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I Was looking in the previous year exam papers for a subject called "Networks", i have exam next month , anyway i found this question that made me confused :

Given the network address 117.223.0.0 and the number of subnets Required is : 16

a. What would be the suitable Subnet Mask ?

b. What is the 1st Subnet Address ?

c. What is the 3rd Subnet Address ?

d. What is the Subnet broadcast address for the before last Subnet ?

this is a Class A ip , and the number in the second octet is 223, they didn't mention if they used the default subnet mask , so if i want to subnet such ip would i start from 117.0.0.0 and forget about the second octet"223" ?! or what ? in other way , should i consider the subnet mask 255.0.0.0 which is the default for Class A , and take 4 bits for the networking subnetting requirements and start from 117.0.0.0 up to 16 network from that address or the second octet should be taken in consideration ?

My answers for the above question are :

a. 255.240.0.0

b. 117.0.0.0

c. 117.32.0.0

d. 117.239.255.255

e. From 117.240.0.1 To 117.255.255.254

Please Help me to understand how to solve such question in case i get any in my exam .

Kind regards

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  • If the course was any good, they should have told you about CIDR and VLSM, and that classfull networking doesn't have any meaning in modern networks anymore.
    – mtak
    May 12, 2017 at 22:41
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    IMO the question is incomplete because it tells you nothing about the amount of addresses allocated to you. Classless Inter Domain Routing makes all of the class A assumptions irrelevant.
    – BrianC
    May 12, 2017 at 22:42
  • You cant subdivide a single address - you can only subdivide a range of addresses. However the classful network which includes the IP you quote - is the class A 117.0.0.0 not 117.240.0.0 so the question is missing something most likely a CIDR. As MTAK said - no one uses Classful anymore
    – Ross
    May 12, 2017 at 22:48
  • The problem isn't CIDR (which affects inter-domain routing) or VLSM (which affects subnet size), it's that you don't know the network mask. May 12, 2017 at 22:49

1 Answer 1

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They gave you a starting network address that uses at least the first 16 bits. So although it’s slightly ambiguous, they almost certainly want you to subnet this /16. To put that in 1980’s classfull networking terms, “treat it like a class B”.

a. You need 4 additional subnet bits to create 16 sinners, so your mask is a /20: 255.255.240.0.

b. First subnet network address is 117.223.0.0/20.

c. Third subnet network address is 117.223.32.0/20.

d. Next-to-last subnet broadcast address is 117.223.239.255.

e. (You never gave us the question for part e.)

Basically, it looks like you knew how to do it right, except you based it on a /8 instead of a /16. The question was poorly worded though if they didn’t specify that it was a /16 that you were starting with. (Thought experiment: What if it was just the first subnet of a /17?)

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