Minimizing wasted addresses

A network administrator needs to create a new network that has 14 computers and two router interfaces. What subnet mask will provide the appropriate number of IPv4 addresses for this network with minimal wasted addresses?

A. 255.255.255.128
B. 255.255.255.192
C. 255.255.255.224 (correct answer)
D. 255.255.255.240
E. 255.255.255.248
F. 255.255.255.252

The back of the book gives the following explanation to why answer C is correct:

255.255.255.224 provides the 16 addresses required. .224 will provide 30. .240 will provide only 14.

I'm thinking... HOLD ON!

My brain went like this:

Each router interfacae must be on a different subnet (I believe) so we borrow one host bit from our original /xx mask to obtain two subnets.

Assuming answer C is correct, 255.255.255.224 equates to /27 so when borrowing one host bit from /27 you obtain two /28 subnets.

Each /28 subnet has 4 host bits which leaves each subnet with 2^4 - 2 = 14 usable hosts. So, adding both subnets' usable host addresses, the total number of hosts is 28 (14 hosts on the router's first interface and 14 on its second interface).

This leads me to question the explanation in the back of the book that simply states 255.255.255.224 provides the 16 addresses required. To that, I would say "No, 255.255.255.224, when subnetted into two networks by borrowing one host bit provides a total of 28 hosts when adding usable hosts across both subnets."

Am I off track here or somehow overthinking this?

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Comments

Forsaken_GA

JJBladester wrote: »

Am I off track here or somehow overthinking this?

You're missing the fact that the router interfaces count as a host, they need addresses too. So you actually need 15 IP's per subnet, so the .240 mask doesn't cut it.

JJBladester

Forsaken_GA wrote: »

You're missing the fact that the router interfaces count as a host, they need addresses too. So you actually need 15 IP's per subnet, so the .240 mask doesn't cut it.

So I was right in realizing that each router interface requires a separate subnet (can't have two interfaces on the same network)?

To reiterate what you're saying above, a .240 mask gives 4 host bits, yielding 2^4-2 = 14 usable IPs (13 for actual computers and 1 for the router interface) for each subnet. This will not cover my need for 14 computers + 1 router interface = 15 total usable IPs.

If we used a .192 mask, we'd have 6 host bits, yielding 2^6-2 = 62 usable IPs (61 for actual computers and 1 for the router interface) for each subnet. This can't be the correct mask because it wastes IP addresses (we only need 14 computers to be addressed per subnet, not 61).

Thus, answer C.

I guess a big part of answering the question is realizing things like "Hmmm, I need two subnets here!"

Forsaken_GA

JJBladester wrote: »

So I was right in realizing that each router interface requires a separate subnet (can't have two interfaces on the same network)?

To reiterate what you're saying above, a .240 mask gives 4 host bits, yielding 2^4-2 = 14 usable IPs (13 for actual computers and 1 for the router interface) for each subnet. This will not cover my need for 14 computers + 1 router interface = 15 total usable IPs.

If we used a .192 mask, we'd have 6 host bits, yielding 2^6-2 = 62 usable IPs (61 for actual computers and 1 for the router interface) for each subnet. This can't be the correct mask because it wastes IP addresses (we only need 14 computers to be addressed per subnet, not 61).

Thus, answer C.

I guess a big part of answering the question is realizing things like "Hmmm, I need two subnets here!"

Yup, cisco is known for tricky wording. They word things in such a way that if you don't mentally catch yourself and understand what they're actually asking for, you'll assume they're asking for something else. And on multiple guess questions, they'll put in answers that play to those misinterpretations, so you'll make an assumption, see your assumption as one of the answer choices, go AHA! and move on. What you really want to do is go 'wait a minute....' and reread the question. Often you'll have a 'you sneaky bastards!' moment as you figure out where the trick is.

That's why it's important to use practice exams correctly - they highlight your weak areas. When you miss a question, it's not enough to know that you missed the question, and what the correct answer to that question is, it's important to know WHY the correct answer is the correct answer. Once you understand that, you've got the core concept down, and then it doesn't matter what they ask you, you'll have the knowledge to figure it out. It's important to do this the first time you take the practice exam - practice exams have always been a one-time use product for me. Once I see a question, I will remember the answer to it, so I need to figure out right then why I was wrong in order to get the best use out of it.

Forsaken_GA

JJBladester wrote: »

So I was right in realizing that each router interface requires a separate subnet (can't have two interfaces on the same network)?

And actually I should clarify this - The same router cannot have two interfaces in the same subnet. However, you can have two separate routers in the same subnet, and each will need an IP. So look at the way the question is worded, it says specifically two router interfaces, it doesn't specify that they're on the same router, so it's possible that they both need IP's.

However for this particular question, it's irrelevant, as long as you recognize at least one router needs an IP. With that one IP + the other 14 for the computers, the requirements cannot be met with the .240 mask, so it's invalid as an answer.