This question
is useful when you are trying to assess the level of experience
of a candidate who claims to have "some" experience with
Cisco router configurations. A candidate with even a minimum
amount of router monitoring experience or some actual router
interface configuration experience should be able to answer
this question. The question also allows you to gauge if
the candidate understands the basics of IP subnetting.
You
want to check the configuration of a LAN port on the router
and so you run the Cisco IOS command: show ip interface
e0.
The
first two lines that appear in the results of this command
read:
Ethernet0
is up. Line Protocol is up.
Internet Address is 130.10.32.1/19
What
does all this mean?
The first thing
the candidate should be able to tell you is that the Ethernet0
interface on the router is up (meaning it is active) and
has been configured correctly with a LAN protocol (in this
case TCP/IP). So basically, the first line of this message
is showing that there is no protocol configuration problem;
the interface appears to be up and running properly.
The next line
of the show ip interface command provides IP addressing
information and just about any candidate (even one that
might be in over their head when it comes to Cisco router
configurations) should be able to tell you the IP address
that has been assigned to the Ethernet0 interface. The tricky
part of the information provided on the second line is the
/19 that appears at the end of the IP address.
The candidate
should be able to tell you that the /19 is the number of
bits that are used for the network addressing plus the number
of bits that have been used to create the subnets on this
network. So the /19 is actually providing you with the subnet
mask that was used to configure the router interface. Asking
the candidate to convert the /19 to an actual subnet mask
in their head might be a little extreme (being part of the
"calculator" generation, I at least need a piece of scratch
paper), but they should be able to provide you with the
following information (at least here is how I would explain
it):
The IP address
is a Class B address. A Class B network with no subnets
would use two octets, 16 bits, to define the network number
(which would be 255.255 in decimal). Take the 19 bits shown
in the router configuration and subtract the 16 bits normally
used. You get 3. This means that 3 bits were used for subnetting.
Take the first three high-order bits and add their decimal
equivalents (128+64+32). You get 224. This means that the
subnet mask used when this router interface was configured
is 255.255.224.0.
If the candidate
can correctly determine the subnet mask they should also
be able to tell you how many subnets the address was divided
into and how many hosts each subnet could have. By taking
3 bits from the 3rd octet for subnetting, 6 subnets are
created from the class B address. This leaves 13 bits to
represent hosts on each subnet or 8,190 hosts.
This question
should definitely provide you with some insight into whether
or not the candidate's experience has been limited to smaller
LANs or if they have had some experience (or at leas thought
and read about) issues related to routed IP environments
involving subnets.
About the author
Joe Habraken has been an active participant in the Information
Technology field for over 15 years. Joe has served as a
network administrator, instructor, and best selling author.
He currently serves as the Director of UNEit, which is the
computer technology training center at the University of
New England in Portland ME. Joe also serves as a Technical
Director for ReviewNet. He recently published two new books
with Que/Pearson Education: Microsoft Office XP 8 in
1, and the Absolute Beginner's Guide to Networking.
