250 Chapter 5: Network Protocols
Keep in mind that all the addresses on the same data link must be in the same subnet. For
example, Ivan and Anna must be in the same subnet, so performing either the easy decimal
algorithm or the more involved binary algorithm on either address will yield the same subnet
number, 180.4.3.0. If the answers are unclear, do several of these using the algorithm used by
computers. For example, using Ivan, the results will be as follows:
For additional practice, you may want to go to the scenarios section at the end of the chapter,
specifically to Scenarios 5-2 and 5-3.
Binary Algorithm for Deriving the Subnet Number, Difficult Subnetting
Difficult subnetting is a term used in this book to denote subnetting when the mask is not all
255s and 0s. The decimal algorithm for calculating the subnet, when basic subnetting is in use,
is more challenging. In fact, several math tricks come in handy so that the result can be
calculated without thinking about binary math. However, starting with the binary algorithm is
helpful.
These difficult masks typically contain one of the values shown in Table 5-17. To speed up the
process of examining these addresses (in case you are taking a timed test), memorizing the
decimal and binary numbers in Table 5-17 will be useful.
Address (Step 1)
180.4.3.3
1011 0100 0000 0100 0000 0011 0000 0011
Mask (Step 2)
255.255.255.0
1111 1111 1111 1111 1111 1111 0000 0000
Result (Steps 3 and 4)
180.4.3.0
1011 0100 0000 0100 0000 0011 0000 0000
Table 5-17
Typical Difficult Mask Values
Decimal
Binary
0
0000 0000
128
1000 0000
192
1100 0000
224
1110 0000
240
1111 0000
248
1111 1000
252
1111 1100
254
1111 1110
255
1111 1111
ch05.fm Page 250 Monday, March 20, 2000 5:06 PM