Networking
Exam drill
Find the network address in 20 seconds
This network address question costs students marks every year
Quick check · tap an answer to continue
Lesson notes
Key points
- A /20 mask is 255.255.240.0, so the interesting octet is the third.
- Block size is 256 minus 240, which gives 16.
- Multiples of 16 in the third octet: 32, 48; 45 falls in the 32 block.
- So the network address is 172.16.32.0, with the fourth octet set to zero.
Takeaway: Find the interesting octet first, then step through block-size multiples.
Quick check: A host has IP 172.16.45.200/20. What is its network address?
- 172.16.32.0
- 172.16.45.0
- 172.16.40.0
Show the mask, block size and the block containing 45 to earn full method marks.
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