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  1. Senior Jr Member
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    #1

    Default Ospf wildcard mask

    Came across this practice ques:

    Which of the following network commands , following the 'router ospf 1' command, tells the router to start using OSPF on interfaces whose ip addresses are 10.1.1.1, 10.1.100.1, and 10.1.120.1?

    A) network 10.1.1.0 255.0.0.0 area 0
    B) network 10.0.0.0 0.255.255.0 area 0
    C) network 10.0.0.0 255.0.0.0 area 0
    D) network 0.0.0.0 255.255.255.255 area 0

    Choose 1 answer.

    More than 1 answer looked correct to me but i'll leave it unanswered if someone wants to try it out.
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  3. Senior Member
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    #2
    I did a similar question to that one in some where too.

    I think the answer is C and D.
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  4. Went to the dark side.... Moderator networker050184's Avatar
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    #3
    The only possible answer is D. The zeros mean must match. The noloy one that matches all is D
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  5. Resident Underachiever EdTheLad's Avatar
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    #4

    Default Re: Ospf wildcard mask

    D is correct, 0 means exact match all other network statements end with .0 and a .0 mask, all of the supplied ip addresses end with .1 and are not .0, hence the only answer an be D.
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  6. Senior Jr Member
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    #5
    Why not B, wouldn't the 0's in the first and last octets of the mask be the do care bits that would match to the 10 in the first and 1 in the last octet of each interface? The 255's would match anything in between. Or am i thinking of the mask entirely wrong
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  7. Went to the dark side.... Moderator networker050184's Avatar
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    #6
    For B the wild card mask is 10.0.0.0 0.255.255.0 that means all addresses have to have a 10 in the first octet and a 0 in the last. All the addresses in the question start with 10 but they do not end with 0 so this mask would not match any of them.
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  8. Senior Member
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    #7
    network 10.0.0.1 0.255.255.0 area 0 would work or

    network 10.1.0.1 0.0.255.0 area 0

    I saw a simlar question in the ICND book.
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  9. Senior Jr Member
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    #8
    Quote Originally Posted by networker050184
    For B the wild card mask is 10.0.0.0 0.255.255.0 that means all addresses have to have a 10 in the first octet and a 0 in the last. All the addresses in the question start with 10 but they do not end with 0 so this mask would not match any of them.
    Should have remembered that, thanks for setting me straight.
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  10. Went to the dark side.... Moderator networker050184's Avatar
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    #9
    No problem!
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  11. Member
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    #10
    Quote Originally Posted by Mikdilly
    Why not B, wouldn't the 0's in the first and last octets of the mask be the do care bits that would match to the 10 in the first and 1 in the last octet of each interface? The 255's would match anything in between. Or am i thinking of the mask entirely wrong
    The last octet is a 1. For B, with the 0 mask, the last octet is a 0. That's why B is not the answer. Hope that helps!
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  12. Senior Member
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    #11
    Oh, I thought C was right too. Because the second, third and fourth octets would cover any number when the same octets are "zero" on the mask. And the "255" on the mask will make sure that the network has to start with "10".
    C) network 10.0.0.0 255.0.0.0 area 0
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  13. Senior Member
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    #12
    It's the other way around. 0 means match exactly.

    Plus 255.0.0.0 is a subnet mask, not an inverse mask which is used for ospf.
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  14. Went to the dark side.... Moderator networker050184's Avatar
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    #13
    0 is match 255 is any, then you can get into the difficult ones!
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  15. Senior Member
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    #14
    Quote Originally Posted by networker050184
    0 is match 255 is any, then you can get into the difficult ones!
    I see, I was confused then. I guess the mask should have been 0.255.255.255

    Thanks for clarifying.
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  16. Junior Member
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    #15
    Refering to the CCNA Study Guide 5th Edition by Todd Lammle

    " A wildcard Review: A 0 octet in the wildcard mask indicates that the corresponding octet in the network must match exactly. On the other hand a 255 indicates that you dont care what the corresponding octet is in the network number."

    as per the question the networks are 10.1.1.1, 10.1.100.1 and 10.1.120.1


    the option in the answers is not there but the correct one must be

    network 10.1.0.0 0.0.255.255 area 0

    or

    network 10.0.0.0 0.255.255.255 area 0

    am i right???
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  17. Senior Member
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    #16
    well you could read the the whole thread and answer that question.
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  18. Junior Member
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    #17
    Quote Originally Posted by Netstudent
    well you could read the the whole thread and answer that question.
    got it right now dude....

    i was thinking of this all nite n early this morning i woke up to check that i was not right abt the masks....

    in this case B is the best choice....
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  19. Junior Member
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    #18
    Quote Originally Posted by networker050184 View Post
    For B the wild card mask is 10.0.0.0 0.255.255.0 that means all addresses have to have a 10 in the first octet and a 0 in the last. All the addresses in the question start with 10 but they do not end with 0 so this mask would not match any of them.
    If u find this query foolish then i'm sorry in advance but the question thats ticking in my mind is that how can the mask 0.255.255.0 means all addresses have to have 10 in first octet and 0 in the last octet.

    either it should mean 0 in the first octet and zero in the last octet or
    it should also support 10 in the first and <b> 1 in the last octet </b> as one is common in the above given ip addresses.
    Last edited by jadenbliss; 06-16-2009 at 01:24 AM.
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  20. CCIE Bound kryolla's Avatar
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    #19
    Quote Originally Posted by jadenbliss View Post
    If u find this query foolish then i'm sorry in advance but the question thats ticking in my mind is that how can the mask 0.255.255.0 means all addresses have to have 10 in first octet and 0 in the last octet.

    either it should mean 0 in the first octet and zero in the last octet or
    it should also support 10 in the first and <b> 1 in the last octet </b> as one is common in the above given ip addresses.
    The IP is 10.0.0.0 with an inverse mask of 0.255.255.0 so that mean the first and last octet has to match which is 10 and 0 the second and third octet can be anything.
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  21. Junior Member
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    #20
    Quote Originally Posted by kryolla View Post
    The IP is 10.0.0.0 with an inverse mask of 0.255.255.0 so that mean the first and last octet has to match which is 10 and 0 the second and third octet can be anything.
    Thanks a lot kryolla i totally got what u wanted to say

    Now i u clearly understand why network 0.0.0.0 255.255.255.255 area 0 could be the only ans to the above asked que

    Jaden
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  22. Member cisconooblet's Avatar
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    #21
    In OSPF your wildcard mask must be all binary 0's followed by all 1's or vise versa. You can't do a mask of 0.255.255.0
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  23. Junior Member
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    #22
    Quote Originally Posted by cisconooblet View Post
    In OSPF your wildcard mask must be all binary 0's followed by all 1's or vise versa. You can't do a mask of 0.255.255.0
    could'nt understand what u meant to say can u pls describe it. What do u mean by mask must be all binary 0's followed by all 1's ...??


    JAden
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  24. CCIE Bound kryolla's Avatar
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    #23
    it has to be contigous
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  25. Junior Member
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    #24
    Quote Originally Posted by kryolla View Post
    it has to be contigous
    does this means that all the bits in an octet can be either 1 or 0

    and this is true for all the 4 octets

    Jaden
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  26. CCIE Bound kryolla's Avatar
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    #25
    it means you cant have 0,1,0s mixed
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