CIT 384: Network Administration

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CIT 384: Network Administration

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1

CIT 384: Network Administration

Subnetting

CIT 384: Network Administration

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2

Topics

1.
IP Addresses

2.
Classful and classless addressing

3.
Subnet Masks and Prefixes

4.
Subnet Math

5.
Subnet Problems

CIT 384: Network Administration

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3

IP Addresses

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

CIT 384: Network Administration

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4

Grouping IP Addresses

Groups of consecutive IP addrs are called
networks
.

Routing table would only need 3 entries below
.

CIT 384: Network Administration

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5

Network and Host Parts

IP addresses are divided into two parts


Network ID (like zip code)


Host ID (like street address)

Network ID

Host ID

Two special IP addresses


Network address (e.g. 130.4.0.0)


Broadcast address (e.g. 130.4.255.255)

CIT 384: Network Administration

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Address Classes

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

CIT 384: Network Administration

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7

CIDR

Classless Inter
-
Domain Routing


Classful routing wastes most IP addresses.


Allocate addresses on bit boundaries instead of
byte boundaries.


Allow ISPs/users to decide on boundaries
instead of basing on IP addresses.

Prefix notation


/
x

indicates that first x bits are shared.


192.168.0.0/16 = 192.168.0.0


192.168.255.255

CIT 384: Network Administration

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8

Public IP Addresses

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

CIT 384: Network Administration

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9

Private RFC1918 IP Addresses

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

CIT 384: Network Administration

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IPv4 vs IPv6 Addresses

Feature

IPv4

IPv6

Size of Address

32 bits

128 bits

Example Address

10.1.1.1

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

Abbreviated
Address

-

::FFFF:FFFF:0A
01:0101

Localhost

127.0.0.1

::1/128

Possible
Addresses

2
32

(~4 billion)

2
128

(~3.4 x 10
38
)

CIT 384: Network Administration

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11

Network Mask

How do we list subnets in routing table?


Ex: addresses 150.150.4.0


150.150.4.255


Table: 155.155.4.0 netmask 255.255.255.0

Subnet mask indicates range


Binary 1s indicate network part of address.


Binary 0s indicate host part of address.


Always consists of 1s followed by 0s.

Prefix notation


Humanly readable form of subnet mask.


Just counts the number of binary 1s in mask.

CIT 384: Network Administration

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12

Classful Address Ranges and Masks

Class A

NNNNNNNN.HHHHHHHH.HHHHHHHH.HHHHHHHH

Class B

NNNNNNNN.NNNNNNNN.HHHHHHHH.HHHHHHHH

Class C

NNNNNNNN.NNNNNNNN.NNNNNNNN.HHHHHHHH

Class

Leading
Bits

Start

End

Subnet Mask

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

CIT 384: Network Administration

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Example


IP Address: 137.201.18.42


Address Class: B since 128 < 137 < 191


Default Netmask: 255.255.0.0


Network Address Part: 137.201.0.0


Host Address Part: 0.0.18.42


Broadcast Address: 137.201.255.255


Host Address Range for Network:


137.201.18.1 through 137.201.255.254

CIT 384: Network Administration

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Why Subnet?

Allows admin to create more networks for:

1.
Address conservation.

2.
Organization of hosts.

3.
Different physical media.

4.
Security.

5.
Performance (smaller broadcast domains.)

CIT 384: Network Administration

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IP Addresses with Subnets

Route on network + subnet part of address.

CIT 384: Network Administration

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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

CIT 384: Network Administration

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How to find network address?

(IP Address) AND (Subnet Mask)


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

CIT 384: Network Administration

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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)


150.150.255.0/24 (ambiguous broadcast 150.150.255.255)

CIT 384: Network Administration

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Choosing Subnet Mask to meet
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

Problem 3: find subnet mask


11111111 11111111 11111111 00000000


255.255.255.0

CIT 384: Network Administration

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20

Multiple Possible Subnet Masks

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)


Subnet masks


/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?

CIT 384: Network Administration

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.