Another Subnetting Question

[ElijahBaley]

Just when I thought I was getting the hang of subnetting, I saw the question below - surely the IP Address and gateway addresses are class C, how can they use a class B subnet mask:


QUESTION:

Users complain that they cannot connect to server4, which was recently added to the network, you check server4's TCP/IP config:

IP Address 192.168.64.100
Subnet Mask 255.255.255.0
Default Gateway 192.168.79.1

You notice that the subnet mask is incorrect, which of the following mask values would be correct?

a. 255.255.252.0
b. 255.255.254.0
c. 255.255.248.0
d. 255.255.224.0

ANSWER:

The correct answer is d. with each of the other mask values. The host and gateway would be on different subnets, a mask of 255.255.224 puts all hosts between 192.168.64.1 and 192.168.95.254 on the same subnet.

Thanks for your comments,

EB

 

[tcvander]

Bad example, should be a Class B address. Unless they are using contiguous Class C addresses, and why would they use contiguous Class C's and not a Class B?; then this is a bad example Todd VanDerwerken, CCNA, CCDA
Technical Consultant

http://www.ciscolinks.net

"If at first you don't succeed...then sky diving isn't for you!"

 

[ElijahBaley]

Phew, thanks

Is this likely to appear on the CCNA exam?

EB

 

[jbaugh]

Elija,

I agree with Todd, it is a bad example of subnetting, and looks more like a supernetting example. In that case, with OSPF(as an example), which depends on a hierarchal IP addressing scheme to work efficiently, the range of Class C addresses (from 192.168.64.0/24 through 192.168.95.0/24) would be in an Autonomous System or an Area x within an AS and that gateway IP would be on either the ASBR or the ABR. The point here is that the ASBR or the ABR would advertise a single route to 192.168.64.0/19 externally for all of the Class C addresses contained within the AS or Area x. I don't think the CCNA exam gets that deep into OSPF, this looks more like a CCNP Router exam type of question, although you should have a basic grasp of the OSPF terminology. However, it has been awhile since I took the CCNA exam and it may have changed significantly, so YMMV.

Good Luck on the exam.
Joe
Joseph B. Baugh, PMP, CISSP, CBM, CCNP, CCDP, CCSE, CUE
Network Services Manager, Sierra Southwest Cooperative

"The road goes on forever and the party never ends." -- Joe Ely: Love And Danger, 1993