Networking and Communications

Oct 24, 2013 (4 years and 6 months ago)

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Slide #
1

Subnetting

Slide #
2

Topics

1.

2.

3.

4.
Subnet Math

5.
Subnet Problems

Slide #
3

32
-
bit integers

One for each network interface.

Dotted decimal notation: ii.jj.kk.ll

172 . 16 . 254 . 1

10101100

00010000

11111110

00000001

1 byte

32 bits = 4 bytes

Slide #
4

Groups of consecutive IP addrs are called
networks
.

Routing table would only need 3 entries below
.

Slide #
5

Network and Host Parts

IP addresses are divided into two parts

Network ID (like zip code)

Network ID

Host ID

Slide #
6

Class A
: 0.0.0.0
-
127.255.255.255

8
-
bit net ID, 24
-
bit host ID

2
24

2 hosts per network; 126 networks

Class B
: 128.0.0.0
-
191.255.255.255

16
-
bit net ID, 16
-
bit host ID

2
16

2 hosts per network; 16,384 networks

Class C
: 192.0.0.0
-
223.255.255.255

24
-
bit net ID, 8
-
bit host ID

(2
8

2) = 254 hosts per network; 2,097,152 networks

Class D
: 224.0.0.0
-
239.255.255.255

28
-
bit multicast group ID

Class E
: 240.0.0.0
-
255.255.255.255

Reserved for future use

Slide #
7

CIDR

Classless Inter
-
Domain Routing

Classful routing wastes most IP addresses.

byte boundaries.

Allow ISPs/users to decide on boundaries

Prefix notation

/
x

indicates that first x bits are shared.

192.168.0.0/16 = 192.168.0.0

192.168.255.255

Slide #
8

ICANN assigns network numbers.

Internet Corporation for Assigned Network
Numbers.

ICANN gives authority to regional orgs, e.g.
ARIN (American Registry for Internet Numbers)

Typically to ISPs, universities, corporations.

ISP assigns IP addresses within network

Slide #
9

Private IP Networks

Network Class

Count of Networks

10.0.0.0

A

1

172.16.0.0 through

172.31.0.0

B

16

192.168.0.0 through
192.168.255.0

C

256

Slide #
10

Feature

IPv4

IPv6

32 bits

128 bits

10.1.1.1

0000:0000:0000:
0000:FFFF:FFFF
:0A01:0101

Abbreviated

-

::FFFF:FFFF:0A
01:0101

Localhost

127.0.0.1

::1/128

Possible

2
32

(~4 billion)

2
128

(~3.4 x 10
38
)

Slide #
11

How do we list subnets in routing table?

150.150.4.255

Binary 1s indicate network part of address.

Binary 0s indicate host part of address.

Always consists of 1s followed by 0s.

Prefix notation

Just counts the number of binary 1s in mask.

Slide #
12

Class A

NNNNNNNN.HHHHHHHH.HHHHHHHH.HHHHHHHH

Class B

NNNNNNNN.NNNNNNNN.HHHHHHHH.HHHHHHHH

Class C

NNNNNNNN.NNNNNNNN.NNNNNNNN.HHHHHHHH

Class

Bits

Start

End

CIDR

A

0

0.0.0.0

126.255.255.255

255.0.0.0

/8

B

10

128.0.0.0

191.255.255.255

255.255.0.0

/16

C

110

192.0.0.0

231.255.255.255

255.255.255.0

/24

D

1110

224.0.0.0

239.255.255.255

N/A

N/A

E

1111

240.0.0.0

255.255.255.0

N/A

N/A

Slide #
13

Example

Address Class: B since 128 < 137 < 191

137.201.18.1 through 137.201.255.254

Slide #
14

Why Subnet?

Allows admin to create more networks for:

1.

2.
Organization of hosts.

3.
Different physical media.

4.
Security.

5.

Slide #
15

Route on network + subnet part of address.

Slide #
16

Subnet Math

Binary <
-
> Decimal Conversion

Convert each byte of dotted quad into binary.

Convert binary byte into 4 decimal values.

Boolean AND operation

0 AND 0 = 0

0 AND 1 = 0

1 AND 0 = 0

1 AND 1 = 1

Convert between dotted quad and prefix.

255.255.255.0 netmask is identical to /24

Slide #
17

137.201.18.42 10001001.11001001.00010010.00101010

AND

255.255.0.0 11111111.11111111.00000000.00000000

---------------------------------------------------

10001001.11001001.00000000.00000000

(convert from binary to decimal)

137.201.0.0

Slide #
18

How to find number of networks?

Address divided between network and host.

If there are s subnet bits and h host bits, then

Number of subnets = 2
s

Number of hosts = 2
h

2

Subnet zero

Classful routing reserves 2 subnets so only have 2
s

2.

Lowest and highest subnet numbers.

For Class B network 150.150.0.0 reserves

150.150.0.0 (ambiguity with address of whole B)

Slide #
19

Design Requirements

Requirements

Class B network 130.1.0.0

Number of subnets: 200

Max hosts per subnet: 200

Problem 1: how many host bits?

Find h, # of host bits, such that 2
h

>= 200.

2
7

= 128, 2
8

= 256, therefore h = 8.

Problem 2: how many subnet bits?

Find s, # of subnet bits, such that 2
s

>= 200, yields s = 8.

NNNNNNNN.NNNNNNNN.SSSSSSSS.HHHHHHHH

11111111 11111111 11111111 00000000

255.255.255.0

Slide #
20

In some problems, many subnet masks exist.

Ex: change # of subnets from 200 to 50.

Find s, # of subnet bits, such that 2
s

>= 50, yields s = 6.

NNNNNNNN.NNNNNNNN.SSSSSSxx.HHHHHHHH

How many possible subnets exist?

11111111 11111111 11111100 (6 subnet bits, 10 host bits)

11111111 11111111 11111110 (7 subnet bits, 9 host bits)

11111111 11111111 11111111 (8 subnet bits, 8 host bits)

11111111 11111111 11111101 (impossible)

/22 255.255.252.0 (6 subnet bits, 10 host bits, 1022 hosts/sub)

/23 255.255.254.0 (7 subnet bits, 9 host bits, 510 hosts/sub)

/24 255.255.255.0 (8 subnet bits, 8 host bits, 254 hosts/sub)

Do you want to maximize # subnets or # hosts/subnet?

Slide #
21

References

1.
James Boney,
Cisco IOS in a Nutshell, 2
nd

edition
, O’Reilly, 2005.

2.
Cisco, Cisco Connection Documentation,
http://www.cisco.com/univercd/home/home.htm

3.
Cisco, Internetworking Basics,
http://www.cisco.com/univercd/cc/td/doc/cisintw
k/ito_doc/introint.htm

4.
Matthew Gast,
802.11 Wireless Networks: The
Definitive Guide
, O’Reilly, 2005.

5.
Wendell Odom,
CCNA Official Exam
Certification Library, 3
rd

edition
, Cisco Press,
2007.