1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Subnet Masks

Discussion in 'Network Infrastructure' started by zimbo, Jul 25, 2006.

  1. zimbo
    Honorary Member

    zimbo Petabyte Poster

    5,215
    98
    181
    Right here we go! I got this covered just want to make sure im doing it correctly and i didnt come to the answer by fluke.

    Im given an address lets say:

    10010010 01101011 00100111 10001000

    and im told to convert that to dotted decimal and determine the subnet mask in CIDR notation so:

    that comes out to 146.107.39.136 which is a class B address whose subnet mask is 255.255.0.0 so the final answer comes out to

    146.107.39.136 /16

    Now the question im asking is I found that /16 because i figured that the address i have is class B and i know the class B subnet mask so 16 is the answer! Is that the only way of working it out? I am correct btw!:biggrin

    Just confirm me while i hit lesson 3 on Chapter 2..... Subnetting and Supernetting of IP Networks! :rolleyes: :blink :blink
     
    Certifications: B.Sc, MCDST & MCSA
    WIP: M.Sc - Computer Forensics
  2. hbroomhall

    hbroomhall Petabyte Poster Gold Member

    6,623
    115
    224
    In the Microsoft world, where, even if they mention CIDR they really haven't come to terms with it, then yes - that is right.

    In the real world the correct answer is that there isn't enough info to determine the mask, as 'Classes' don't exist any longer!
    (CIDR = Classless Inter-Domain Routing - i.e. no classes)

    EDIT: I suppose the getout here is the phrase
    . This means they can ask for it in modern terms while using the mechanisms of last century!

    Harry.
     
    Certifications: ECDL A+ Network+ i-Net+
    WIP: Server+
  3. ffreeloader

    ffreeloader Terabyte Poster

    3,661
    106
    167
    I don't know Zimbo. It's pretty difficult to say what the network address is in CIDR notation as there is no subnet mask. Network classes A, B, C, and D do not exist by definition in CIDR (Classless Inter Domain Routing).
     
    Certifications: MCSE, MCDBA, CCNA, A+
    WIP: LPIC 1
  4. zimbo
    Honorary Member

    zimbo Petabyte Poster

    5,215
    98
    181
    so im right in how i got to /16?:D
     
    Certifications: B.Sc, MCDST & MCSA
    WIP: M.Sc - Computer Forensics
  5. simongrahamuk
    Honorary Member

    simongrahamuk Hmmmmmmm?

    6,199
    125
    199
    Not with CIDR you're not! :blink

    You have determined that it is a /16 address using classfull methods, not classless.

    With CIDR you could use the 146.107.39.136 address with a /24 address if you liked.

    Confused? You will be! :ohmy

    :biggrin
     
  6. zimbo
    Honorary Member

    zimbo Petabyte Poster

    5,215
    98
    181
    the answer is /16 so how would you work it out? using just the binary?
     
    Certifications: B.Sc, MCDST & MCSA
    WIP: M.Sc - Computer Forensics
  7. simongrahamuk
    Honorary Member

    simongrahamuk Hmmmmmmm?

    6,199
    125
    199
    First question - how do you know that the answer is /16?

    As freddy and Harry have said it is difficult to determine the mask based purely on the IP address unless you have alreasy been told that it is /16 or 255.255.0.0.

    In classfull addressing this is easy, in classless its extremely difficult without more info.

    P.S. Boyce, If you're reading this I havent forgot about you!

    8)
     
  8. Boycie
    Honorary Member

    Boycie Senior Beer Tester

    6,281
    85
    174
    Zim,

    An IP address is worthless unless you have a mask with it. /16 means that the first sixteen bits belong to the network.

    291 looks good! :eek:

    Si
     
    Certifications: MCSA 2003, MCDST, A+, N+, CTT+, MCT
  9. Boycie
    Honorary Member

    Boycie Senior Beer Tester

    6,281
    85
    174
    yeah, there seems to be some overlap/confusion when it comes to Classful and classless addressing. I bet the CCNA is a real belter!

    Si
     
    Certifications: MCSA 2003, MCDST, A+, N+, CTT+, MCT
  10. zimbo
    Honorary Member

    zimbo Petabyte Poster

    5,215
    98
    181
    K i think let me try re-word what the question is asking:

    1. Determine the dotted-decimal - we figured that one
    2. Use CIDR notation to show the default subnet mask

    all the info you are given is that binary

    now i got the right answer BUT you guys are seriously confusing me!
     
    Certifications: B.Sc, MCDST & MCSA
    WIP: M.Sc - Computer Forensics
  11. Boycie
    Honorary Member

    Boycie Senior Beer Tester

    6,281
    85
    174
    Zim,

    As far as i can see you haven't been given the mask.

    You have been given the IP address in binary and the /16 means the first 16 bits are for network allocation.
     
    Certifications: MCSA 2003, MCDST, A+, N+, CTT+, MCT
  12. zimbo
    Honorary Member

    zimbo Petabyte Poster

    5,215
    98
    181
    and the way i worked it out was the address is class B which is subnet mask 255.255.0.0 OR /16

    so the answer is 146.107.39.136 /16 i got that and the book had that...:blink
     
    Certifications: B.Sc, MCDST & MCSA
    WIP: M.Sc - Computer Forensics
  13. zimbo
    Honorary Member

    zimbo Petabyte Poster

    5,215
    98
    181
    like you said the 1st 16 bits is class B - 255.255.0.0
     
    Certifications: B.Sc, MCDST & MCSA
    WIP: M.Sc - Computer Forensics
  14. Boycie
    Honorary Member

    Boycie Senior Beer Tester

    6,281
    85
    174
    Zim,

    As Harry pointed out, strictly speaking that address isn't in a class; it is classless, hence the /16

    Si
     
    Certifications: MCSA 2003, MCDST, A+, N+, CTT+, MCT
  15. zimbo
    Honorary Member

    zimbo Petabyte Poster

    5,215
    98
    181
    so thats what im asking... if my method of finding the answer is wrong what is the right method using the given info?
     
    Certifications: B.Sc, MCDST & MCSA
    WIP: M.Sc - Computer Forensics
  16. ffreeloader

    ffreeloader Terabyte Poster

    3,661
    106
    167
    Zimbo,

    Take a look at my first post on this thread. There are no classes of networks in CIDR. It's that way by definition. Since CIDR has, by design, no classes, how can anyone say from an IP address alone what the subnet mask is?

    What the question is asking you to do is take a binary IP address figure out what the dotted decimal equivalent is, figure out what the network address would be in the old classful networking scheme, and then put the CIDR equivalent at the end of the IP address. That's the only way possible to arrive at that answer. It's not possible using only CIDR notation.

    Once again, look at the acronym, CIDR, and then what it stands for. Then you will start to understand what people are saying to you.

    There are no default networks in CIDR other than the private address ranges that have been set aside. 146.107.39.136 could be in 146.107.39.136/9, or 146.107.39.136/28, or 146.107.39.136/21, or any other combination you can think of since CIDR doesn't start, by default, with /8, /16, and /24 when creating subnets.
     
    Certifications: MCSE, MCDBA, CCNA, A+
    WIP: LPIC 1
  17. Boycie
    Honorary Member

    Boycie Senior Beer Tester

    6,281
    85
    174
    Zim,

    The purpose of classless addressing is for more scope.
    I have attached a subnetting spreadsheet that the Transender technician sent me for N+.
    Hope it helps. He pointed out to me that subnetting is one of those *specialist* areas. 8)

    Si
     

    Attached Files:

    Certifications: MCSA 2003, MCDST, A+, N+, CTT+, MCT
  18. Boycie
    Honorary Member

    Boycie Senior Beer Tester

    6,281
    85
    174
    Great explanation, thanks Freddie

    Si
     
    Certifications: MCSA 2003, MCDST, A+, N+, CTT+, MCT
  19. zimbo
    Honorary Member

    zimbo Petabyte Poster

    5,215
    98
    181
    so on that part im ok? right?

    this is the part if you could explain please....


    btw thanks to all of you guys... really hot here and got more confused!:biggrin
     
    Certifications: B.Sc, MCDST & MCSA
    WIP: M.Sc - Computer Forensics
  20. Boycie
    Honorary Member

    Boycie Senior Beer Tester

    6,281
    85
    174
    Zim,

    You could only refer it to *a class* if it were a classful address, which it isn't. When you see the / try and forget about A, B and C.

    Not sure if you saw the list i attached; i double posted :oops:

    Si
     
    Certifications: MCSA 2003, MCDST, A+, N+, CTT+, MCT

Share This Page

Loading...