Subnetting a Class B Network Using Binary

Class B Network Subnetting Using Binary
Andrey Volkov

Andrey Volkov

System, network administration + DBA. And a little programmer!)) See Author profile.

You have an address of 172.16.0.0 /16. You need nine subnets. What is the IP plan of network numbers, broadcast numbers, and valid host numbers? What is the subnet mask needed for this plan?

You cannot use N bits, only H bits. Therefore, ignore 172.16. These numbers cannot change.

Step 1. Determine how many H bits you need to borrow to create nine valid subnets.

2N ≥ 9

N = 4, so you need to borrow 4 H bits and turn them into N bits.

Start with 16 H bits

HHHHHHHHHHHHHHHH (Remove the decimal point for now)

Borrow 4 bits

NNNNHHHHHHHHHHHH

Step 2. Determine the first valid subnet in binary (without using decimal points).

0000HHHHHHHHHHHH

 

0000000000000000

Subnet number

0000000000000001

First valid host

 

0000111111111110

Last valid host

0000111111111111

Broadcast number

Step 3. Convert binary to decimal (replacing the decimal point in the binary numbers).

00000000.00000000 = 0.0

Subnetwork number

00000000.00000001 = 0.1

First valid host number

 

00001111.11111110 = 15.254

Last valid host number

00001111.11111111 = 15.255

Broadcast number

Step 4. Determine the second subnet in binary (without using decimal points).

0001HHHHHHHHHHHH

 

0001000000000000

Subnet number

0001000000000001

First valid host

 

0001111111111110

Last valid host

0001111111111111

Broadcast number

Step 5. Convert binary to decimal (returning the decimal point in the binary numbers).

00010000.00000000 = 16.0

Subnetwork number

00010000.00000001 = 16.1

First valid host number

 

00011111.11111110 = 31.254

Last valid host number

00011111.11111111 = 31.255

Broadcast number

Step 6. Create an IP plan table.

Subnet

Network Number

Range of Valid Hosts

Broadcast Number

1

0.0

0.1–15.254

15.255

2

16.0

16.1–31.254

31.255

3

32.0

32.1–47.254

47.255

Notice a pattern? Counting by 16.

Step 7. Verify the pattern in binary. (The third subnet in binary is used here.)

0010HHHHHHHHHHHH

Third valid subnet

00100000.00000000 = 32.0

Subnetwork number

00100000.00000001 = 32.1

First valid host number

 

00101111.11111110 = 47.254

Last valid host number

00101111.11111111 = 47.255

Broadcast number

Step 8. Finish the IP plan table.

Subnet

Network Address (0000)

Range of Valid Hosts (0001–1110)

Broadcast Address (1111)

1 (0000)

172.16.0.0

172.16.0.1–172.16.15.254

172.16.15.255

2 (0001)

172.16.16.0

172.16.16.1–172.16.31.254

172.16.31.255

3 (0010)

172.16.32.0

172.16.32.1–172.16.47.254

172.16.47.255

4 (0011)

172.16.48.0

172.16.48.1–172.16.63.254

172.16.63.255

5 (0100)

172.16.64.0

172.16.64.1–172.16.79.254

172.16.79.255

6 (0101)

172.16.80.0

172.16.80.1–172.16.95.254

172.16.95.255

7 (0110)

172.16.96.0

172.16.96.1–172.16.111.254

172.16.111.255

8 (0111)

172.16.112.0

172.16.112.1–172.16.127.254

172.16.127.255

9 (1000)

172.16.128.0

172.16.128.1–172.16.143.254

172.16.143.255

10 (1001)

172.16.144.0

172.16.144.1–172.16.159.254

172.16.159.255

11 (1010)

172.16.160.0

172.16.160.1–172.16.175.254

172.16.175.255

12 (1011)

172.16.176.0

172.16.176.1–172.16.191.254

172.16.191.255

13 (1100)

172.16.192.0

172.16.192.1–172.16.207.254

172.16.207.255

14 (1101)

172.16.208.0

172.16.208.1–172.16.223.254

172.16.223.255

15 (1110)

172.16.224.0

172.16.224.1–172.16.239.254

172.16.239.255

16 (1111)

172.16.240.0

172.16.240.1–172.16.255.254

172.16.255.255

Quick Check

Always in form even #.0

First valid host is always even #.1

Last valid host is always odd #.254

Always odd #.255

Use any nine subnets—the rest are for future growth.

Step 9. Calculate the subnet mask. The default subnet mask for a Class B network is as follows:

Decimal

Binary

255.255.0.0

11111111.11111111.00000000.00000000

1 = Network or subnetwork bit

0 = Host bit

You borrowed 4 bits; therefore, the new subnet mask is the following:

11111111.11111111.11110000.00000000

255.255.240.0

Read more: How to a Class C Network  Subnetting Using Binary

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