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  1. Senior Member
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    #1

    Default Broadcast address on Class B

    Can someone give me an example of how to find the broadcast address of a specific subnet on a class B subnet? Specifically, a class B subnet which only borrows part of the third octet. I'm still fuzzy on how to find that. Thanks.
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  3. Senior Member aquilla's Avatar
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    #2

    Default Re: Broadcast address on Class B

    Quote Originally Posted by Tricon7
    Can someone give me an example of how to find the broadcast address of a specific subnet on a class B subnet? I'm still fuzzy on how to find that. Thanks.
    It's very similar to working with a class C subnet. Let's assume the following:

    172.16.0.0 with a subnet of 255.255.224.0

    We've borrowed three bits (to get the 224) which means we've got 8 subnets in blocks of 32 (2 x 2 x 2)

    So you would have:

    172.16.0.0 < Network Address
    172.16.0.1 - 172.16.31.254 < Usable Hosts
    172.16.31.255 < Broadcast Address

    With a class C you would work in the last octet. With a class B you would normally work in the third octet (unless your subnet mask tells you otherwise).
    Regards,

    Stuart
    Oct 2007: CCNA 640-801 (Recert'd in Oct 2010)
    WIP: CCNP -> CCNA:Voice -> CCNP: Voice (My personal development plan according to my employer)
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  4. Senior Member
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    #3

    Default Re: Broadcast address on Class B

    Quote Originally Posted by aquilla
    Quote Originally Posted by Tricon7
    Can someone give me an example of how to find the broadcast address of a specific subnet on a class B subnet? I'm still fuzzy on how to find that. Thanks.
    It's very similar to working with a class C subnet. Let's assume the following:

    172.16.0.0 with a subnet of 255.255.224.0

    We've borrowed three bits (to get the 224) which means we've got 8 subnets in blocks of 32 (2 x 2 x 2)

    So you would have:

    172.16.0.0 < Network Address
    172.16.0.1 - 172.16.31.254 < Usable Hosts
    172.16.31.255 < Broadcast Address

    With a class C you would work in the last octet. With a class B you would normally work in the third octet (unless your subnet mask tells you otherwise).
    Thanks. However, my question really dealt with, say, what is the broadcast address (in this case) of the 3rd subnet on this network (172.16.0.0)?
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  5. Senior Member aquilla's Avatar
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    #4

    Default Re: Broadcast address on Class B

    Quote Originally Posted by Tricon7
    Quote Originally Posted by aquilla
    Quote Originally Posted by Tricon7
    Can someone give me an example of how to find the broadcast address of a specific subnet on a class B subnet? I'm still fuzzy on how to find that. Thanks.
    It's very similar to working with a class C subnet. Let's assume the following:

    172.16.0.0 with a subnet of 255.255.224.0

    We've borrowed three bits (to get the 224) which means we've got 8 subnets in blocks of 32 (2 x 2 x 2)

    So you would have:

    172.16.0.0 < Network Address
    172.16.0.1 - 172.16.31.254 < Usable Hosts
    172.16.31.255 < Broadcast Address

    With a class C you would work in the last octet. With a class B you would normally work in the third octet (unless your subnet mask tells you otherwise).
    Thanks. However, my question really dealt with, say, what is the broadcast address (in this case) of the 3rd subnet on this network (172.16.0.0)?
    To answer that sort of question, you need to know the subnet mask. Once you know that you can do the binary math.
    Regards,

    Stuart
    Oct 2007: CCNA 640-801 (Recert'd in Oct 2010)
    WIP: CCNP -> CCNA:Voice -> CCNP: Voice (My personal development plan according to my employer)
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  6. Senior Member
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    #5
    Apparantly you can do this by "OR"ing the subnet mask but I've had far too many beers to explain it that way tonight.

    Aquilla is right, without the mask you can't do anything so ..


    say ip 150.0.0.1 SN 255.255.240.0


    Quickest way I would say is that as you know it's a class B you're dealing with last two bytes of SN so 240.0

    240.0 (not just 240 - this is important!)
    256 Rule kicks in - (Very useful rule this)
    256-240 => 16.0 subnet boundary - not to be confused with 0.16 which is given by SN 255.255.255.240 - very easy to confuse and lead to lots of hair pulling. 255.255.255.240 using 256 rule leads to a class B subnet increments of 0.16 (remember the class IP your working with)

    Now you have the 16.0 you count out the subnets to find out the subnet, subnet host start, subnet host end and broadcast for that subnet.

    [SN ... HostStart ... HostEnd ... BC]

    1) 150.0.0.0 ... 150.0.0.1 ... 150.0.15.254 ... 150.0.15.255 .. <whether "Subnet Zero" is used. If not SN 2 is first "Usable Subnet">
    2) 150.0.16.0 ... 150.0.16.1 ... 150.0.31.254 ... 150.0.31.255
    3) 150.0.32.0 ... 150.0.32.1 ... 150.0.47.254 ... 150.0.47.255
    4) 150.0.48.0 ... 150.0.48.1 ... 150.0.63.254 ... 150.0.47.255
    5) 150.0.64.0 ... etc

    So, quickest way is work out the 256 rule. Then in your head work out how the SN will pan out. (16.0, 32.0, 48.0, 64.0, etc.) Find your IP address and what SN it lies within and then work out the broadcast.

    It gets pretty quick with practice and way less chances of messing it up with ORing.

    As always, I would point you in the direction of subnettingquestions.com for lots and lots of practice and when your wrong, work out why your wrong don't just skip. That's how it got drilled home for me.

    Do not be afraid of Class A's and Class B's ... you're just wasting time when you should be worrying about getting CIDR & VLSM wrong.
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