Subnetting Calculation

SUBNETTING CALCULATION



In this section, you are expected to show the calculation on how to identify the IP address that was assigned for the Ulearn System Server based on the information given in the project scenario. Explain the concept and show the calculation to identify the IP address.


192.168.1.0/24: Ranges between 192.168.1.0 and 192.168.1.255 with 254 possible hosts

    The number following the IP address is the abbreviation for the subnet mask, and the number of ones in the subnet mask when expressed in the binary notation is equal to the number of the abbreviation.

For example, the /16 subnet would have 16 ones in a row, and the rest of the numbers being zeros: 11111111.11111111.11111111.00000000, which equals to 255.255.255.0 when converted from binary into decimal.

    Subnet is a division of large IP Networks into multiple Small Logical Networks. Subnet mask is a 32-bit number used to identify the Network portion and the Host portion in the IP Address. There are 2 types of Subnetting, FLSM (Fixed Length Subnet Mask) and VLSM (Variable Length Subnet Mask). In our project we are using FLSM to calculate the subnetting based on the number of the faculties.


FLSM

  • Dividing an IP Network with the same or equal size.
  • The Subnet remains within its class Boundary
  • Formula 2^n >= N (Requirements)
  • Binary to Decimals

Group Size

128

64

32

16

8

4

2

1

Subnet

128

192

224

240

248

252

254

255

CIDR

/25

/26

/27

/28

/29

/30

/31

/32

**Each CIDR increment doubles the number of networks


Private addressing scheme used in UTeM network :

  • 192.168.1.0/24


The number of faculties:

  • 8 faculties = 8 subnets

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STEP 1


How many sub-networks could I need to create for 8 IP addresses starting with a /24?

Taking into consideration that there are 8 /27 networks inside a  /24.

2^n = IP addresses

n

1

2

3

4

5

6

7

8

9

IP Addresses

2

4

8

16

32

64

128

256

512


/24

/25

/26

/27

x2



x4


x8

From the table above, /27 - /24 = 3

    To calculate the number of possible subnets, use the formula 2^n, where n equals the number of host bits borrowed. If three host bits are borrowed, then n=3. 2^3=8, so eight subnets are possible if three host bits are borrowed.


STEP 2


Possible Number of Hosts per Subnets

To calculate the number of possible hosts per subnet, use the formula 2^n - 2, where h equals the number of host bits. The reason two addresses must be subtracted is because of the network address and the broadcast address.

Determine the number of remaining host bits based on the class of address and the number of host bits borrowed.

Given IP Address Space

C

Number of Host Bits Available

8

Number of Host Bits Borrowed

3

Number of Host Bits Remaining (n)

8 - 3 = 5

Number of Possible Hosts

2^5 = 32


32-2 =30

New Subnet Mask

255.255.255.224

New Prefix

/27



How 224 is discovered:

255.255.255.224

Base

2^7

2^6

2^5

2^4

2^3

2^2

2^1

2^0

Place Value

128

64

32

16

8

4

2

1

Binary Value

1

1

1

0

0

0

0

0

Meaning

Network Bit

Network Bit

Network Bit

Host Bit

Host Bit

Host Bit

Host Bit

Host Bit


255

255

255

x

x

x

192

168

0

2^7

2^6

2^5

-

-

-

128

64

32

From the tables above, Total = 128 + 64 + 32 = 224



STEP 3

Before proceeding to the fourth step of the subnetting process, we must calculate the decimal and prefix values of the new subnet mask.


Assigned IP Address Space

192.168.0.0

Class of Assigned IP Address Space

C

Default Subnet Mask (Dotted Decimal Format)

255.255.255.0

Default Subnet Mask in Binary

11111111.11111111.11111111.11111111.00000000

Default Subnet Mask (Prefix Format)

/24

Number of Host Bits Borrowed

3

New Subnet Mask (Dotted Decimal Format)

255.255.255.224

New Subnet Mask in Binary

11111111.11111111.11111111.11100000

New Subnet Mask (Prefix Format)

/27


Subnet Number

Network ID

Host ID Range

Broadcast Address

0

192.168.1.0/27

192.168.1.1/27 to 192.168.1.30/27

192.168.1.31/27

1

192.168.1.32/27

192.168.1.32/27 to 192.168.1.62/27

192.168.1.63/27

2

192.168.1.64/27

192.168.1.65/27 to 192.168.1.94/27

192.168.1.95/27

3

192.168.1.96/27

192.168.1.97/27 to 192.168.1.126/27

192.168.1.127/27

4

192.168.1.128/27

192.168.1.129/27 to 192.168.1.158/27

192.168.1.159/27

5

192.168.1.160/27

192.168.1.161/27 to 192.168.1.190/27

192.168.1.191/27


Number of Possible Hosts : 2^5 = 32

                                            32 - 2 = 30


The 5th available address as the address of the ULearn System Server is 192.168.1.165, which is also the IP address of the server. We do not start from 192.168.1.161 since it will be assigned to the router IP address as we need to identify the 5th accessible address. So, it will begin at 192.168.1.162 and so on.

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