Serial interface addresing

Mikdilly

From the cisco prep practice exams a question says use the most efficient addressing scheme and vlsm to configure a serial connection between two routers, which address can be configured on one of the serial interfaces?

A) 192.168.16.63/27
192.168.16.158/27
C) 192.168.16.192/27
D) 192.168.16.113/30
E) 192.168.16.145/30
F) 192.168.16.193/30

They say F is correct, why wouldn't D and E be correct as well?

Netstudent

good question.
Looks to me like 145 is in the 144 subnet.

r_durant

Hmmmmm...good question!! From what I calculated, D, E & F seem to be valid addresses in the scheme of the question asked...

Paul Boz

Idiologically speaking, you want to reserve /30 masks for point to point links, as they only afford two usable ip addresses, which means absolutely no waste with your IP allocation. That should answer r_durant's question. [edit] r_durant edited his reply. For the sake of anyone that doesn't know this, he asked what the point is in using a /30 over the other choices, since the others would technically work.

As far as the original poster:

A) 192.168.16.63/27
192.168.16.158/27
C) 192.168.16.192/27
D) 192.168.16.113/30
E) 192.168.16.145/30
F) 192.168.16.193/30

A, B, and C are instantly out because using wasteful addressing space is against Cisco best practice (and common sense).

Between D, E, and F,

D is correct because 192.168.16.113 is a part of the 112 subnet. It breaks down like this:

192.168.16.112 is the subnet
192.168.16.113-114 are the valid usable host addresses
192.168.16.115 is the broadcast address
192.168.16.116 is the next subnet.

E is correct because 192.168.16.145 is a part of the 144 subnet. It breaks down like this:

192.168.16.144 is the subnet
192.168.16.145-146 are the valid usable host addresses
192.168.16.147 is the broadcast address
192.168.16.148 is the next subnet.

F is correct because 192.168.16.193 is a part of the 192 subnet. It breaks down like this:

192.168.16.192 is the subnet
192.168.16.193-194 are the valid usable host addresses
192.168.16.195 is the broadcast address
192.168.16.196 is the next subnet

r_durant

Paul Boz wrote:

That should answer r_durant's question. [edit] r_durant edited his reply. For the sake of anyone that doesn't know this, he asked what the point is in using a /30 over the other choices, since the others would technically work.

I did??

I agreed with D, E & F...

Mikdilly

It'd be helpful if they could give an explanation for their answers within the practice exam. Was thinking of submitting this question to them, would i get an answer?

Pash

I totally agree Mikdilly, questions should always be backed up with explanations. Ever done a google search looking for a topic that is quiet hard to find specific information on...you find a post by some chap somewhere who has EXACTLY the same issue/problem you had....you follow his thread down to the end where he kindly puts.....got it working, thanks everyone for your help......no explanation on how. Yup, i bet most have.

So please, always always post an explanation to a an issue or problem, just like paul did for us. Back on topic.....yes that question is wrong and yup you probably should let them know.

Paul Boz

r_durant wrote:

Paul Boz wrote:

That should answer r_durant's question. [edit] r_durant edited his reply. For the sake of anyone that doesn't know this, he asked what the point is in using a /30 over the other choices, since the others would technically work.

I did??

I agreed with D, E & F...

Hmm, could have sworn you did. Oh well, at any rate it's still good info

Mikdilly

Sorry, my mistake, there was a graphic on the ques that showed the 2 routers with subnets of 192.168.16.64/27, 192.168.16.96/27, and 192.168.16.128/27. Answers D and E would fall into those ranges. I thought since they used a diff mask they wouldn't apply.

Netstudent

ya be very careful of overlaps when using VLSM and manual route summarization.