Information Technology Reference
In-Depth Information
Determining Quantities of Subnets and Hosts
The general technique described in the previous section is also useful when trying to
determine the total number of subnets and hosts produced by a given mask with respect to
the default mask of the class of address in question.
For example, consider the Class B address 172.16.0.0 with a subnet mask of
255.255.254.0.
This is a prefix length of 23 bits. When you subtract the default prefix length for a Class
B address of 16 from 23, you get the value 7. Raising 2 to the 7th power results in the value
128, which is the number of subnets you get when you subnet a Class B address with the
255.255.254.0 mask.
Determining the number of hosts available in each of these 128 subnets is simple
because you always subtract the prefix length that the subnet mask produces, 23 in this
example, from the value 32, which represents the total number of bits in any IP address.
The difference, 9, represents the remaining number of 0s, or host bits, in the subnet mask.
Raising 2 to this value produces the total possible number of host IDs per subnet that this
subnet mask allows. Remember to subtract 2 from this result to account for the subnet and
broadcast addresses for each subnet. This gives you the actual number of usable host IDs
per subnet. In this case, this value is 2 9 - 2 = 510.
Repeated practice with this technique will reduce your time to obtain the desired answer
to mere seconds, leaving time for the more challenging tasks in each question. You have
a wealth of examples and scenarios in this chapter, as well as in the review questions, on
which to try your technique and build your trust in this faster method.
Supernetting
Let's take a look at a different type of subnetting. Class B addresses give you 65,534
addresses, but let's say that you have 1,000 users. Would you really need a Class B address?
Not if you use supernetting.
Supernetting allows you to have two or more blocks of contiguous subnetwork
addresses. So what does that actually mean? Class C addresses give you 254 useable
addresses. So if you needed 1,000 users, you could set up supernetting of 4 Class C
addresses that are contiguous.
Example:
192.168.16.0
192.168.17.0
192.168.18.0
192.168.19.0
When you set up supernetting for a Class C, you would use a Class B subnet mask.
When you set up supernetting for a Class B, you would use a Class A subnet mask. This
allows you to use multiple classes to get a larger number of hosts without taking up an
entire class.
So the subnet mask for the above example would be 255.255.252.0 or /22. The reason
we used this subnet mask is because a 252 subnet mask allows for 4 subnets. Each of the
above Class C numbers would equal one subnet on this network.
 
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