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Network Configuration
An Introductory Tutorial
Network Parameters

The following are common parameters that
need to be set in a networked computer:

DNS Servers (e.g. 137.207.32.32)



The logical (e.g. IP) address of the DNS servers to be
used to resolve domain names
These can be local (e.g. frankie.csfac8.uwindsor.ca) or
remote (e.g. www.google.ca)
Host and domain information

This includes the computer’s name (e.g. elvis) and the
domain on which it resides (e.g. csfac8.uwindsor.ca)
Network Parameters

Logical address (e.g. IP address: 137.207.132.21)


Subnet mask (e.g. 255.255.0.0)


This is the binary mask that should be used to extract the
network and subnet information from the host’s address
Default gateway (e.g. 137.207.32.2)


This address is required to participate on a WAN, such as
the Internet
This is the logical (e.g. IP) address of the router which
should receive the host’s Internet-bound packets
All of these values would be given to a
computer’s user by a network administrator
DNS Servers

DNS servers use the DNS protocol to
determine the logical (e.g. IP) address of a
host, given its domain name


e.g. www.google.ca: 64.233.167.99
Many servers are given (in a specific order) so
that local DNS servers are first

This is because the main DNS servers (Internic)
would not know local domain names


e.g. An Internic server would know about uwindsor.ca, but
not frankie.csfac8.uwindsor.ca)
These server addresses would usually be wellknown
Host and Domain Info

Host name:

This is the name of the host on the network


Domain name:

This is the name of the domain on which the host resides



A domain is a named group of computers, which may correspond
to a subnet of addresses
e.g. csfac8.uwindsor.ca refers to the LAN on the 8th floor of
Lambton Tower
Given these two pieces of information, the host’s DNS
name would be elvis.csfac8.uwindsor.ca


e.g. elvis
Depending on the operating system, the domain may require
that you log on first
The network admin will create one or more domains, but each host’s
user can determine the host name, provided it is unique
Logical Address

This is typically an IP address
This address is necessary for all WAN
(including Internet) communication
 Routing uses the logical address for finding
the path for packets


These addresses are normally assigned
automatically by a DHCP/BOOTP server

They can be random addresses on the
network, or sequentially assigned
Subnet Mask

The subnet mask that it used by the host
is the same subnet mask used by the
nearest routers


e.g. If using 2-level subnetting of 2 and 3
bits, this mask would be the last subnet
mask used (137.207.248.0)
A network admin would know the subnet
masks he/she assigned to each network
Default Gateway

The default gateway (router) is the router to
which all Internet-bound traffic should be sent




Typically, a LAN will have exactly one router, and
this will be its address
If there is more than one router, and only one leads
to the Internet, the Internet-connected router’s
address is used here
If there is more than one router that connects to the
Internet, either address could be used
The network admin would know the address of
the routers
Troubleshooting Networks
How to Determine Network
Configuration or Hardware
Problems
Troubleshooting

A network admin’s job is not just to set
up a network, but maintain it

Thus, if anything goes wrong, the admin
should track down the problem somehow
The problem may be a configuration error, which
the admin should fix
 The problem may be a hardware failure, which
can only be fixed by repairing or replacing the
hardware


Nevertheless, the admin must determine which
hardware has failed, so this can happen
A Typical Scenario

A user calls up the network admin,
saying their network connection is down
For most non-technical users, even this
clear of a statement of the problem might
not be possible
 A network admin will eventually figure at
least this much out from the user’s
comments

Step 1: Test Local Connectivity

First, an admin will try to connect to a local computer
cmd% ping frankie.csfac8.uwindsor.ca
ping: unknown host frankie.csfac8.uwindsor.ca

This could mean DNS servers are unavailable


The admin should check the DNS server addresses, and the servers
Next, the admin will make sure the IP address works
cmd% ping 137.207.140.93
Destination host unreachable.
Destination host unreachable.
Destination host unreachable.

This could mean no connectivity


The admin should check the configuration settings (ipconfig, netconfig, networkadmin)
However, if the ping command responds from a router that a host is unreachable,
the node may actually be unreachable, or the address may not be bound

Obviously, we have connectivity since the router sent a message to us
How Ping Works
Ping works by sending an ICMP Echo
Reqest to the destination
 The destination sends back an ICMP
Echo Reply to the host
 The host records the round trip time, and
repeats the process a few times

A Problem With Ping

ICMP Echo Requests are often disabled
in some networks


This is due to denial of service attacks that
use ICMP Echo Request/Reply
In these networks, ping will not work
Check Configuration Settings

In Windows (2000/XP):
C:\> ipconfig
(success)
Windows IP Configuration
Ethernet adapter Wireless Network Connection:
Connection-specific DNS Suffix . : csfac8.uwindsor.ca
IP Address. . . . . . . . . . . . : 192.168.137.71
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 192.168.137.2
(failure)
Windows IP Configuration
Ethernet adapter Wireless Network Connection:
Media State . . . . . . . . . . . : Media disconnected
Edit Configuration Settings

In Windows (2000/XP), editing your network
configuration settings can be done from
TCP/IP properties




Open control panel, select Network Connections
Select your connection, click Properties
Select Internet Protocol (TCP/IP), click Properties
From here you can edit your configuration settings
manually, or specify that a DHCP should provide
the values for you
Check/Edit Configuration Settings
In Linux:
Linux% network-admin
 In Solaris:
Solaris% netconfig


These bring up a graphical dialog where
configuration can be viewed and edited
Step 2: Test Remote Connectivity
 First,
an admin will try to connect to
a local computer
cmd% ping www.google.ca
ping: unknown host www.google.ca
 This


could be a routing problem
Most likely the routing problem is within the
organization
The network administrator should check the
route using the traceroute (traceroute, tracert)
command
Testing Routes

The traceroute command can give the logical
addresses of each router in a route

It may also tell you if there is a problem with the
route

In Windows (2000/XP):
tracert www.google.ca

In Linux/Solaris:
traceroute www.google.ca
Windows Tracert Output
C:\>tracert www.google.ca
Tracing route to www.l.google.com [66.102.7.104]
over a maximum of 30 hops:
1 1 ms 1 ms 1 ms 192.168.1.1
2 23 ms 16 ms 20 ms 10.71.96.1
3 11 ms 13 ms 13 ms d226-4-1.home.cgocable.net [24.226.4.1]
4 30 ms 15 ms 18 ms cgowave-0-157.cgocable.net [24.226.0.157]
5 30 ms 17 ms 17 ms cgowave-0-101.cgocable.net [24.226.0.101]
6 29 ms 18 ms 17 ms gw-google.torontointernetxchange.net [198.32.245.6]
7 19 ms 37 ms 17 ms 66.249.94.75
8 44 ms 23 ms 31 ms 64.233.175.96
9 53 ms 40 ms * 64.233.175.96
10 98 ms 92 ms 86 ms 66.249.95.215
11 98 ms 89 ms 94 ms 216.239.49.142
12 88 ms 95 ms 90 ms 216.239.49.142
13 104 ms 98 ms 89 ms 216.239.49.142
14 99 ms 94 ms 88 ms 66.102.7.104
Trace complete.
Windows Tracert Output

The output contains lines like the following:
5 30 ms 17 ms 17 ms cgowave-0101.cgocable.net [24.226.0.101]
5: The 5th hop
30,17,17ms: Round trip times to/from the router
cgowave-0-101.cgocable.net [24.226.0.101]:
The domain name and address of the router
Linux traceroute Output
rfortier@elvis:~ $ traceroute www.google.ca
traceroute: Warning: www.google.ca has multiple addresses; using 64.233.167.147
traceroute to www.l.google.com (64.233.167.147), 30 hops max, 38 byte packets
1 ssr68 (137.207.140.65) 0.395 ms 0.302 ms 0.250 ms
2 ssr (137.207.140.66) 0.597 ms 0.508 ms 0.472 ms
3 cs-ssr-main.newcs.uwindsor.ca (137.207.76.254) 1.038 ms 0.901 ms 0.692 ms
4 passport1.uwindsor.ca (137.207.32.1) 1.701 ms 1.245 ms 1.225 ms
5 internet.uwindsor.ca (137.207.92.14) 1.434 ms 1.136 ms 0.967 ms
6 acs7204.uwindsor.ca (137.207.232.3) 2.494 ms 1.563 ms 1.852 ms
7 f0-1.na03.b011027-0.yyz01.atlas.cogentco.com (38.112.19.97) 10.358 ms 10.811 ms 9.377 ms
8 g9-1.core01.yyz02.atlas.cogentco.com (66.28.66.53) 10.628 ms 9.289 ms 10.427 ms
9 p15-0.core01.yyz01.atlas.cogentco.com (154.54.2.221) 10.265 ms 15.687 ms10.139 ms
10 p13-0.core02.ord01.atlas.cogentco.com (66.28.4.213) 24.619 ms 24.274 ms 25.259 ms
11 p15-0.core01.ord01.atlas.cogentco.com (66.28.4.61) 24.502 ms 23.292 ms 24.221 ms
12 p6-0.core01.ord03.atlas.cogentco.com (154.54.2.238) 24.316 ms 24.378 ms 24.401 ms
13 google.ord03.atlas.cogentco.com (154.54.11.186) 22.021 ms 22.070 ms 21.698 ms
14 66.249.94.193 (66.249.94.193) 22.524 ms 22.307 ms 66.249.94.195 (66.249.94.195) 24.050 ms
15 64.233.175.38 (64.233.175.38) 26.037 ms 64.233.175.42 (64.233.175.42) 64.185 ms
64.233.175.38 (64.233.175.38) 25.547 ms
16 64.233.167.147 (64.233.167.147) 23.303 ms 24.924 ms 22.002 ms
How To Interpret the Output

The round trip times can tell you if there
is a problem
A round trip time of * means a timeout has
occurred, which is a definite problem
 A round trip time > 500 ms usually means
there is a problem with that router


The router could be dropping packets (due to
overloaded buffer or routing loops typically)
How Does Traceroute Work?

Traceroute sends either a UDP or ICMP Echo
packet to the destination

Initially, the hop limit is set to 1, which causes the
first router to drop the packet

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Routers, however, will respond with an ICMP Time
Exceeded error message, which contains the router’s IP
address
The process is repeated a few times to get a few round trip
times
The domain name of the IP address is found (for
convenience of the network admin)
Next, the process is repeated with a hop limit of 2

Then 3, 4, etc. until the destination host is reached
A Problem with Traceroute

One problem with traceroute is that it could read a false
path

If the routers reconfigure (e.g. due to congestion) during its
execution, the readings could be inaccurate
A1
A2
A2
S
D
B2
B2
A Problem with Traceroute

One problem with traceroute is that it could read a false
path

If the routers reconfigure (e.g. due to congestion) during its
execution, the readings could be inaccurate
1
A1
A2
A2
S
D
B2
B2
A1
A Problem with Traceroute

One problem with traceroute is that it could read a false
path

If the routers reconfigure (e.g. due to congestion) during its
execution, the readings could be inaccurate
A1
A2
A2
S
D
B2
B2
1
A1
2
A2
A Problem with Traceroute

One problem with traceroute is that it could read a false
path

If the routers reconfigure (e.g. due to congestion) during its
execution, the readings could be inaccurate
A1
A2
A2
S
D
B2
B2
1
A1
2
A2
3
D
A Problem with Traceroute

For networks with ICMP Echo Request/Reply
disabled, the UDP format must be used

Some traceroute commands do not support the
UDP message format, however
LAN Setup
10BaseT Ethernet
802.11g Wireless
10BaseT Ethernet Setup

To connect a 10BaseT Ethernet network:


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Hub:


Install a network interface card (NIC) into each computer
(host)
Connect each NIC to a hub or switch, using unshielded
twisted pair (UTP) cabling
Every frame received by a hub is reproduced on all of it’s
ports
Switch (or Cable/DSL Router):


Every frame received by a switch is examined, and the correct
port is determined for the frame
A Cable/DSL also provides network address translation (NAT)
802.11g Wireless Setup

To setup a 802.11g wireless network:
Install an 802.11g (or 802.11b) NIC into
each host
 Plug a wireless router or base station into
your Internet connection (e.g. cable modem)
 Configure the SSID and security protocols
on the wireless router and each of the NICs


e.g. WPA encryption, and specify the key
Ad Hoc Wireless Networks

If you have only a few hosts (2-3), you
can create an ad hoc network
This type of network does not have a base
station (or wireless router)
 The hosts communicate directly with one
another

Wireless Networks Comment

A wireless network connection is often
not useful for a desktop PC
For this reason, wireless routers typically
contain a few Ethernet ports
 It is typical to have desktop PCs wired, and
notebooks and PDAs will have wireless


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e.g. Notebooks with 802.11g and PDAs with
802.11b
Wireless networks will be discussed in
depth later
Packet Sniffers

Packet sniffers are a useful tool for exploration
of networking

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They typically examine many protocols

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They can also be used by network programmers to
diagnose packet format errors
This includes the header information
e.g. Wireless, Ethernet, TCP/IP, HTTP
Some common packet sniffers are:
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Ethereal (Linux/Mac/Windows, multi-protocol)
TCPDump (Linux/Windows, TCP/IP)
Kismet (Linux, wireless)
Ethereal Tutorial
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Here is how to sniff packets using Ethereal:
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Start Ethereal
Select the correct network interface
Start capturing packets, stop when done
Select a packet from the list
Examine the packet hierarchy and header
information, such as:
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Ethernet or wireless frame headers
IP datagram headers
TCP datagram headers
SMTP message