13-5Lesson 1 Configuring and Troubleshooting TCP/IP
A custom subnet mask is not restricted by the same rules used in the classful method.
Remember that a subnet mask consists of a set of four numbers, similar to an IP
address. Consider the default subnet mask for a class B network (255.255.0.0), which
in binary format would be the following:
11111111 11111111 00000000 00000000
This mask specifies that the first 16 bits of an IP address are to be used for the network
ID and the second 16 bits are to be used for the host ID. To create a custom subnet
mask, you would just extend the mask into the host ID portion. However, you must
extend this by adding ones from left to right. Remember that a subnet mask must be an
unbroken string of ones followed by an unbroken string of zeroes. For example, a cus-
tom subnet mask might look like this:
11111111 11111111 11111000 00000000
The value 11111000 in decimal format would be 248, making this IP address
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