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10.3 | |
The Mechanics of Subnetting | |
| 10.3.3 | |
Establishing the subnet mask address |
Selecting the number of bits to use in the
subnet process will depend on the maximum number of hosts required per
subnet. An understanding of basic binary math and the position value
of the bits in each octet is necessary when calculating the number of
subnetworks and hosts created when bits were borrowed.
 The last two bits in the last octet, regardless of the IP address class, may never be assigned to the subnetwork. These bits are referred to as the last two significant bits. Use of all the available bits to create subnets, except these last two, will result in subnets with only two usable hosts. This is a practical address conservation method for addressing serial router links. However, for a working LAN this would result in prohibitive equipment costs. The subnet mask gives the router the
information required to determine in which network and subnet a
particular host resides.
To determine the number of bits to be
used, the network designer needs to calculate how many hosts the
largest subnetwork requires and the number of subnetworks needed. As
an example, the network requires 30 hosts and five subnetworks. A
shortcut to determine how many bits to reassign is by using the
subnetting chart.
The method that was used to create the
subnet chart
can be used to solve all subnetting problems.
Number of usable subnets= two to the power of the assigned subnet bits or borrowed bits, minus two (reserved addresses for subnetwork id and subnetwork broadcast)
Number of usable hosts= two to the power of the bits remaining, minus two (reserved addresses for subnet id and subnet broadcast)
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The subnet mask is created by using binary ones in the host octet
or octets. The subnet octet or octets are determined by adding the
position value of the bits that were borrowed. If three bits were
borrowed, the mask for a Class C address would be 255.255.255.224.
This mask may also be represented, in the slash format, as /27. The
number following the slash is the total number of bits that were used
for the network and subnetwork portion. 
By consulting the row titled ”Usable hosts”, the chart indicates that
for 30 usable hosts three bits are required. The chart also shows that
this creates six usable subnetworks, which will satisfy the
requirements of this scheme. The difference between usable hosts and
total hosts is a result of using the first available address as the ID
and the last available address as the broadcast for each subnetwork. The ability to use these subnetworks is not provided with
classful routing. However, classless routing, which will be covered
later in the course can recover many of these lost addresses. 
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