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Chapter 3

  • IP Subnetting and Variable Length Subnet Masks (VLSMs)

  • 14.

    A. A /25 mask is 255.255.255.128. Used with a Class B network, the third and fourth octets are used for subnetting with a total of 9 subnet bits, 8 bits in the third octet and 1 bit in the fourth octet. Since there is only 1 bit in the fourth octet, the bit is either off or on—which is a value of

    • 0

      or 128. The host in the question is in the 0 subnet, which has a broadcast address of 127 since 128 is the next subnet.

  • 15.

    C. Start with 255.255.255.0. This provides 256 subnets, each with 254 hosts. Move the subnet bits right if you need more subnets; move them left if you need more hosts. Since we need more hosts, we’re going to take away subnet bits. The next mask then is 255.255.254.0, which pro- vides 7 subnet bits and 9 host bits, or 128 subnets, each with 510 hosts.

  • 16.

    C. The default gateway is the subnet address and not a valid host ID.

  • 17.

    E. A Class C subnet mask of 255.255.255.224 is 3 bits on and 5 bits off (11100000) and pro- vides 8 subnets, each with 30 hosts. However, if the command ip subnet-zero is not used, then only 6 subnets would be available for use.

  • 18.

    E. A Class B network ID with a /22 mask is 255.255.252.0, with a block size of 4 in the third octet. The network address in the question is in subnet 172.16.16.0 with a broadcast address of 172.16.19.255. Only answer E even has the correct subnet mask listed, and

        • 172.16.18.255

          is a valid host.

  • 19.

    C. A Class B /20 mask is 255.255.240.0, which is 4 subnet bits and 12 host bits. Eight host bits is 256 (minus 2 is 254 hosts). Nine hosts bits is 256 + 256 = 512 (minus 2 is 510 hosts per subnet). Ten host bits is 512 + 512 = 1024 (minus 2 is 1022 hosts per subnet). Eleven hosts bits is 1024 + 1024 = 2048 (minus 2 is 2046 hosts per subnet). Twelve host bits is 2048 + 2048 = 4096 (minus 2 is 4094 hosts per subnet).

20. B, C, D. This question only looks somewhat difficult because Class A, B, and C network addresses are listed. But they are only there to confuse you. What is a /27? 255.255.255.224. This is the mask you will use regardless of the class of address. Take out all the subnet and broadcast addresses and you’re left with just the valid hosts.

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