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The TCP/IP Guide  9  Networking Fundamentals       9  Backgrounder: Data Representation and the Mathematics of Computing

Boolean Logic and Logical Functions 1 2 3 The Open System Interconnection (OSI) Reference Model

Clearing Groups of Bits Using the AND Function

To clear a certain pattern of bits, you do a similar masking operation, but using the AND function instead. If you AND a bit with zero, it will clear it to zero regardless of what the bit was before, while ANDing with one will leave the bit unchanged. So, to take the same example above and clear the middle six bits, we AND with the reverse bit mask, 111000000111. This is shown in Table 15 and illustrated in Figure 10.

We can also look at this “clearing” function a different way. We are clearing the bits where the mask is a zero, and in so doing “selecting” the bits where the mask is a one. Thus, ANDing with a bit mask means that you “keep” the bits where the mask is a one and “remove” the bits where it is a zero.

Boolean Logic and Logical Functions 1 2 3 The Open System Interconnection (OSI) Reference Model