# Subnetting Questions

Registered Users Posts: 1 ■□□□□□□□□□
OK...so I've been trying to study on subnetting so I've been doing practice questions myself. I had a friend check my answers and said it was wrong...can someone please help clarify why?

a) What is the smallest IP address accessible on this network?
b) What is the highest address accessible on this network?
c) /21 is shorthand for the subnet mask. What is the subnet on this network fully spelled out?

a) 10.0.1.0
b) 10.0.254.254
c) 255.255.248.0

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Can you explain why you need to use a 1 in the 3rd octet?
a) 10.0.1.0

10.9.0.1 - 10.9.7.254

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• Member Posts: 60 ■■□□□□□□□□
Let's analyze the IP address 10.9.5.50/21. This is just some random host number since, /21 = 255.255.248.0 or in binary form
11111111.11111111.11111000.00000000
Understand "Interesting octet" - This is the octet(a byte) containing the 'Least significant 1' in the subnet mask (written in binary). And that 'Least significant 1' will become your "Block size". Block size is the increment value for subnets.

In our case, "target number" a.k.a "Block size" is 8 corresponding to the subnet mask value or the Least significant '1' bit.

Hence the networks would become -
10.9.0.0 (first one starts with 0)
10.9.8.0 (added Block size to the Interesting Octet)
10.9.16.0 (added Block size to the Interesting Octet)
10.9.24.0 (added Block size to the Interesting Octet)
10.9.32.0 (added Block size to the Interesting Octet)
10.9.40.0 (added Block size to the Interesting Octet)
.
.
. (so on and so forth)
.
.
Total = 32 subnets (25 = 32)

Our IP address 10.9.5.50 lies in the first subnet.