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Given:
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Host IP address = 192.168.33.1
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Subnet mask length = 26 bits (255.255.255.192)
We know that 192.168.33.1 is a Class C address because the top three bits are "110". The natural network mask length for a Class C network is 24 bits (255.255.255.0). However, the actual mask length for this subnet is 26 bits. These two masks are shown in the diagram below:
In classful networks, the difference between the lengths of the natural and actual masks is the subnet portion of the address. In this case, that is bits 25 and 26, which are highlit in green.
The rules for classful routing require that the subnet portion of an address may be neither all-zeroes nor all-ones. Because all the bits in the subnet portion of 192.168.33.1/26 are zero (highlit in red), this address is illegal in any classful environment.
Note that modern networking equipment generally supports CIDR. If your network is built using such equipment, you can safely enable the CIDR button and ignore this type of problem.
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