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Consider an enterprise network with two Ethernet segments, a web server and a firewall, connected via three routers as shown below.

What is the number of subnets inside the enterprise network?

  1. $3$
  2. $12$
  3. $6$
  4. $8$
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3 Answers

19 votes
19 votes

Answer C

For each interface, router has one entry in routing table. 

Each interface out of a router = 1 subnet

So, here 2 + 3 + 2 = 7 

but 1 is common (router to router) so 7 - 1 = 6

 

below image ref- https://www.cisco.com/c/en/us/support/docs/ip/routing-information-protocol-rip/13788-3.html

 

Also Please find screenshot from Kurose 

edited by

4 Comments

The function of a firewall is both to regulate and filter the data.
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@nitin95 Firewalls at the edge of the network usually have routing capabilities. In fact , firewalls are typically configured on routers itself, in general. (You can refer to CISCO routers). In this case, the firewall that we have is at at the edge of enterprise network, connecting it with Internet. If it doesn't have routing capability and say it receives a packet, how it will decide whether it has to forward the packet to router connecting the web server or  to the router connecting the first Ethernet segment.

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This was the most beautiful question in the paper, they tried to confuse student that 6 can be wrong by giving exactly 6 layer 2 (Datalink layer)devices (ethernet)and 3 layer 3(Network layer) devices so that student will confuse 6 as wrong and 3 as right answer bcoz up untill now we got used to question /answers like “network layer is common for a group of Datalink layer devices on 1 router”

refer this link for more clarity and similar question https://learningnetwork.cisco.com/s/question/0D53i00000KsuHACAZ/number-of-subnets-in-this-topology

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10 votes
10 votes
According to me answer should be 3 as routers performs a logical duvision based on ip and hence the each area connected to Ethernet would have one subnet each and upper portion constitute a one subnet

 

Thus the topmost router constitute a T point for subnet division which is 3

3 Comments

@Yash Singhal can we challenge this question as official answer key has given 6 as the answer, I too feel both should be accepted.

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Ya me too agree with you kindly share the updates.
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Every interface of device with layer 3 or more contitutes a network ,total 7 interfaces,considering common networks here answer is 6 only.

3 will be answer only when 3 interfaces on single layer 3 device or all layer 3 device have common 3 interface. this ia very beautiful question, 3 was given option to confuse student so that they will think 6 is wrong considering layer 2 in the Ethernet in the diagram. and 3 is right answer becoz network layer is common in one ethernet in the pic.

refer this link for similar question. https://learningnetwork.cisco.com/s/question/0D53i00000KsuHACAZ/number-of-subnets-in-this-topology

 

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3 votes
3 votes

Thanks @  for posting the reference

After reading the explanation from Kurose and Ross, finally arrived this answer 6. This is very nice and very fundamental question in computer networks where I didn’t know the subtle notion hidden inside this fundamental question and learnt that now.

1 comment

@felics moses 1 if you are counting subnet 1 which is between two routers then in this way pic 4.17 of book has 9 subnets. correct me if am wrong
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