Information Technology Reference
In-Depth Information
Once the network administrator has created the subnet mask and has assigned it to
each machine, the IP software applies the subnet mask to the IP address to determine its
subnet address. The word
mask
carries the implied meaning of “lens” in this case; that is,
the IP software looks at its IP address through the lens of its subnet mask to see its subnet
address. Figure 8.8 illustrates an IP address being viewed through a subnet mask.
fIgure 8.8
Applying the subnet mask
Subnet mask code
1s = Positions representing network or subnet addresses
0s = Positions representing the host address
Positions relating to the subnet address
Subnet mask:
1111 1111.1111 1111. 11111111.00000
00
0
1000 0 .
01
0011 1001. 00000001.00111
00
0
IP address of a machine on subnet 1:
(Decimal: 130.57.1.56)
Bits relating to the subnet address
In this example, the IP software learns through the subnet mask that, instead of being
part of the host address, the third byte of its IP address is now going to be used as a subnet
address. The IP software then looks in its IP address at the bit positions that correspond to
the mask, which are 0000 0001.
The final step is for the subnet bit values to be matched up with the binary numbering
convention and converted to decimal. In the Widget Inc. example, the binary-to-decimal
conversion is simple, as illustrated in Figure 8.9.
fIgure 8.9
Converting the subnet mask to decimal
Binary numbering convention
Position/value:
(continued)
128
64
32
16
8
4
2 1
Widget third byte:
0
0
0
0
0
0
0 1
Decimal equivalent:
0 + 1 = 1
Subnet address:
1
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