Adder Technology 5000 User Manual

To really understand the operation of a net mask it is necessary to delve deeper 
into the life blood of computers – binary; this is native digital, where everything 
is either a 1 (one) or 0 (zero), on or off, yes or no. 
The net mask operation described on the 

 is known as a ‘bit-wise 

AND function’. The example of 255.255.255.0 is handy because the last octet 
is completely zero and is “clean” for illustrative purposes. However, actual net 
mask calculations are carried out, not on whole decimal numbers, but bit by bit 
on binary numbers, hence the term ‘bit-wise’. In a real local network, a net mask 
might be 

255.255.255.240. Such an example would no longer be quite so clear, 

until you look at the net mask in its binary form:

 

11111111.11111111.11111111.

11110000

In this case, the four zeroes at the end of the net mask indicate that the local 
part of the address is formed by only the last four bits. If you use the diagram 
from the previous example and insert the new net mask, it will have the 
following effect on the final result:

192

192

168

168

142

142

154

154

144

1

1

1

0

0

0

0

1

1

1

0

0

0

1

0

0

0

1110 0 0 0

144

Thus, when 154 is bit-wise ANDed with 240, the result is 144. Likewise, any 
local address from 192.168.142.

144 through to 192.168.142.159 would 

produce exactly the same result when combined with this net mask, hence they 
would all be local addresses. However, any difference in the upper three octets 
or the upper four bits of the last octet would slip through the mask and the 
address would be flagged as not being local.    

When you “open up” the last octet 
of the net mask and look at the 
binary inside, you can see the last 
four zero bits preventing any 1’s in 
the address from falling through.