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Lab
Activity 9.2.1: Creating subnets
The table shows an
example, which is covered in this activity.
Host
IP Address
|
Class
|
Subnet
Mask
|
No.
of
Subnet
Bits
|
Maximum
#
of Subnets
s
=
sn –
2
|
Ordinal
Number of this Subnet
|
138.101.114.250
|
|
255.255.255.192
|
|
|
|
|
|
|
Subnet
Address of This Subnet or Wire
|
Range
of Host Addresses
For
this subnet
|
Broadcast
Address of This Subnet
|
|
|
|
Step 1: Translate Host IP Address and Subnet Mask into
binary notation.
|
138. |
101. |
114. |
250 |
IP Address |
10001010 |
01100101 |
01110010 |
11111010 |
Mask |
11111111 |
11111111 |
11111111 |
11000000 |
|
255. |
255. |
255. |
192 |
Step 2: Determine the
Network (or Subnet) where this Host address lives:
-
Draw a line under the
mask
-
Perform a bit-wise AND
operation on the IP Address and the Subnet Mask
Note: 1 AND 1 results in a 1, 0 AND anything results in a 0
-
Express the result in
Dotted Decimal Notation
-
The result is the
Subnet Address of this Subnet or "Wire" which is
138.101.114.192
|
138.
|
101.
|
114.
|
250
|
IP
Address
|
10001010
|
01100101
|
01110010
|
11111010
|
Mask
|
11111111
|
11111111
|
11111111
|
11000000
|
Network
|
10001010
|
01100101
|
01110010
|
11000000
|
|
138
|
101
|
114
|
192
|
Step 3: Determine which
bits in the address contain Network information and which contain Host
information:
- Draw the "Great Divide"
(G.D) as a wavy line where the 1's in the Default Subnet Mask
would end for this class address (if no subnetting occurred). In
our example, the IP Address is a Class B address, and so the
Default Subnet Mask is 255.255.0.0.
- Draw the "Small Divide"
(S.D.) as a straight line where the 1's in the given mask
actually end. The network information ends where the 1's in the
mask end
- The result is the
"Number of Subnet Bits" may be determined by
simply counting the number of bits between the G.D. and S.D.,
which in this case is 10 bits.
Step 4: Determine bit
ranges that are for subnets and for hosts:
- Label the "subnet counting
range" between the G.D. and the S.D. (these are the bits
that are being incremented to make the subnet numbers or
addresses).
- Label the "host counting
range" between the S.D. and all of the way to the end on
the right (these are the bits that are being incremented to make
the host numbers or addresses).
Step 5: Determine the
range of host addresses available on this subnet, and the broadcast
address on this subnet:
- Copy down all of the network/subnet
bits of the Network Address (i.e. all bits before the S.D.)
- In the host portion (to the right of
the S.D.) make the host bits all 0's except for the right most bit
(or least significant bit), which you make a 1. This gives you the
first Host IP Address on this subnet, which is the first part of
the result for "Range of Host Addresses for This
Subnet," or in our example 138.101.114.193.
- Now, in the host portion (to the
right of the S.D.) make the host bits all 1's except for the right
most bit (or least significant bit), which you make a 0. This
gives you the last Host IP Address on this subnet, which is the
last part of the result for "Range of Host Addresses for This
Subnet," or in our example 138.101.114.254.
- In the host portion (to the right of
the S.D.) make the host bits all 1's. This gives you the Broadcast
IP Address on this subnet. This is the result for "Broadcast
Address of This Subnet," or in our example 138.101.114.255.
Step 6: Determine the
maximum number of subnets and usable subnets:
The maximum number of subnets is determined by how many bits are
in the subnet counting range (in this example, 10 bits).
Two methods:
Exponents
- Use the formula 2n - 2, where n is the number of
bits borrowed form the host field.
- 210 = 1024.
- Subtract 2 for the number of usable subnets.
1024 - 2 = 1022
Charting
1024 |
512 |
256 |
128 |
64 |
32 |
16 |
8 |
4 |
2 |
1 |
0 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
- Make 10 columns for the 10 subnet bits
- Convert from binary to decimal.
This produces the mask address. 1023 in our example.
- Subtract 2 for the number of usable subnets
Put the result in "Maximum
# of Subnets s = sn - 2," where n is the total number of
subnets and s is the number of usable subnets. In our example 1022
= (1024 - 2)
Step 7: Determine the ordinal
number of this subnets (i.e. which one of this subnet, out of the
maximum number of subnets available:
- Write down all of the bits in the
subnet counting range (i.e. between the G.D. and the S.D).
- Convert this number to decimal.
0111001011 = 459
Or use the chart
1024 |
512 |
256 |
128 |
64 |
32 |
16 |
8 |
4 |
2 |
1 |
|
0 |
1 |
1 |
1 |
0 |
0 |
1 |
0 |
1 |
1 |
256 + 128 + 64 + 8 + 2 + 1 = 459
This is the result for the "Number of This Subnet,"
which is the 459th subnet
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