Step 1 Step 2

navybeansvietnameseNetworking and Communications

Oct 24, 2013 (3 years and 9 months ago)

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dgooch

7 Jan
2005

From the
table on the right; to
obtain 500 useable subnets.


We need to borrow 9 host bits
as subnet bits.


This will give us ;

512


㈠㴠㔱〠2sea扬e su扮ets



(oemem扥r y潵 may en搠u瀠
with unuse搠 su扮ets)

T漠摥termine the num扥r 潦
usa扬e h潳ts 灥r⁳u扮et we
su扴ract the 扯br潷e搠扩ts (㤩
fr潭 the t潴al num扥r 潦 h潳t 扩ts
(ㄶ1 wh
ich gives us 㜠7emai湩湧
h潳t 扩ts⸠Using the f潲mulae

e潳ts 㴠′
n


㈠‽′
7



㈠⸠2r潭
the ta扬e 2
7

= 128. Thus usable
hosts = 128


㈠㴱㈶

(oemem扥r the first an搠last
a摤desses are f潲 netw潲欠an搠
扲潡摣astⰠhence the minus ㈠2n
the f潲mula)

A Step by Step Guide to Subnetting



Address 172.16.0.0/16


=

default subnet 255.255.0.0

ie Class B address









Network


Host

IP Address


10101100.00010000
.
00000000.00000000



Mask



11111111.11111111
.
00000000.00000000



You may be asked to sup
ply
500

useable subnets ;

using the formula Subnets = 2
n



2







Where n = number of bits to be borrowed





Step 1




Step 2















Step 3


Mark the borrowed bits as subnet bits






Network

Subnet


Host

IP Address


10101100.00010000
.
00000000.0
0000000



Mask



11111111.11111111
.
11111111
.
1
0000000

n

2
n

0

1

1

2

2

4

3

8

4

16

5

32

6

64

7

128

8

256

9

512

10

1024

11

2048

12

4096

13

8192

14

16384

15

32768

16

65536

16 network bits

Remember the borrowed bits in the mask ch
ange
from 0 (zero) to 1 (one)

dgooch

7 Jan
2005

Step 4


Define
the subnetted network;


Address 1
72.16.0.0/25


=

subnet mask 255.255.255.128









We are now in a position to calculate the subnet addresses and usable hosts for each
subnet.


Step 5






Network

Subnet


Host

IP Address


10101100.00010000
.
00000000.0
0000000



Mask



111
11111.11111111
.
11111111
.
1
0000000








00000000.0
0000000







First subnetwork not
used


First usable network







Last usable network




Last subnetwork not used
(Broadcast)



Thus first usable subnet

(Subnet
1
) =
10101100.00010000
.
00
000000.
1
0000000









=

172 . 16 . 0 . 128




the first usable host (Subnet
1
) =
10101100.00010000
.
00000000.
1
0000001







172 . 16 . 0 . 129

There are now 25 network bits.

(16 original + 9 borrowed)

The first 3 octets (24 bits) are network
bits, hence each octet = 255. The
final octet has only the MSB (Most
Significant Bit) as a network bit, hence
128.

We are

only interested in this
part of the address when
calculating the subnets


For each subnet, the binary value
of the subnet bits will increment
by 1, so the first 5 subnets and
last 2 are; *See appendix


00000000.0

00000000.1

00000001.0

00000001.1

00000010
.0

…etc.


ㄱㄱㄱㄱ⸰

ㄱㄱㄱㄱ⸱


dgooch

7 Jan
2005


the last usa
ble host (Subnet
1
) =
10101100.00010000
.
00000000.
1
1111110







172 . 16 . 0 . 254




the broadcast address (Subnet
1
) =
10101100.00010000
.
00000000.
1
1111111







172 . 16 . 0

. 255



Thus host range
(Subnet
1
)
=



10101100.00010000
.
00000000.
1
000000
1


to

10101100.00010000
.
00000000.
1
1111110


172 . 16 . 0 . 129


172 . 16 . 0 . 254





Thus 2nd
us
able
subn
et
(Subnet
2
)


=
10101100.00010000
.
0
0000001
.
0
0000000









=

172 . 16 .
1

.

0




the first usable host (Subnet
2
) =
10101100.00010000
.
0000000
1
.
0
0000001







172 . 16

.
1



.


1


the last usable host (Subnet
2
) =
10101100.00010000
.
0000000
1
.
0
1111110







172 . 16 .
1



.
126


the broadcast address (Subnet
2
) =
10101100.00010000
.
0000000
1
.
0
1111111







172 . 16 .
1

.

127


Thus host range (Subnet
2
) =



10101100.00010000
.
0000000
1
.
0
0000001
to

10101100.00010000
.
0000000
1
.
0
1111110


172 . 16 .
1

.

1


172 . 16

.
1

.
126




the host range (Subnet
3
) =


10101100.00010000
.
0000000
1
.
1
0000001
to

10101100.00010000
.
0000000
1
.
1
1111110


172 . 16 . 1 . 129


172 . 16 . 1 .

254




and the host range (Subnet
4
) =


10101100.00010000
.
000000
10
.
0
0000001
to

10101100.00010000
.
000000
10
.
0
1111110


172 . 16 . 1 . 129


172 . 16 . 1 . 126


and so on………

Good

Luck with your subnetting
dgooch

7 Jan 2005

Appendix



IP Value

128

64

32

16

8

4

2

1

128

64

32

16

8

4

2

1

Subnet Number

256

128

64

32

16

8

4

2

1















Subnet 0( Not Used)

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Subnet 1 Address

0

0

0

0

0

0

0

0

1

0

0

0

0

0

0

0

Subnet 1
Host 1

0

0

0

0

0

0

0

0

1

0

0

0

0

0

0

1

Subnet 1 Host 126

0

0

0

0

0

0

0

0

1

1

1

1

1

1

1

0

Subnet 1 Broadcast

0

0

0

0

0

0

0

0

1

1

1

1

1

1

1

1

Subnet 2 Address

0

0

0

0

0

0

0

1

0

0

0

0

0

0

0

0

Subnet 2 Host 1

0

0

0

0

0

0

0

1

0

0

0

0

0

0

0

1

Subnet 2 Host
126

0

0

0

0

0

0

0

1

0

1

1

1

1

1

1

0

Subnet 2 Broadcast

0

0

0

0

0

0

0

1

0

1

1

1

1

1

1

1

Subnet 3 Address

0

0

0

0

0

0

0

1

1

0

0

0

0

0

0

0

Subnet 3 Host 1

0

0

0

0

0

0

0

1

1

0

0

0

0

0

0

1

Subnet 3 Host 126

0

0

0

0

0

0

0

1

1

1

1

1

1

1

1

0

Subnet 3 Broadcas
t

0

0

0

0

0

0

0

1

1

1

1

1

1

1

1

1



etc….

Subnet 510 Address

1

1

1

1

1

1

1

1

0

0

0

0

0

0

0

0

Subnet 510 Host 1

1

1

1

1

1

1

1

1

0

0

0

0

0

0

0

1

Subnet 510 Host 126

1

1

1

1

1

1

1

1

0

1

1

1

1

1

1

0

Subnet 510 Broadcast

1

1

1

1

1

1

1

1

0

1

1

1

1

1

1

1

Su
bnet 511(Not Used)

1

1

1

1

1

1

1

1

1

0

0

0

0

0

0

0


Note
:

O
nly the final two octets are shown for clarity.