

APIPA addresses (autoconﬁguration IPv4 address of 169.254.x.x) because the DHCP

server is unavailable.

Now, return to the DHCP on the Internal 192.168.12.10 Windows Server, click the +

next to the w2k8r2internal1.netplus.com and IPv4 to expand them. Highlight Scope

[192.168.12.0] Internal 1, right-click, and select Activate.

Return on the Internal 192.168.12.11 Windows Server, on the command promt,type

ipconfig /renew

and press Enter to reset the TCP/IP conﬁguration settings.

Ping the DHCP server machine (Internal 192.168.12.10 Windows Server) by typing

ping 192.168.12.10

and press Enter to verify that the settings are correct and

there is network connectivity.

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Note: Press the STOP button to complete the lab.

CONCLUSION:

The DHCP protocol allows a server to dynamically distribute IP addressing and conﬁguration

information to clients. Normally, the DHCP server provides the client with at least this basic

information: IP address, subnet mask, and default gateway. By using DHCP server computers

to centrally manage IP addresses and other related conﬁguration parameters, network

administrators are relieved of the trouble of manually conﬁguring TCP/IP settings on all

network hosts.