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IP Subnetting Step #4: Determining Subnet Identifiers and Subnet Addresses
(Page 1 of 5)
The network ID assigned to our network
applies to the entire network. This includes all subnets and all hosts
in all subnets. Each subnet, however, needs to be identified with a
unique subnet identifier or subnet ID, so it can be differentiated
from the other subnets in the network. This is of course the purpose
of the subnet ID bits that we took from the host ID bits in
subnetting. After we have identified each
subnet we need to determine the address of each subnet, so we can use
this in assigning hosts specific IP addresses.
This is another step in subnetting
that is not really hard to understand or do. The key to understanding
how to determine subnet
IDs and subnet addresses is to always
work in binary form, and then convert to decimal later. We will also
look at a shortcut for determining addresses in decimal
directly, which is faster but less conceptually simple.
Let's go directly to our examples
to see how subnet IDs and addresses are determined. We number the subnets
starting with 0, and then going to 1, 2, 3 and so on, up to the highest
subnet ID that we need.
Note: I assume in this description that we will be using the all-zeroes and all-ones subnet numbers. In the original RFC 950 subnetting system, those two subnets are not used, which changes most of the calculations below. See here for an explanation.
We determine the subnet IDs and addresses
- Subnet ID: This is just the
subnet number, and can be expressed in either binary or decimal form.
- Subnet Address: This is the
address formed by taking the address of the network as a whole, and
substituting the (binary) subnet ID in for the subnet ID bits. We need
to do this in binary, but only for the octets where there are subnet
ID bits; the ones where there are only network ID bits or only host
ID bits are left alone.
Seem complicated? Let's go back to
our examples and well see that its really not.
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Version 3.0 - Version Date: September 20, 2005
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