Which subnet is host 10.20.1.23 255.240.0.0 in?

We have taken four bits (240 in binary is 11110000 or 128+64+32+16) to make subnets from since class A addresses normally have the default eight bits (255.0.0.0 is 11111111.0000000.0000000.00000000 in binary).

1.  How many subnets?

2^4= 16

2.  How many hosts per subnet?

We have 20 bits left for hosts so:

2^20-2= 1048574 hosts per subnet

3.  What are the valid subnets?

256-240 = 16 (the increment).  Our subnets go up in increments of 16.

10.0.0.0 (the zero subnet)

10.16.0.0*

10.32.0.0

10.48.0.0   ….all the way up to

10.224.0.0

10.240.0.0

4.  What are the valid hosts per subnets?

10.0.0.0 Hosts 10.0.0.1 to 10.15.255.254

10.16.0.0  Hosts 10.16.0.1 to 10.31.255.254*   10.20.1.23 is in this subnet

10.32.0.0  Hosts 10.32.0.1 to 10.47.255.254

10.48.0.0  Hosts 10.48.0.1 to 10.63.255.254

10.64.0.0 and so on

10.224.0.0  Hosts 10.224.0.1 to 10.239.255.254

10.240.0.1 Hosts 10.240.0.1 to 10.255.255.254

5.  What are the broadcast addresses?

This is the last address before each subnet.

10.15.255.254 is the last host on the first subnet

10.15.255.255 is the broadcast address

10.16.0.0 is the next subnet

10.16.0.1 is the first host on the next subnet

10.31.255.254 is the last host

10.31.255.255 is the broadcast address

10.32.0.0 is the next subnet

10.47.255.255 broadcast the third subnet

10.48.0.0 next subnet etc.

You will learn how to shorten this step down shortly.

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