Quick Subnetting NOTE
Given:
192.168.1.0/24
Need 30 usable hosts per subnet.
Step 1 — One Rule
Memorize this host table:
- 2 hosts → /30
- 6 hosts → /29
- 14 hosts → /28
- 30 hosts → /27
- 62 hosts → /26
- 126 hosts → /25
You don’t calculate this every time. You know it.
Since they want 30 usable hosts, you immediately know:
30 usable → /27
Done. No math needed.
Step 2 — How Many Subnets?
You started with /24.
Now you’re using /27.
Difference:
27 − 24 = 3 bits borrowed
2³ = 8 subnets
Again — mental math.
Step 3 — Block Size Trick (Fastest Way)
To find subnet increments, do this:
Look at the last octet of the mask.
For /27, mask is 255.255.255.224
Block size =
256 − 224 = 32
So subnets increment by 32.
That’s it.
Step 4 — Write Subnets in 10 Seconds
Just count by 32:
0
32
64
96
128
160
192
224
You already know each subnet spans 32 addresses.
So:
Subnet = start value
Broadcast = start + 31
Next subnet = start + 32
Example:
192.168.1.0
Broadcast = 0 + 31 = 31
Usable = 1–30
Next:
192.168.1.32
Broadcast = 32 + 31 = 63
Usable = 33–62
And repeat.
No formulas. Just counting.
Why This Works
Because every subnet size is a power of 2.
32 addresses per subnet
→ 30 usable
→ 1 network
→ 1 broadcast
Practice Question:
192.168.10.0/24
Need 62 usable hosts
Answer these immediately:
- What prefix?
- Block size?
- How many subnets?