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How to subnet in third octet IPv4

  1. Jan 2, 2012 #1
    I already know how to subnet Class C addresses. Class B subnetting isn't proving to be very difficult, except for one little thing. I'm confused about how to space out the address blocks in the third octet when dealing with networks below /24.

    Here's an example that I'm having trouble with:

    Given the IP address provide:
    Network Number:
    First Usable Host:
    Last Usable Host:
    The reason this confuses me is that the third and fourth octets are already mostly made of 1's (binary), therefore doesn't that mean not many addresses are available right off the rip?
    Also, if anyone has some easy-to-memorize tricks to subnet Class B and A addresses that I can do with paper and pencil I would greatly appreciate the help!
  2. jcsd
  3. Jan 2, 2012 #2

    I like Serena

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    Homework Helper

    Welcome to PF, itlivesthere! :smile:

    The first 19 bits identify the network (subnet), the last 13 bits identify the client.
    To find the network number, you need to set all bits from bit 20 up to bit 32 to zero.
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