# 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

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

(0000)

Range of Valid Hosts
(00011110)

(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.

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 Portable Command Guide
ISBN: 1587201585
EAN: 2147483647
Year: 2006
Pages: 261
Authors: Scott Empson