Exam 300-420 topic 1 question 173 discussion

Actual exam question from Cisco's 300-420

Question #: 173
Topic #: 1

An engineer is designing an IPv4 addressing plan for an enterprise with 1000 branches. Each branch requires a prefix for data and a prefix for voice. Each prefix must accommodate up to 128 hosts, and prefixes must facilitate summarization at aggregation points in the network. The security team requires a simple method for identifying voce prefixes. Which allocation does the engineer recommend from the RFC1918 address space?

A. /24 prefixes for data from 10.0.0.0/15 and /24 prefixes for voice from 172.16.0.0/15
B. /24 prefixes for data from 10.0.0.0/8 and /24 prefixes for voice from the next contiguous /24 prefix per site
C. /25 prefixes for data from 10.0.0.0/8 end /25 prefixes for voice from the next contiguous /25 prefix per branch
D. /24 prefixes for data from 10.0.0.0/8 and /24 prefixes for voice from 172.16.0.0/12

Show Suggested Answer

Suggested Answer: B 🗳️

by Reinier_veen at Oct. 17, 2022, 2:31 p.m.

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andrewChan

Highly Voted 1 year, 9 months ago

Selected Answer: B

B is correct For example: Site 0001 Data:10.0.0.0/24 Voice: 10.0.1.0/24 summary route : 10.0.0.0/23 Site 0002 Data:10.0.2.0/24 Voice: 10.0.3.0/24 summary route: 10.0.2.0/23 ....cont... site 0129 Data:10.1.0.0/24 Voice: 10.1.1.0/24 summary route: 10.1.0.0/23 site 0130 Data:10.1.2.0/24 Voice: 10.1.3.0/24 summary route: 10.1.2.0/23 so 3rd octet is odd number assigned to voice, and even number assigned to data; for security team to recongize voice prefix, use an ACL with wildcast to filter odd number on third octet, started from 10.0.1.0 0.0.254.255, 10.1.1.0 0.0.254.255......, 10.1.1.0 0.0.254.255 etc; for 10.0.1.0 0.0.254.255, any IP in binary that started with 00001010.00000000.xxxxxxx1.xxxxxxxx will be matched (x = either 0 or 1), covert 3rd octet into dec, for example, 10000001 = 129 which is a voice VLAN.

upvoted 6 times

andrewChan

1 year, 9 months ago

BTW, when the question ask 128/64/32/16/8/4 hosts in a subnet, this type of magic number always misleading people to pick a wrong answer..... cos the first and last address (network address and broadcast address) cannot be used, and also you have to assign at least a deafult gateway that question may not describe....

upvoted 3 times

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Sickcnt

Most Recent 1 year, 9 months ago

How is this possible? If we got with Option D -> Then the prefixes won't be able to be summarized at aggregation points. But if we go with Option B -> Security won't have a simple method to "Identify voice packets" Any tipps lads? :\

upvoted 1 times

Sickcnt

1 year, 9 months ago

Maybe put ODD /24 sites for data and EVEN /24 sites for Voice or something? Could they mean something like that?

upvoted 1 times

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Eards

1 year, 9 months ago

128 hosts will not fit a /25. Broadcast and network addresses mean >128 so has to be a /24. Cisco sneakiness...

upvoted 1 times

Eards

1 year, 9 months ago

And Gateway address

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Reinier_veen

1 year, 9 months ago

any idea's why B and not C? /25 seems logical for maximum of 128 hosts, and gives a easy way to identufy voice from data. (<128 = data ; > 128=voice). Or do I have to read something in the difference between "sites" and "branches"? Any ideas?

upvoted 1 times

mgiuseppe86

10 months ago

If you are asking this question on a frigging CCNP ENSLD Question you need to back to learning CCNA or Network+. Come on man, a /25 does not get you 128 hosts. Still need 1 for broadcast, 1 for network, and 1 for gateway.

upvoted 2 times

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leo_591

1 year, 9 months ago

1000 branches -> 2exp(10)=1024 branches, need 10bits 128 host -> 2exp(8)-2 =254 host, need 8 bits

upvoted 1 times

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