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IP Custom Subnet Masks
(Page 3 of 4)
Determining the Custom Subnet Mask
Once we have determined how many
bits we are going to devote to the subnet ID and the host ID, we can
figure out the subnet mask. This is fairly easy to do, now that we understand
how subnetting works. We begin with the default subnet mask in binary
for the appropriate class of our network. We then start with the left-most
zero in that mask and change as many bits to 1 as we have dedicated
to the subnet ID. We can then express the subnet mask in dotted decimal
form. Figure 69
shows how the custom subnet mask can be determined for each of the subnetting
options of a Class C network, in both binary and decimal.
Figure 69: Custom Subnet Masks for Class C Networks
Since there are 8 host ID bits in a Class C network address, there are six different ways that the network can be subnetted. Each corresponds to a different custom subnet mask, which is created by changing the allocated subnet ID bits from zero to one.
So, to take the example
in that figure, consider the Class C network 18.104.22.168. There are
8 bits in the original host ID, which gives us six different subnetting
options (we cant use 7 or 8 bits for the subnet ID, for reasons
we will discuss shortly.) Suppose we use three of these for the subnet
ID and five are left for the host ID. To determine the custom subnet
mask, we start with the Class C default subnet mask:
11111111 11111111 11111111
We then change the first three zeroes
to ones, to get the custom subnet mask:
11111111 11111111 11111111
In dotted decimal format, this is
Key Concept: Once the choice of how to subnet has been made, the custom subnet mask is determined simply, by starting with the default subnet mask for the network and changing each subnet ID bit from a 0 to a 1.
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