Kindly i need an explanation

[Amr Ali]

Dear Sirs,

Kindly , i need to understand this question but i can't get this explanation at all , so i will be glad if you assist me

Refer to the exhibit. Which three statements correctly describe Network Device A? (Choose three.)

A.  With a network wide mask of 255.255.255.128, each interface does not require an IP address.

B.  With a network wide mask of 255.255.255.128, each interface does require an IP address on a unique IP subnet.

C.  With a network wide mask of 255.255.255.0, must be a Layer 2 device for the PCs to communicate with each other.

D.  With a network wide mask of 255.255.255.0, must be a Layer 3 device for the PCs to communicate with each other.

E.  With a network wide mask of 255.255.254.0, each interface does not require an IP address.

Answer: BDE

Explanation:

If Subnet Mask is 255.255.255.128 the hosts vary from x.x.x.0 -  x.x.x.127 & x.x.x.128- x.x.x.255,so the IP Addresses of 2 hosts fall  in different subnets so each interface needs an IP an address so that  they can communicate each other.

If Subnet Mask is 255.255.255.0 the 2 specified hosts fall in differnt subnets so they need a Layer 3 device to commicate.

If Subnet Mask is 255.255.254.0 the 2 specfied hosts are in same subnet  so are in network address and can be accomidated in same Layer 2 domain  and can communicate with each other directly using the Layer 2 adddress.

Thanks for your time - amr

[tony.henry_2]

Amr,

Whats troubling you with the answers? B and D are a bit tricky in that they both have the same problem. That is that in order to communicate it would have to pass through a layer 3 device.

IMO it's not a really good question.

Tony

[riyaaz000 · Accepted solution]

Hi amr

The question says what kind of device the ' Network Device A' can be based on the correct answers. This question is testing your knowledge on subnetting, how routers basicly works, and how switches basicly works.

So, based on only the IP address, they can be on the same subnet or they may not be on the same subnet. To break the tie,we need to know what is the subnet mask.

Lets analyse each statement:

A - With a network wide mask of 255.255.255.128. Based on this subnet mask, the first 3 octet of the both IP addresses should be the same, but they are clearly not the same. each interface does not require an IP address, indeed you will need a router, because the IP addresses are in two different network. furthermore, in a router, each interface should have an IP address on a unique IP subnet

B - With a network wide mask of 255.255.255.128, each interface does require an IP address on a unique IP subnet. This is correct, at that means 'Network Device A' would be a router as explained above.

C - With a network wide mask of 255.255.255.0, must be a Layer 2 device for the PCs to communicate with each other.

Still refering to explaination at A, a layer2 device (switch) cannot do routing. Furthermore, a switch cares only where hosts are within the same network, whereas a router cares only where networks are. So this answer is incorrect.

D - With a network wide mask of 255.255.255.0, must be a Layer 3 device for the PCs to communicate with each other.

This is correct, as explained above. 'Network Device A' should be a router because as per this subnet mask, both PC are in diferrent network.

E - With a network wide mask of 255.255.254.0, each interface does not require an IP address. This one test your subnetting. based on this subnet mask the IP address range would be 10.1.0.0 through 10.1.3.255. Meaning that both PC are on the same subnet, thus a switch is needed, and on a switch, we dont assign IP address on their ports.

Hope that helps

Regards

Riyaaz

[Amr Ali]

Thanks Mr. Riyaaz for your clarification , but can you explain point (E) again , as i can't get what's the relation between this mask 255.255.254.0 , and these IPs 10.1.0.0 , 10.1.3.255 , and where you got these IPs

Thanks once again

[tony.henry_2]

Amr,

It would be 1.255.

255.255.254.0 subnet mask

network.-----------------------------| Host

11111111.11111111.11111110.00000000

00001010.00000001.00000000.00000000

10.1.0.0

the end of the subnet would be

00001010.00000001.00000001.11111111

10.1.1.255

The next subnet would run from 2.0 through to 3.255

Tony

[Amr Ali]

Thanks for replying , can you suggest me a good ccna book which i can study from it

[tony.henry_2]

Amr,

Try this

http://www.learntosubnet.com/

Tony

[riyaaz000]

Hi amr,

tony is right, i made typo error.

Read CCNA study guide by Todd Lammle, I passed my CCNA with it.

Regards

Riyaaz

[Amr Ali]

Hello Riyaaz

Kindly can you give me a link & i need to know which is preferred , studying this book or studying the pass4sure ?

Warmest Regards - amr

[riyaaz000]

Hi,

I bought the book on ebay. I would say better read books, understand the fundamental of networking, know how it works.

Regards

Riyaaz

[asdvads vwe]

Due to "E" section. Just want to mention that the Subnet mask 255.255.254.0 = /23. IP addresses range within the first network address which is 10.1.0.0/23 will be 10.1.0.1 up to 10.1.1.254. The 10.1.3.255 it's the broadcast address of the next subnet which is 10.1.2.0/23 with the available host range of 10.1.2.1 up to 10.1.2.254. So if PC on the right was in the second subnet of 10.1.2.0/23 it will the answer that "With a network wide mask of 255.255.254.0, each interface does not require an IP address", would be false, because again the PC on the right would be in another subnet than PC on the left, so in this case we would need a Router to do its routing job between different networks.