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Subnetting Basics

109

24

= 16

25

= 32

26

= 64

27

= 128

28

= 256

29

= 512

210

= 1,024

211

= 2,048

212

= 4,096

213

= 8,192

214

= 16,384

Before you get stressed out about knowing all these exponents, remember that it’s helpful to know them, but it’s not absolutely necessary. Here’s a little trick since you’re working with 2s: each successive power of 2 is double the previous one. For example, all you have to do to remember the value of 29 is to first know that 28 = 256. Why? Because when you double 2 to the eighth power (256), you get 29 (or 512). To determine the value of 210, simply start at 28 = 256, and then double it twice.

You can go the other way as well. If you needed to know what 26 is, you just cut 256 in half two times: once to reach 27 and then one more time to reach 26.

Subnet Masks

For the subnet address scheme to work, every machine on the network must know which part of the host address will be used as the subnet address. This is accomplished by assigning a subnet mask to each machine. A subnet mask is a 32-bit value that allows the recipient of IP packets to distinguish the network ID portion of the IP address from the host ID portion of the IP address.

The network administrator creates a 32-bit subnet mask composed of 1s and 0s. The 1s in the subnet mask represent the positions that refer to the network or subnet addresses.

Not all networks need subnets, meaning they use the default subnet mask. This is basically the same as saying that a network doesn’t have a subnet address. Table 3.1 shows the default subnet masks for Classes A, B, and C. These default masks cannot change. In other words, you can’t make a Class B subnet mask read 255.0.0.0. If you try, the host will read that address as invalid and usually won’t even let you type it in. For a Class A network, you can’t change the first byte in a subnet mask; it must read 255.0.0.0 at a minimum. Similarly, you cannot assign 255.255.255.255, as this is all 1s—a broadcast address. A Class B address must start with 255.255.0.0, and a Class C has to start with 255.255.255.0.

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