Mitsunori:
I was looking through the Sybex CCNA Study Guide 2nd edition, and couldn't
find the question you reference--I assume it's from a different book in
their CCNA series...
I recently completed a project with Sybex that is due out in January. The
one thing my editor and I really worked on was ensuring that the terminology
used was correct and that didn't confuse the test taker. In my opinion,
when I read "12-bit subnet mask" I assume exactly that -- there are 12 bits
in the subnet mask. If I as the author of the test question wanted you to
calculate a subnet where you would tack on 12 bits of subnetting to the
subnet mask, I would use the terminology "12 bits of subnetting". Travis
was on the right track when he said that "a subnet is the 'borrowed bits'".
Those borrowed bits are "the bits of subnetting".
Often you can deduce the response being sought by looking at the options
available. In this case, the author of the question is looking for a 28 bit
subnet mask response. Same applies for whether when determining the number
of networks or hosts available, whether ip subnet zero is expected as a
response or not. For the CCNA exam, the formula to calculate hosts is
always 2^n-2 (no ip subnet zero). It's really unfortunate that a question
such as this exists, but sometimes we authors don't think about
terminology... I got caught on BRI - I thought it was Basic Rate ISDN. My
editor pointed out BRI is actually Basic Rate Interface. I checked with
CCITT (the organization responsible for ISDN standards). Sure enough, she
was correct!
As an author, I feel that it's very important for authors to think about how
someone less experienced will interpret a question. In this case, I'd say
Sybex dropped the ball - but as long as they keep the editor I worked with
on the CCNA Virtual Test Center project, I'd expect the number of errors or
unclear statements to decrease drastically. If Sybex can keep people THAT
good (and as long as my editor IS the editor), I'd definitely recommend
their future Cisco reference materials!
As you work through your CCNA, a great reference is certificationzone.com.
CertificationZone strives for an extremely high level of technical accuracy
and that really benefits the beginner because you DON'T learn things that
are wrong. Been there, done that--it's no fun. It's so hard to unlearn
things you've learned wrong... I've also heard good things about ccprep.com
(to plug someone I'm not affiliated with...) Yes, both sites charge for
their services, but you get what you pay for with Internet sites...
-- Leigh Anne
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
JL
Sent: November 23, 2000 11:37 AM
To: [EMAIL PROTECTED]
Subject: RE: IP Address Calculation [1:1010]
It's important to remember your default mask for all 3 address ranges when
looking at a question like this. A class B address has a default mask of 16
bits, so if " You have an IP address of 172.16.4.58 with a 12-bit subnet
mask." you would add an additional 12 bits to the default mask and from
there determine your valid hosts. 16 bits (default mask) + 12 bits
(additional bits) = 28 = 255.255.255.240; then 256 - 240 = 16. You will have
valid ranges in multiples of 16, 1-16, 17-32, 33-48, 49-64, etc... you do
have to remember to leave out the high and low addresses (network and
broadcast) leaving 14 valid host id's per subnet. Using a 172.16.4.58/28
notation would accomplish the same thing and is just an easier way to say "I
have a class B address with a 12 bit subnet mask". I personally struggled
with subnetting before it finally clicked, it's just important to remember
that there are only just so many combinations available. I would suggest
approaching this from multiple different angles until you find the one that
clicks for you, once it does you'll be amazed at how simple it all seems.
Gragg Vaill
MCP CCNA
NOS Contractor
Sprint ION NOC
Kansas City, Ks.
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
Timothy R Estes
Sent: Thursday, November 23, 2000 8:19 AM
To: [EMAIL PROTECTED]
Subject: Re: IP Address Calculation [1:1010]
Group,
That's pretty confusing. (IMHO). If we are going to refer to subnet masks
with the /30 notation, then we need to stick to it. How would we know if
someone meant with, or without the default mask included?
my $0.02
Timothy Estes
CCNA
Tampa FL
""Travis O'Hara"" wrote in message
news:[EMAIL PROTECTED]...
> Hi,
>
> It's important to understand the wording
>
> 172.16.4.58 is a Class B address. A subnet is the 'borrowed bits' from
the
> host portion of the address. In this case it is stated as being 12bits
>
> So by default it has a 16 bit network mask a Class a has 8 and a Class C
24
>
> 255 255 0 0
> 11111111 11111111 00000000 00000000 = 16Bit netmask
> |---------------| |---------------|
> network host
>
> 255 255 255 240
> 11111111 11111111 11111111 11110000 = 16bit netmask + 12bit subnet mask
+
> 4bit host portion
> |---------------| |-----------||--|
> network subnet host
>
>
> 172.16.4.58 255.255.255.240 is the ip address and network mask
>
> (anyone got a better way of explaining this bit?)
> As networks must start on a border of the subnet that they are divisible
by
> (anyone got a better way of explaining this bit?)
>
> we can figure out the the network address for this IP address with this
> subnet mask starts at 172.16.4.48 (this is not a host address it's a
> network/cable/segment address) first available host address is 172.16.4.49
> your network size is 16 IP addresses so counting from and including
> 172.16.4.48 you reach 172.16.4.63. The last address of the network is
> discarded as being a host address as well as this will be used as the
> broadcast address for the network so the available IP's assignable to
hosts
> are 172.16.4.49-62
>
> That explanation probably sounds as clear as mud but hopefully it's
helpful.
> It's important to read the questions carefully for the syntax they use and
> how it is applied to the subject.
>
> Trav.
>
> > -----Original Message-----
> > From: Mitsunori Sagae [mailto:[EMAIL PROTECTED]]
> > Sent: Thursday, 23 November 2000 3:46 PM
> > To: [EMAIL PROTECTED]
> > Subject: IP Address Calculation [1:1010]
> >
> >
> > Hi, and I am kinda lost with the following question on CCNA
> > sybex book.
> > It's on the Ch4 Review question, and says
> >
> > You have an IP address of 172.16.4.58 with a 12-bit subnet
> > mask. What are
> > your valid hosts?
> >
> > The answer to this question is 172.16.4.49 to 172.16.4.62
> > but I can never get this result,
> >
> > Can someone help me on this, explaining the logic behind it?
> >
> > Thanks
> >
> > mitzs
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