MS Self-paced 70-291 query...

Squirrel23

I was finishing Chapter 2 and going through the exercises, but I can't figure out how to spot the configuartion error on questions 2 and 3 on pages 2-54 and 2-55. I looked up the answer, but they don't tell you how they got it, or how I SHOULD have spotted it!!

Any help would be greatly appreciated!! Cheers.

tawnos

Questions of that sort are tedious and the only way i can find to answer them is to painstakingly check each facet. For each router, figure out what subnet each interface corresponds to (expand the CIDR notation for each into a readable range). Then compare each host attached to that interface and make sure the host's IP is within the range. After that, check to make sure the gateway for each host is configured to the interface to which it is physically attached.

For those self-paced book questions, #3 is pretty straight forward. The IP falls just outside of the range of the network the host is on, which seems to be a pretty common trick they like to pull. This is why i've had to get out of the habit of "eyeballing it" for these kinds of questions, since i might erroneously look at 131.107.120.1/21 and 131.107.128.1/21 and think it looks fine. If you expand the notation for all of them (which is definitely tedious) its much harder to miss that the IP is outside of 131.107.120.0-131.107.127.255.

I would say the answer to #2 isn't the only answer one could give. The 2 subnets overlap, and their solution of changing the masks to /28 is just the most elegant way of fixing it. You could also just change the addresses of all of the hosts and the router interface of the higher numbers network by adding 16 to the last octet and keep the masks the same. On a real test question like that, there would be multiple choice answers to select from, rather than having to guess which of the possible solutions to the question they were looking for.

Squirrel23

Thank you! Most helpful!!

I really appreciate you taking the time to look into this. I don't think MS's approach of just giving the answer really helped me understand the best method for tackling the scenario!

Cheers!