Need explanation from a cisco.com practice test question....

zero-g-smith

Ok I cant copy the graphic but here is the question.

Using the most efficent addressing scheme and VLSM, which address can be configured on one of the serial interfaces.

Ok the graphic is of 2 routers connected by a serial cable, they both have a ethernet network attached via E0


RouterA E0 =192.168.16.64 /27

RouterB E0 =192.168.16.96 /27
RouterB E0/1 =192.168.16.128 /27

Ok my answer choices are as follows:

A. 192.168.16.63 /27
B 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


Now I know that using VLSM it is advisable by cisco to use /30 since your not wasting network numbers on unusable hosts.

I assumed that a couple of the /30 answers would either be subnet numbers or broadcast numbers but all 3 /30 are legit host numbers. I choose D. figuring that it was the smallest subnet number out of the choices. but they said the correct answer is F.

Why would this be?
Thanks

dmaftei

You should chose a /30 address to satisfy the "most efficient" requirement. Of the three /30 addresses, D and E fall within subnets already configured: D is in 192.168.16.96/27, E is in 192.168.16.128/27. That leaves you with F.

zero-g-smith

ok I got ya. For some reason I forgot about the same subnets. I was concentrated on the mask.

Danman32

Yup, you can't use an IP set that's already allocated. Not much different than memory mapping when doing low level programming or system design