Transcript Introduction - Eastern Illinois University

School of Business
Eastern Illinois University
Review for Exam 4
© Abdou Illia, Spring 2005
School of Business
Eastern Illinois University
The Internet
3
The Internet addressing system

Network deliver messages based on network
addresses
–
Host
The Internet has two addressing systems for hosts
 IP addresses. Example: 139.67.8.3
 Host names (or domain names or Unique Resource
Locators). Example: eiu.edu
Host
IP Address

4
IP addresses
–
–
–
Are really strings of 32 bits (1s and 0s)
 Example: 10000000101010100001000100001101
Usually represented by four number segments separated
by dots: dotted decimal notation
 Example: 128.171.17.13
Official addresses for hosts
127.18.47.145
127.47.17.47
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Assigning Parts

Most Organizations have multiple Segments
within the Organizational Network

So, usually Local Part is broken in two parts
–
–
a Segment Part to represent each segment
Remaining Bits are the Host Part, designating a
particular station on that segment
Local Part
Network Part
Segment Part
IP Address (32 bits total)
Host Part
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IP address
Position Place Value
Bit Decimal
(N)
(2N)
Binary
10100011
=
Decimal
163
Note: Starts with 0
7
6
5
4
3
2
1
0
128
64
32
16
8
4
2
1
1
0
1
0
0
0
1
1
128
0
32
0
0
0
2
1
163
7
Network classes

The value of the first octet in an IP address
determines the Network class
Class
Leftmost
bits
Class A 0xxx
Class B 10xx
Class C 110x
Network Part
Length
Address range
8 bits
16 bits
24 bits
0.x.x.x to 127.x.x.x
128.0.x.x to 191.255.x.x
192.0.0.x to 223.255.255.x
Position
(N)
1)
For each of the following IP
addresses, give the class and the
network bits.
10101010111110000101010100000001
01010100111110000101010100000001
2)
To which class belong Eastern’s
network? (Net. Part =139.67)
7
6
5
4
3
2
1
0
Place Value
(2N)
128
64
32
16
8
4
2
1
Bit
Decimal
Summary Questions
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1.
a) Distinguish between IP address and host name. b)
Which is the official address of a host? c) Does a
server host need an IP address? d) Does your home
PC need an IP address when you are on the Internet?
e) Does a server host need a host name? f) Does
your home PC need a host name when you are on
the Internet?
2.
Using the conversion system on slide #10, convert
the following IP address to dotted decimal notation:
10101010 11110000 11001100 01010101. (Spaces
are included to facilitate reading.)
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Summary Questions
3.
a) What are the three parts in IP addresses? b) What part(s)
do border routers look at to determine whether the
destination host is within the network or outside of it?
4.
a) Who assigns the Network part? b) The Segment part? c)
The Host part?
5.
a) When do we need DNS? b) What information do you send
in a DNS request message? c) What information do you
receive in a DNS response message?
4.
a) What is autoconfiguration? b) What information do we get
back, at a minimum, in an autoconfiguration response
message? c) What other information may we get back?
School of Business
Eastern Illinois University
Security
11
Summary Questions

Name a few kinds of systems attacks
Answer (Security, Slide #3): Denial of Service attacks, Malicious
content attacks and Intercepting messages.

Distinguish between Denial-of-Service (DoS) attack and Distributed
Denial-of-Service (DDoS) attack
Answer (Security, Slides #4, 5): In DoS attack, the attacker sends a
stream of messages to the victim (usually a server) from a single
computer connected to the network. In DDoS, the attacker hacks into
multiple clients and plants Zombie programs on them. Then, he sends
commands to Zombie programs which execute the attacks.

Name some techniques usually used to protect a communication
against Intercepting messages.
Answer: Symmetric key encryption/decryption and Public key
encryption/decryption
Summary Questions

12
Jason sends a message to Kristin using public key encryption. (a) What
key will Jason use to encrypt the message? (b) What key will Kristin
use to decrypt the message? (c) What key will Kristin use to encrypt
the reply? (d) What key will Jason use to decrypt the reply? (e) Can the
message and reply be long messages? Explain.
Answer:
a) Jason will encrypt the message with Kristin’s public key.
b) Kristin will use her own private key to decrypt the message.
c) Kristin will use Jason’s public key to encrypt the reply.
d) Jason will use his own private key to decrypt the reply.
e) No, public key encryption can only encrypt short messages.

Does public key encryption have a problem with secure key exchange
for the public key? Explain.
Answer: There is no problem distributing the public key, because it does
not have to be distributed securely. You can even find companies’
public keys on their website.
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Summary Questions
1.
Briefly describe the various types of malicious content
attacks
Answer: See slide #6
2.
What headers and messages do Packet filter firewalls
examine?
Answer:
3.
See slide #10
a) What part of a packet do Application firewalls examine?
b) What do they look for?
Answer: a) Application layer messages.
b) They look for illicit content.
Network Management 1 & 2
Availability
15

Availability: probability that a particular component
or system will be available during a fixed time period

Availability is function of:
–
–
Mean time between failures (Given by manufacturer or
generated based on past performance)
Mean time to repair (Found in studies or in our archives)

Mean time between failures (MTBF) is the average
time a device or system will operate before it fails.

Mean time to repair (MTTR) is the average time
necessary to repair a failure
Availability
 Standard equation:
A(t) = a/(a+b) + b/(a+b) x e-(a+b)t
in which:
a = 1/MTTR
b = 1/MTBF
e = natural log function
t = the time interval
 Approximation equation:
Availability% = (Total available time – Downtime)/Total available time
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Availability
A(t) = a/(a+b) + b/(a+b) x e-(a+b)t
17
Suppose we want to calculate the availability of a modem that has a
MTBF of 3000 hours and a MTTR of 1 hour. The availability of this
modem for an 8-hour period is:
a = 1/1
b = 1/3000 = 0.00033
A(8 hours) =1/(1 + 0.00033) + 0.00033/(1 + 0.00033) x e-(1 + 0.00033)8
= 0.9997 + 0.00033 x 0.000335
= 0.9997
Q: What will be the availability of the modem if the Approximation equation is used?
Availability

18
A component has been operating continuously for
three months. During that time, it has failed twice,
resulting in downtime of 4.5 hours. Calculate the
availability of the component during that threemonth period using the Approximation method.
Availability

To calculate the availability of a system of
components:
–
–

Calculate the availability of each component
Find the product of all availabilities
Example: If a network has tree devices with
availabilities of 0.992, 0.894, and 0.999, the
availability of the network is:
0.992 x 0.894 x 0.999 = 0.886
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Reliability
20

Reliability: probability that a component or system
will be operational for the duration of a transaction
time t.

Reliability is function of:
–
–
Mean time between failures
Transaction time

Mean time between failures (MTBF) is the average
time a device or system will operate before it fails.

Transaction time is the time interval of operation to
complete a given transaction.
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Reliability
Reliability is defined by the equation:
R(t) = e -bt
in which:
b = 1/MTBF
t = the time interval of the operation
Reliability
22
What is the reliability of a modem if the MTBF is 3000 hours
and a transaction takes 20 minutes, or 1/3 of an hour (0.333
hours):
R(t) = e -bt
b = 1/MTBF = 1/3000
t = 0.333
R(0.333 hours) = e -(1/3000)(0.333) = e -0.000111 = 0.99989
Q: If a component has a MTBF of 500 hours and a transaction
takes 4 seconds, calculate the reliability of the component
Summary Questions (Part 1)
23
1) List the main elements in centralized network
management
2) Does the Manager communicate directly with the
managed devices? Explain.
3) Explain the difference between a managed device
and objects.
4) Where is the MIB (database) stored?
Summary Questions (Part 2)
1) In Manager-Agent communications, which
device creates commands? Responses? Traps?
2) Explain the two types of commands.
3) What is a trap?
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