Subnetting Questions 30 | Calculate the Subnet Mask from IP Address

Subnetting Questions 30

What subnet mask would you use for the 172.28.0.0 network, such that you can get 280 subnets and 70 hosts per subnet?

Hence, the IP Address starts from 172 it indicates it is a Class B Address. By default, in Class B Address 16 bits are reserved for the network and 16 bits are reserved for the host.

A/Q we need 280 Subnets with minimum 70 host on each.

Therefore, we have to borrow 9 bits from the third and fourth octet 2⁹ = 512 Subnets

11111111 . 11111111 . 11111111 . 10000000

(2⁷ + 2⁶ + 2⁵ + 2⁴ + 2³ + 2² + 2¹ + 2⁰) . (2⁷ + 2⁶ + 2⁵ + 2⁴ + 2³ + 2² + 2¹ + 2⁰) . (2⁷ + 2⁶ + 2⁵ + 2⁴ + 2³ + 2² + 2¹ + 2⁰) . 2⁷

So, the subnet mask is 255.255.252.128

Additional Information

Let’s count the number of host on each network

2ⁿ – 2 (n is the number of host bits, subsctrated 2 because on every network 1 bit is reserved for the Network ID and 1 bit is reserved for the broadcast ID)

= 2⁷ – 2

= 128 – 2

= 126 host is available on each network