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Wireless Networks Thread, The dreaded topic of subnetting... in Technical; Hi all, I'm preparing for 070-291, and am going over some test prep. I thought I had a pretty firm ...
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    Question The dreaded topic of subnetting...

    Hi all,

    I'm preparing for 070-291, and am going over some test prep. I thought I had a pretty firm grasp on subnets, but I'm caught up on an idea, and need some clarification.

    Lets say my network is 10.0.0.0/20

    That makes my mask

    11111111 . 11111111 . 11110000 . 00000000 ( 255.255.240.0 )

    So my networks can range from

    00001010 . 00000000 . 00000000 . 00000000 ( 10.0.0.0 )
    to
    00001010 . 11111111 . 11110000 . 00000000 ( 10.255.240.0 )

    My question involves the 3rd octet.

    If the network in use is

    00001010 . 00000001 . 10110000 . 00000000 ( 10.1.176.0 )

    and the host portion in binary is

    xxxxxxxx . xxxxxxxx . xxxx0110. 00000010 ( x.x.6.2 )

    Is the resultant IP address for the host

    00001010 . 000000001 . 10110110 . 00000010 ( 10.1.182.2 )

    I'm mentally caught up on combining the network and host portion of the binary digits into a final decimal. Just want to make sure I got this straight in my head.

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    SYNACK's Avatar
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    Yes you are correct, it does combine that way adding to 182.

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    Resolver (25th April 2010)

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    mac_shinobi's Avatar
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    Are there any good books that go through cidr / vlsm / subnetting / super netting etc ??

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    Quote Originally Posted by mac_shinobi View Post
    Are there any good books that go through cidr / vlsm / subnetting / super netting etc ??
    the cisco ccna book covers all of that

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    Thanks Synack.

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    The study guide I'm using shows a diagram with the following information...

    Clients
    IPs 10.10.10.66-10.10.10.70
    SM 255.255.255.192
    GW 10.10.10.65

    Then another portion of the network with
    10.10.10.94
    255.255.255.192
    10.10.10.93

    The question the diagram is asking is irrelevant to my question. As I understand it, the only IPs available for a 255.255.255.192 subnet would be x.x.x.0-63

    Subnet 11111111 . 11111111 . 11111111 . 11000000
    allows for hosts xxxxxxxx . xxxxxxxx . xxxxxxxx . xx000000 in the ending zeros. Those zeros wouldn't allow a host number high enough to access a gw at 10.10.10.65, or 93, or be an IPs at 66-70 or 94.

    Is this yet another poorly written question in my guide, or am I at a misunderstanding?

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    Quote Originally Posted by Resolver View Post
    As I understand it, the only IPs available for a 255.255.255.192 subnet would be x.x.x.0-63
    Nope this is not true.

    lets break down the last octect 192 to binary -> 11000000
    now lets break it down to the possible number of network id's (the first 2 digits are important here)

    00000000 = invalid
    01000000
    10000000
    11000000 = invalid

    You broke it down to 2 possible network ids. 01 = 64 and 10 = 128.
    Thus your 2 network ID's are 10.10.10.64 and 10.10.10.128
    Now the host ip adress will be 10.10.10..65 (GW ip) to 10.10.10.127 (broadcast ip) for network id 10.10.10.64
    Now the host ip adress will be 10.10.10..129 (GW ip) to 10.10.10.191 (broadcast ip) for network id 10.10.10.128

    Learn To Subnet: is a great resource to learn

    bio..

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    mac_shinobi (28th April 2010)

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    If the subnet is 11111111 . 11111111 . 11111111 . 11000000, that means the network portion of the address is where the 1s are. It cannot be all 0's or all 1's, as you mention, but having two 0's in the last octet doesn't violate that. The network could be 10.0.10/26, in which case the two digits in the 4th octet would be zeros, but the network address wouldn't be ALL 0s, and therefore ok ....correct?

    Assuming the last two octets are 00 (Yes, I should have said 1 to 62, but for sake of math, I'm including the entire range) the range is 0-63, right?

    What you said makes sense to me, the question in the book doesn't specify if the network portion includes any digits in the last octet. I guess it's up to me to figure that out based on the given subnet and host addresses.

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    Quote Originally Posted by mac_shinobi View Post
    Are there any good books that go through cidr / vlsm / subnetting / super netting etc ??
    Most of the books are as good as each other, you (or I did at least) just have to keep reading them over and over until it sinks in :P

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    Quote Originally Posted by Resolver View Post
    If the subnet is 11111111 . 11111111 . 11111111 . 11000000, that means the network portion of the address is where the 1s are. It cannot be all 0's or all 1's, as you mention, but having two 0's in the last octet doesn't violate that. The network could be 10.0.10/26, in which case the two digits in the 4th octet would be zeros, but the network address wouldn't be ALL 0s, and therefore ok ....correct?

    Assuming the last two octets are 00 (Yes, I should have said 1 to 62, but for sake of math, I'm including the entire range) the range is 0-63, right?

    What you said makes sense to me, the question in the book doesn't specify if the network portion includes any digits in the last octet. I guess it's up to me to figure that out based on the given subnet and host addresses.
    In practice you could use the 00 and 11 networks ID's but your routers should be able to cope with RFC1812. I also remember from my cisco exams that the cisco books considers 00 and 11 to be invalid (for exam taking ). So your way of thinking is good

    bio..

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    Quote Originally Posted by ChrisH View Post
    Most of the books are as good as each other, you (or I did at least) just have to keep reading them over and over until it sinks in :P
    I read about subnetting in at least 3 different books. The thing that confused me the most were the different classes (A, B, C). Most books I've seen, even new books, still treat the internet as if we're all using public addresses, and that no one subnets.

    I didn't really grasp subnetting until I took some MCSA night classes, and even now I still need to check my answers with you guys.

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    I found Advanced Subnet Calculator from Solarwinds very useful in understanding this topic further.

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