Subnetting a Class B Network Using Binary


You have a Class B 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 2 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).

0001HHHHHHHHHHHH

 

0001000000000000

Subnet number

0001000000000001

First valid host

.

 

.

 

.

 

0001111111111110

Last valid host

0001111111111111

Broadcast number


Step 3.

Convert binary to decimal (replacing 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 4.

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

0010HHHHHHHHHHHH

 

0010000000000000

Subnet number

0010000000000001

First valid host

.

 

.

 

.

 

0010111111111110

Last valid host

0010111111111111

Broadcast number


Step 5.

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

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 6.

Create IP plan table.

Valid Subnet

Network Number

Range of Valid Hosts

Broadcast Number

1

16.0

16.131.254

31.255

2

32.0

32.147.254

47.255

3

48.0

48.163.254

63.255


Notice a pattern? Counting by 16.

Step 7.

Verify pattern in binary (third valid subnet in binary used here).

0011HHHHHHHHHHHH

Third valid subnet

00110000.00000000 = 48.0

Subnetwork number

00110000.00000001 = 48.1

First valid host number

.

 

.

 

.

 

00111111.11111110 = 63.254

Last valid host number

00111111.11111111 = 63.255

Broadcast number


Step 8.

Finish IP plan table.

Subnet

Network Address
(0000)

Range of Valid Hosts
(00011110)

Broadcast Address
(1111)

0 (0000) invalid

172.16.0.0

172.16.0.1172.16.15.254

172.16.15.255

1 (0001)

172.16.16.0

172.16.16.1172.16.31.254

172.16.31.255

2 (0010)

172.16.32.0

172.16.32.1172.16.47.254

172.16.47.255

3 (0011)

172.16.48.0

172.16.48.1172.16.63.254

172.16.63.255

4 (0100)

172.16.64.0

172.16.64.1172.16.79.254

172.16.79.255

5 (0101)

172.16.80.0

172.16.80.1172.16.95.254

172.16.95.255

6 (0110)

172.16.96.0

172.16.96.1172.16.111.254

172.16.111.255

7 (0111)

172.16.112.0

172.16.112.1172.16.127.254

172.16.127.255

8 (1000)

172.16.128.0

172.16.128.1172.16.143.254

172.16.143.255

9 (1001)

172.16.144.0

172.16.144.1172.16.159.254

172.16.159.255

10 (1010)

172.16.160.0

172.16.160.1172.16.175.254

172.16.175.255

11 (1011)

172.16.176.0

172.16.176.1172.16.191.254

172.16.191.255

12 (1100)

172.16.192.0

172.16.192.1172.16.207.254

172.16.207.255

13 (1101)

172.16.208.0

172.16.208.1172.16.223.254

172.16.223.255

14 (1110)

172.16.224.0

172.16.224.1172.16.239.254

172.16.239.255

15 (1111) invalid

172.16.240.0

172.16.240.1172.16.255.254

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 subnetsthe 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





CCNA Self-Study(c) CCNA Portable Command Guide
CCNA Portable Command Guide
ISBN: 1587201585
EAN: 2147483647
Year: 2006
Pages: 261
Authors: Scott Empson

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