What is Subnetting/Supernetting?
Note : IP addresses are designed with
two levels of hierarchy.
 |
Addresses in a network with
and without subnetting..
Hierarchy concept in a telephone number
Finding the Subnet Address
Given an IP address, we can find the
subnet address by applying the mask to the
address. We can do this in two ways:
straight or short-cut.
Straight Method
In the straight method, we use binary
notation for both the address and the
mask and then apply the AND operation
to find the subnet address.
Example 1
What is the subnetwork address if the
destination address is 200.45.34.56 and the
subnet mask is 255.255.240.0?
Solution
11001000 00101101 00100010 00111000
11111111 11111111 11110000 00000000
11001000 00101101 00100000 00000000
The subnetwork address is 200.45.32.0.
Short-Cut Method
** If the byte in the mask is 255, copy
the byte in the address.
** If the byte in the mask is 0, replace
the byte in the address with 0.
** If the byte in the mask is neither 255
nor 0, we write the mask and the address
in binary and apply the AND operation
Example 2
What is the subnetwork address if the
destination address is 19.30.80.5 and the
mask is 255.255.192.0?
Solution
See Figure 5.6
a power of 2.
Example 3
A company is granted the site address
201.70.64.0 (class C). The company needs
six subnets. Design the subnets.
Solution
The number of 1s in the default
mask is 24 (class C).
The company needs six subnets. This number
6 is not a power of 2. The next number that is
a power of 2 is 8 (23
). We need 3 more 1s in
the subnet mask. The total number of 1s in
the subnet mask is 27 (24 + 3).
There is 5 , 0s (32 - 27).
The mask is
11111111 11111111 11111111 11100000
or
255.255.255.224
The number of subnets is 8.
The number of addresses in each subnet
is 25
(5 is the number of 0s) or 32.
See Figure 5.8
Example 4
A company is granted the site address
181.56.0.0 (class B). The company needs
1000 subnets. Design the subnets.
Solution
The number of 1s in the default mask is 16
(class B).
The company needs 1000 subnets. This
number is not a power of 2. The next number
that is a power of 2 is 1024 (210). We need 10
more 1s in the subnet mask.
There are 26 , 1s in the subnet mask (16 + 10).
The total 0s is 6 (32 - 26).
The mask is
11111111 11111111 11111111 11000000
or
255.255.255.192.
The number of subnets is 1024.
The number of addresses in each subnet is 2
6
(6 is the number of 0s) or 64.
See Figure 5.9
SUPERNETTING
Rules:
** The number of blocks must be a power of 2 (1, 2,
4, 8, 16, . . .).
** The blocks must be contiguous in the address
space (no gaps between the blocks).
** The first address's third byte in the
superblock must be evenly divisible by the number
of blocks. In other words, if the number of blocks is
N, the third byte must be divisible by N.
Example 5
A company needs 600 addresses. Which of
the following set of class C blocks can be
used to form a supernet for this company?
198.47.32.0 198.47.33.0 198.47.34.0
198.47.32.0 198.47.42.0 198.47.52.0 198.47.62.0
198.47.31.0 198.47.32.0 198.47.33.0 198.47.52.0
198.47.32.0 198.47.33.0 198.47.34.0 198.47.35.0
Solution
1: No, there are only three blocks.
2: No, the blocks are not contiguous.
3: No, 31 in the first block is not divisible by 4.
4: Yes, all three requirements are fulfilled.
In subnetting,
we need the first address of the
subnet and the subnet mask to
define the range of addresses.
In supernetting,
we need the first address of
the supernet
and the supernet mask to
define the range of addresses.
Example 6
We need to create a 16-member supernetwork.
class C blocks. What is the supernet mask?
Solution
We need 16 blocks. For 16 blocks we should
place four 1s to 0s in the default mask. So the
mask is
11111111 11111111 11110000 00000000
or
255.255.240.0
Example 7
A supernet has a first address of 205.16.32.0 and a
supernet mask of 255.255.248.0. A router receives three
packets with the following destination addresses:
205.16.37.44
205.16.42.56
205.17.33.76
Which packet belongs to the supernet?
Solution
The supernet mask is used to see if we can locate
the beginning address.
205.16.37.44 AND 255.255.248.0 205.16.32.0
205.16.42.56 AND 255.255.248.0 205.16.40.0
205.17.33.76 AND 255.255.248.0 205.17.32.0
Only the first address belongs to this supernet.
Example 8
A supernet has a first address of 205.16.32.0 and a
supernet mask of 255.255.248.0. How many blocks are in
this supernet, and what is its address range?
Solution
The supernet has 21 1s. The default mask has 24 1s.
Since the difference is 3, there are 2
3 or 8 blocks in
this supernet. The blocks are 205.16.32.0 to
205.16.39.0. The first address is 205.16.32.0. The
last address is 205.16.39.255.
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