255.255.248.0. Table 13-2 shows the possible values for an octet in a custom subnet
mask.
Table 13-2 Custom Subnet Mask ValuesBinary Value Decimal Value10000000 128
11000000 192
11100000 224
11110000 240
11111000 248
11111100 252
11111110 254
In the classful method, each of the four numbers in a subnet mask can be only the
maximum value 255 or the minimum value 0. The four numbers are then arranged as
contiguous octets of 255, followed by contiguous octets of 0. For example, 255.255.0.0
is a valid subnet mask, whereas 255.0.255.0 is not. The 255 octets identify the network
ID, and the 0 octets identify the host ID. For example, the subnet mask 255.255.0.0
identifies the network ID as the first two numbers in the IP address.
When subnetting an existing network ID to create additional subnets, you can use any
of the preceding subnet masks with any IP address or network ID. So the IP address
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