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IP Custom Subnet Masks
(Page 1 of 4)
It's important to understand what
default
subnet masks are and how they work. A
default subnet mask doesn't really represent subnetting, however, since
it is the case where we are assigning zero bits to the subnet ID. To
do real subnetting we must dedicate at least one of the
bits of the pre-subnetted host ID to the subnet ID, as shown in the
example in the topic that introduced subnet masks.
Since we have the ability to customize
our choice of dividing point between subnet ID and host ID to suit the
needs of our network, this is sometimes called customized subnetting.
The subnet mask that we use when creating a customized subnet is, in
turn, called a custom subnet mask. The custom subnet mask is
used by network hardware to determine how we have decided to divide
the subnet ID from the host ID in our network.
Deciding How Many Subnet Bits to Use
The key decision in customized subnetting
is how many bits to take from the host ID portion of the IP address
to put into the subnet ID. Recall that the number of subnets possible
on our network is two to the power of the number of bits we use to express
the subnet ID, and the number of hosts possible per subnet is two to
the power of the number of bits left in the host ID (less two, which
I will explain later in this topic).
Thus, the decision of how many bits
to use for each of the subnet ID and host ID represents a fundamental
trade-off in subnet addressing:
- Each bit taken from the host ID for the subnet
ID doubles the number of subnets that are possible in the network.
- Each bit taken from the host ID for the subnet
ID (approximately) halves the number of hosts that are possible within
each subnet on the network.
Subnetting Bit Allocation Options
Let's take a brief example or two
to see how this works. Imagine that we start with a Class B network
with the network address 154.71.0.0. Since this is Class B, 16 bits
are for the network ID (154.71) and 16 are for the host ID. In the default
case there are no subnets (well, one subnet that is the
whole network) and 65,534 hosts total. To subnet this network, we have
a number of choices:
- We can decide to use 1 bit for the
subnet ID and 15 bits for the host ID. If we do this, then the total
number of subnets is 21 or 2: the first subnet is 0 and the
second is 1. The number of hosts available for each subnet is 215-2
or 32,766.
- We can use 2 bits for the subnet ID
and 14 for the host ID. In this case, we double the number of subnets:
we now have 22 or 4 subnets: 00, 01, 10 and 11 (subnets 0,
1, 2 and 3). But the number of hosts is now only 214-2 or
16,382.
- We can use any other combination of
bits that add up to 16, as long as they allow us at least 2 hosts per
subnet: 4 and 12, 5 and 11, and so on.
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Version 3.0 - Version Date: September 20, 2005
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