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Chapter 3

  • IP Subnetting and Variable Length Subnet Masks (VLSMs)

The next subnet is 64, so the broadcast address is 63. (Remember that the broadcast address of a subnet is always the number right before the next subnet.) The valid host range is 33–62. This is too easy! No, it’s not?

Okay, then let’s try another one. We’ll subnet another Class C address:

        • 192.168.10.33

          = Node address

        • 255.255.255.240

          = Subnet mask

What subnet and broadcast address is the above IP address a member of? 256 – 240 = 16. 0, 16 + 16 = 32. 32 + 16 = 48. And bingo—the host address is between the 32 and 48 subnets. The subnet is 192.168.10.32, and the broadcast address is 47. The valid host range is 33–46.

Okay, we need to do more, just to make sure you have this down. You have a node address of 192.168.10.174 with a mask of 255.255.255.240. What is the valid host range?

The mask is 240, so we’d do a 256 – 240 = 16. This is our block size. Just keep adding 16 until we pass the host address of 174: 0, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176. The host address of 174 is between 160 and 176, so the subnet is 160. The broadcast address is 175, so the valid host range is 161–174. That was a tough one.

One more—just for fun. This is the easiest one of all Class C subnetting:

        • 192.168.10.17

          = Node address

        • 255.255.255.252

          = Subnet mask

What subnet and broadcast address is the above IP address a member of? 256 – 252 = 0, 4, 8, 12, 16, 20, etc. You’ve got it! The host address is between the 16 and 20 subnets. The subnet is 192.168.10.16, and the broadcast address is 19. The valid host range is 17–18.

Now that you’re all over Class C subnetting, let’s move on to Class B subnetting. But before we do, let’s have a quick review.

What Do We Know?

Okay—here’s where you can really apply what you’ve learned so far, and begin committing it all to memory. This is a very cool section that I’ve been using in my classes for years. It will really help you nail down subnetting!

When you see a subnet mask of slash notation (CIDR), you should know the following:

/26 What do we know about a /26?

  • 192 mask

  • 2 bits on and 6 bits off (11000000)

  • Block size of 64

  • 4subnets, each with 62 hosts

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