Information Technology Reference
In-Depth Information
Heavily subnetting a network results in the loss of a progressively greater
percentage of addresses to the network number, broadcast address, and
router address.
The second subnet would have the network number 200.211.192.128, router address
200.211.192.129, and broadcast address 200.211.192.255.
why It's best to use routers That support subnet 0
When subnetting a Class C network using the method in Table 8.5, if you use the 2 x - 2
calculation, the subnet 128 in the table doesn't make sense. It turns out that there's a
legitimate and popular reason to do it this way, however.
Remember that using subnet 0 is not allowed according to the RFC standards, but by
using it you can subnet your Class C network with a subnet mask of 128. This uses
only 1 bit, and according to your calculator 2 1 - 2 = 0, giving you zero subnets.
By using routers that support subnet 0, you can assign 1-126 for hosts and 129-254
for hosts, as stated in the table. This saves a bunch of addresses! If you were to stick
to the method defined by the RFC standards, the best you could gain is a subnet
mask of 192 (2 bits), which allows you only two subnets (2 2 - 2 = 2).
Determining the Subnet Numbers for a Class C Subnet
The first subnet always has a 0 in the interesting octet. In the example, it would be
200.211.192.0, the same as the original nonsubnetted network address. To determine the
subnet numbers for the additional subnets, first you have to determine the incremental value:
1.
Begin with the octet that has an interesting value (other than 0 or 255) in the subnet
mask. Then subtract the interesting value from 256. The result is the incremental value.
If again you use the network 200.211.192. x and a mask of 255.255.255.192, the
example yields the following equation: 256 - 192 = 64. Thus, 64 is your incremental
value in the interesting octet—the fourth octet in this case. Why the fourth octet?
That's the octet with the interesting value, 192, in the mask.
2.
To determine the second subnet number, add the incremental value to the 0 in the
fourth octet of the first subnet.
In the example, it would be 200.211.192.64.
3.
To determine the third subnet number, add the incremental value to the interesting
octet of the second subnet number.
In the example, it would be 200.211.192.128.
4.
Keep adding the incremental value in this fashion until you reach the actual subnet
mask number.
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