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Mark Bedingfield
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 Posted: Sat Aug 09, 2003 8:55 am Post subject: Calculating hosts in a Subnet: CONTRADICTION - Help please!! |
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Mark Bedingfield
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| Posted: Sat Aug 09, 2003 8:55 am Post subject: Re: Calculating hosts in a Subnet: CONTRADICTION - Help plea |
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The number of subnets is 2n-2 for the purposes of the Exam. But the RFC says
2n. Cisco believe otherwise. So always use 2n - 2 for subs.
Mark
"martin" <bajelisNO@SPAM4MEozemail.com.au> wrote in message
news:3f345f41$0$23595$5a62ac22@freenews.iinet.net.au...
| Quote: |
Wendell Odom (In CCNA Certification guide 640-507) states:
" The number of subnets per network is defined by this formula:
2 ^ number of subnet bits
In some documents the number of subnets is shown as 2 less than 2^ number
of
subnet bits, implying two reserved cases. These two cases are no longer
reserverd, however and can be used."
Tod Lammle (In CCNA Study Guide 3rd Edition 640-607) states:
" How many subnets ? 2 ^x - 2 = number of subnets. x is the number of
masked
bits. "
So my quesiton is:
Which is correct FOR THE PURPOSES OF THE EXAM ???
Thanks,
Martin.
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André Franke
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| Posted: Tue Aug 12, 2003 5:49 pm Post subject: Re: Calculating hosts in a Subnet: CONTRADICTION - Help plea |
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For the exam:
If the question doesn't mention "IP SUBNET ZERO" enabled, Cisco calculates
still the old formula of 2^n-2.
But if "IP SUBNET ZERO" should be enabled (mentioned or shown in any kind
of a listing) it is 2^n.
The RFC 1878 states:
| Quote: | *Subnet all zeroes and all ones excluded. (Obsolete)
*Host all zeroes and all ones excluded. (Obsolete).
You can enable usage of the zero-subnet/host and the broadcast-subnet/host |
by issuing the "ip subnet zero" command, but by default it is set to "no
ip subnet zero". So unless explicitly enabled it will exclude the
subnetaddress and the broadcastaddress for every subnet. Thus default is
2^n-2.
regards
André |
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