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Hello and a little help ... if you please.

Discussion in 'New Members Introduction' started by sp0o0oky, Apr 17, 2008.

  1. sp0o0oky

    sp0o0oky New Member

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    Hello,

    Techies.

    I am a new member and am here to introduce myself.

    But besides saying hello I am grateful to whomever may be able to shed some light on the matter of subnetting for me. While I have been doing the work of a network engineer for some time (mostly as a CITRIX Administrator - CCA) I am now working my way through the 70-293 book with the 70-290 down I am finding that the 293 a bit more challenging.

    With that said I am noting one of the questions on page 2-32 that is giving me a read arse. The question regards the network address of 10.0.0.0/19. The first part of the answer to this question seemed easy but i found how wrong I was.

    What are the number of bits in the subnet identifier? I answered 19.

    Wrong!

    According to the book it should have been 11.

    After reading the material again and then in the Cisco CCNA 640-802 Sybex guide I am still at a loss and am unable to wrap my head around what the subnet id is if it isn't 19 then what is it and why? How can I realize this concept for myself?

    Thank you all indvance for your help.

    Blessings.
     
  2. greenbrucelee
    Highly Decorated Member Award

    greenbrucelee Zettabyte Poster

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    Hi Welcome :)

    I can't answer your query but a more experienced person will be along shortly.
     
    Certifications: A+, N+, MCDST, Security+, 70-270
    WIP: 70-620 or 70-680?
  3. Sparky
    Highly Decorated Member Award

    Sparky Zettabyte Poster Moderator

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    Certifications: MSc MCSE MCSA:M MCSA:S MCITP:EA MCTS(x5) Security+ Network+ A+
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  4. sp0o0oky

    sp0o0oky New Member

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    Thank you.
     
  5. Notes_Bloke

    Notes_Bloke Terabyte Poster

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    Hi & welcome to CF:D

    NB
     
    Certifications: 70-210, 70-215, A+,N+, Security+
    WIP: MCSA
  6. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    Hi and welcome to CF!

    There must be part of the question missing - otherwise I can't see why that answer is correct either.

    But a Cisco expert may be along to tell us why...

    Harry.
     
    Certifications: ECDL A+ Network+ i-Net+
    WIP: Server+
  7. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    OK - after a bit of thought (difficult after a bottle of wine) I think I know what is happening here.

    The *assumption* (and be careful here) is that we are talking about old classfull addressing. If that is the case then 10. is a class A - so 8 bits in the network address. 10 - 8 = 11 so there are 11 bits in the subnet address.

    My own view is that when you write /19 you are implying CIDR, so then demanding classfull addresses is bad form.

    But this may all be wide of the mark....

    Harry.
     
    Certifications: ECDL A+ Network+ i-Net+
    WIP: Server+
  8. BosonMichael
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    BosonMichael Yottabyte Poster

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    Harry's got it. The difference between a classful /8 for a Class A address and a /19 is 11 bits: 19-8=11.

    It'd be different for a Class B address... those start out as a /16. So if you had a /19 on a Class B address, there'd be 3 bits in the subnet identifier (19-16=3).
     
    Certifications: CISSP, MCSE+I, MCSE: Security, MCSE: Messaging, MCDST, MCDBA, MCTS, OCP, CCNP, CCDP, CCNA Security, CCNA Voice, CNE, SCSA, Security+, Linux+, Server+, Network+, A+
    WIP: Just about everything!
  9. GiddyG

    GiddyG Terabyte Poster Gold Member

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    Welcome to CF! 8)
     
  10. Xenophon

    Xenophon Bit Poster

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    Its taken me ages to figure out how to do subnetting for the 70-291 but I have cracked it now (I think check with me again after next thursday, exam day :blink)

    Had a look at your question and 11 is correct. The slash notation is just another way of writing subnet masks (SM)
    Its easy to remember that:
    /8 = the old A class
    /16 = the old B class
    /24 = the old C class
    /19 in binary is
    11111111.11111111.11100000.00000000
    The SM gives you the number of host bits as opposed to subnet bits (1's are subnet bits. 0's are hosts bits)
    The question stated for a subnet in the 10.0.0.0 network. So you ignore the first octet.
    19 - 8 = 11
    or in other words this is the only bit that counts for the subnet in question
    11111111.11100000.00000000
    The secret to all Subnetting questions is.
    1. Think in binary
    2. Use the calculator if like me your maths is crap! :oops:
    Too be fair to your good self though sir I have not seen a question before that has asked for "the number of bits in the subnet identifier" having said that it is normally something you work out in most subnetting questions so you can calulate the maximum number of possible subnets available for your chosen or given SM.
    Which is of course 2 to the power of the number of subnet bits. (again easy to work out on a calculator)
    You are allowed to use the calculator in MS exams.
    Hope all this helps somebody. Its been more revision for me anyway! :biggrin
     
    Certifications: A+, N+, 70-210, 70-270, MCSA 2003
    WIP: CCNA or 70-293 (not sure yet)
  11. UCHEEKYMONKEY
    Honorary Member

    UCHEEKYMONKEY R.I.P - gone but never forgotten. Gold Member

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    I concur with Harry:rocks

    Greetings & welcome to CF:sunny
     
    Certifications: Comptia A+
    WIP: Comptia N+

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