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Exam SY0-501 topic 1 question 172 discussion

Actual exam question from CompTIA's SY0-501
Question #: 172
Topic #: 1
[All SY0-501 Questions]

A security administrator is creating a subnet on one of the corporate firewall interfaces to use as a DMZ which is expected to accommodate at most 14 physical hosts.
Which of the following subnets would BEST meet the requirements?

  • A. 192.168.0.16 255.25.255.248
  • B. 192.168.0.16/28
  • C. 192.168.1.50 255.255.25.240
  • D. 192.168.2.32/27
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Suggested Answer: B 🗳️

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Bartin8tor
Highly Voted 4 years, 10 months ago
14 is 1110 in binary, so you need 4 bits of the ip-adres to indicate the host, so you have 32-4 bits left for the subnetmask => /28
upvoted 15 times
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exiledwl
Highly Voted 3 years, 11 months ago
How is this sec+???
upvoted 14 times
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mxh778872
Most Recent 3 years ago
why not: C. 192.168.1.50 255.255.25.240
upvoted 1 times
slackbot
1 day, 16 hours ago
because 192.168.1.50 is not a network address in this case, you need the network address
upvoted 1 times
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Cindan
3 years, 9 months ago
2^subnetmask-2
upvoted 1 times
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Mohawk
3 years, 9 months ago
i hate subnetting. I can never figure it out!!
upvoted 2 times
Dedutch
3 years, 9 months ago
Hey. This is my dumb head way. /24 is a 255.255.255.0 subnet so thats 256 addresses. If you make the / smaller the subnet gets more addresses and the mask gets smaller. Use this as your base, and just adjust from /24. Also remember the available addresses is always 2 lower than the total addresses. a /23 would double the addresses to 512 and the subnet mask would become 255.255.254.0 a /22 would double the addresses available again to 1024 and the mask would get 1 smaller, 255.255.253.0. On the flip side a /25 is going to halve the addresses from 256 to 128. Figuring out the subnet for /## greather than /24 you instead take 256-[addresses]. So a /25 would be 255.255.255.[256-128) A /26 would be 256/2/2 so 64 addresses available, and the mask would be 255.255.255.[256-64). You can always look up tables, or figure out the binary math... but for doing it in my head i just revert back to a /24 and adjust.
upvoted 4 times
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AlexChen011
3 years, 9 months ago
This is CCNA knowledge, /28 = 2^4=16 IPs
upvoted 1 times
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Jaak
4 years, 2 months ago
http://www.subnet-calculator.com/
upvoted 1 times
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dieglhix
4 years, 2 months ago
32 - 28 = 4 4 to square root = 16 -2 =14
upvoted 2 times
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dinosan
4 years, 4 months ago
This is how to quickly count it on your fingers. 1 - 2 - 4 - 8 - 16 hosts (five fingers). subnet backward from 32 - 31 - 30 - 29 - 28 (five fingers).
upvoted 6 times
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Mara03
4 years, 4 months ago
The answer is D. You have at most 14 devices in the subnet plus the firewall port itself, plus Broadcast and NetID makes 17.
upvoted 1 times
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abdulmian
4 years, 4 months ago
IP Address / Mask 192.168.0.16 / 28 Results Address: 192.168.0.16 11000000.10101000.00000000.00010000 Netmask: 255.255.255.240 11111111.11111111.11111111.11110000 Wildcard: 0.0.0.15 00000000.00000000.00000000.00001111 Network Address: 192.168.0.16 / 28 11000000.10101000.00000000.00010000 Broadcast Address: 192.168.0.31 11000000.10101000.00000000.00011111 First host: 192.168.0.17 11000000.10101000.00000000.00010001 Last host: 192.168.0.30 11000000.10101000.00000000.00011110 Total host count: 14
upvoted 1 times
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MagicianRecon
4 years, 5 months ago
32-28=4 2 to the power 4 is 16. 16-2 = 14 atmost hosts
upvoted 5 times
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kdce
4 years, 5 months ago
B - 192.168.0.16/28
upvoted 1 times
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SINGINGWITHME
4 years, 7 months ago
https://www.youtube.com/watch?v=ZxAwQB8TZsM start at 6:35 it will break down how to solve this in the first chart
upvoted 2 times
SINGINGWITHME
4 years, 6 months ago
Actually, this is what helped me more so than anythign ive seen. 1. Subtract number after the slash by 32 (ex. 32-28=4) 2. Now take 2 raised to the power of that answer (2^4 =16) 3. Now subtract that answer from 2 (16-2=14) I have no prior BG in IT whatsoever and I learned this recently
upvoted 22 times
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Stefanvangent
5 years, 3 months ago
IPv4 Subnet Network 192.168.0.16/28 (Class C) Netmask 255.255.255.240 Specified Network Host Address Range 192.168.0.17-192.168.0.30 (14 hosts) Broadcast 192.168.0.31
upvoted 9 times
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mrlee
5 years, 3 months ago
http://www.exampointers.com/ccna/sub.php
upvoted 3 times
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