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Chapter 3

  • IP Subnetting and Variable Length Subnet Masks (VLSMs)

Answer: 256 – 252 = 4. The subnet is 172.16.10.8, with a broadcast of 172.16.10.11. Question: What is the subnet and broadcast address of the host 172.16.88.255/20?

Answer: What is a /20? If you can’t answer this, you can’t answer this question, can you? A /20 is 255.255.240.0, which gives us a block size of 16 in the third octet, and since no subnet bits are on in the fourth octet, the answer is always 0 and 255 in the fourth octet. 0, 16, 32, 48, 64, 80, 96…bingo. 88 is between 80 and 96, so the subnet is 80.0 and the broadcast address is 95.255.

Subnetting Class A Addresses

Class A subnetting is not performed any differently from Classes B and C, but there are 24 bits to play with instead of the 16 in a Class B address and the 8 in a Class C address. Let’s start by listing all the Class A subnets:

255.128.0.0

(/9)

255.192.0.0

(/10)

255.224.0.0

(/11)

255.240.0.0

(/12)

255.248.0.0

(/13)

255.252.0.0

(/14)

255.254.0.0

(/15)

255.255.0.0

(/16)

255.255.128.0

(/17)

255.255.192.0

(/18)

255.255.224.0

(/19)

        • 255.255.255.128

          (/25)

        • 255.255.255.192

          (/26)

        • 255.255.255.224

          (/27)

        • 255.255.255.240

          (/28)

        • 255.255.255.248

          (/29)

        • 255.255.255.252

          (/30)

255.255.240.0

(/20)

255.255.248.0

(/21)

255.255.252.0

(/22)

255.255.254.0

(/23)

255.255.255.0

(/24)

That’s it. You must leave at least 2 bits for defining hosts. And I hope you can see the pattern by now. Remember, we’re going to do this the same way as a Class B or C subnet. It’s just that, again, we simply have more host bits.

Subnetting Practice Examples: Class A Addresses

When you look at an IP address and a subnet mask, you must be able to distinguish the bits used for subnets from the bits used for determining hosts. This is imperative. If you’re still struggling with this concept, please reread the preceding “IP Addressing” section. It shows you how to determine the difference between the subnet and host bits, and should help clear things up.

Practice Example #1A: 255.255.0.0 (/16)

Class A addresses use a default mask of 255.0.0.0, which leaves 22 bits for subnetting since you must leave 2 bits for host addressing. The 255.255.0.0 mask with a Class A address is using 8 subnet bits.

  • Subnets? 28 = 256.

  • Hosts? 216 – 2 = 65,534.

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