# Step 1 Step 2

Networking and Communications

Oct 24, 2013 (4 years and 6 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

the f潲mula)

A Step by Step Guide to Subnetting

=

default subnet 255.255.0.0

Network

Host

10101100.00010000
.
00000000.00000000

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

10101100.00010000
.
00000000.0
0000000

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;

72.16.0.0/25

=

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

Step 5

Network

Subnet

Host

10101100.00010000
.
00000000.0
0000000

111
11111.11111111
.
11111111
.
1
0000000

00000000.0
0000000

First subnetwork not
used

First usable network

Last usable network

Last subnetwork not used

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
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

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

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

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

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

0

0

0

0

0

0

0

0

1

1

1

1

1

1

1

1

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

0

0

0

0

0

0

0

1

0

1

1

1

1

1

1

1

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

t

0

0

0

0

0

0

0

1

1

1

1

1

1

1

1

1

etc….

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

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.