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. 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. 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. 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. The method that was used to create the
subnet chart can be used to solve all subnetting problems. This
method uses the following formula: 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) (2 power of borrowed bits) – 2
= usable subnets (23) – 2
= 6 Number of usable hosts= two to the power of
the bits remaining, minus two (reserved addresses for subnet id
and subnet broadcast) (2 power of remaining host bits)
– 2 = usable hosts (25)
– 2 = 30 Web Links IP
Addressing Subnetting Tutorial http://www.ralphb.net/IPSubnet/
Content 10.3 The Mechanics of
Subnetting 10.3.4 Applying the subnet mask
Once the subnet mask has been established it then can be used
to create the subnet scheme. The chart in the Figure is an
example of the subnets and addresses created by assigning three
bits to the subnet field. This will create eight subnets with
32 hosts per subnet. Start with zero (0) when numbering
subnets. The first subnet is always referenced as the zero
subnet. When filling in the subnet chart three of the fields
are automatic, others require some calculation. The subnetwork
ID of subnet zero is the same as the major network number, in
this case 192.168.10.0. The broadcast ID for the whole network
is the largest number possible, in this case 192.168.10.255.
The third number that is given is the subnetwork ID for subnet
number seven. This number is the three network octets with the
subnet mask number inserted in the fourth octet position. Three
bits were assigned to the subnet field with a cumulative value
of 224. The ID for subnet seven is 192.168.10.224. By inserting
these numbers, checkpoints have been established that will
verify the accuracy when the chart is completed. When
consulting the subnetting chart or using the formula, the three
bits assigned to the subnet field will result in 32 total hosts
assigned to each subnet. This information provides the step
count for each subnetwork ID. Adding 32 to each preceding
number, starting with subnet zero, the ID for each subnet is
established. Notice that the subnet ID has all binary 0s in the
host portion. The broadcast field is the last number in each
subnetwork, and has all binary ones in the host portion. This
address has the ability to broadcast only to the members of a
single subnet. Since the subnetwork ID for subnet zero is
192.168.10.0 and there are 32 total hosts the broadcast ID
would be 192.168.10.31. Starting at zero the 32nd sequential
number is 31. It is important to remember that zero (0) is a
real number in the world of networking. The balance of the
broadcast ID column can be filled in using the same process
that was used in the subnetwork ID column. Simply add 32 to the
preceding broadcast ID of the subnet. Another option is to
start at the bottom of this column and work up to the top by
subtracting one from the preceding subnetwork ID. Web
Links IP Addressing Subnetting Tutorial
http://www.ralphb.net/IPSubnet/
Content
10.3 The Mechanics of Subnetting
10.3.5 Subnetting Class A and B networks The
Class A and B subnetting procedure is identical to the process
for Class C, except there may be significantly more bits
involved. The available bits for assignment to the subnet field
in a Class A address is 22 bits while a Class B address has 14
bits. Assigning 12 bits of a Class B address to the subnet
field creates a subnet mask of 255.255.255.240 or /28. All
eight bits were assigned in the third octet resulting in 255,
the total value of all eight bits. Four bits were assigned in
the fourth octet resulting in 240. Recall that the slash mask
is the sum total of all bits assigned to the subnet field plus
the fixed network bits. Assigning 20 bits of a Class A address
to the subnet field creates a subnet mask of 255.255.255.240 or
/28. All eight bits of the second and third octets were
assigned to the subnet field and four bits from the fourth
octet. In this situation, it is apparent that the subnet mask
for the Class A and Class B addresses appear identical. Unless
the mask is related to a network address it is not possible to
decipher how many bits were assigned to the subnet field.
Whichever class of address needs to be subnetted, the following
rules are the same: Total subnets = 2 to the power of
the bits borrowed
Total hosts= 2 to the power of the
bits remaining
Usable subnets = 2 to the power of
the bits borrowed minus 2
Usable hosts= 2
to the power of the bits remaining minus 2 Lab
Activity Lab Exercise: Basic Subnetting This lab is to
identify reasons to use a subnet mask and to distinguish
between a default subnet mask and custom subnet mask. Lab
Activity Lab Exercise: Subnetting a Class A Network This
lab is to analyze a Class A network address with the number of
network bits specified in order to determine subnet mask,
number of subnets, hosts per subnet, and information about
specific subnets. Lab Activity Lab Exercise: Subnetting
a Class B Network This lab is to provide a subnetting scheme
using a Class B network. Lab Activity Lab Exercise:
Subnetting a Class C Network This lab is to provide a
subnetting scheme using a Class C network. Web Links IP
Addressing Subnetting Tutorial http://www.ralphb.net/IPSubnet/
Content 10.3 The Mechanics of
Subnetting 10.3.6 Calculating the resident
subnetwork through ANDing Routers use subnet masks to