Practical TCP/IP and Ethernet Networking for Industry

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For class C, the number of NetIDs is determined by octets ‘w’, ‘x’ and ‘y’. The first

three bits (110) are used to indicate class C and hence cannot be used. This leaves twenty-

two usable bits. Twenty-two bits allow 2

22

= 2 097 152 combinations. The number of

HostIDs is determined by octet ‘z’. From these 8 bits, 2

8

= 256 combinations are

available. Once again, all zeros and all ones are not permissible which leaves 254 usable

combinations.

Figure 6.4

Hosts and subnets per class

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Strictly speaking, one should be referring to ‘netmasks’ in general, or to ‘subnet masks’ ...