8/8/2019 Calculating Subnet Masks
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Calculating Subnet Masks
By Joshua Erdman
Digital Foundation, inc.
Subnet Masks in Binary
The easiest way to explain a subnet mask is by looking at the IP
address and subnet mask in its
binary format. Before you continue, be sure you have a good grip
on counting with binary, wehave provided a quickbinary primerif you
need a refresher. If you do not care for the intricacies
of calculating a subnet mask and just need to know the correct
mask for your situation, we haveprovided a reference table.
Not to mention you should already be familiar with the IP
address. This article explains the basic
use of an IP address and why we need a subnet mask.
A regular IP address when converted to binary is 32 bits in
length, each segment being 8 bits
long. Refer to ourfirst address example of 63.26.15.5 with a
subnet mask of 255.255.255.0 inbinary.
Network and Host Addresses
In this example, it is only the last segment that changes from
one host to another; this lastsegment is known as the host address;
the first 3 segments, for this example, make up the
network address. If we were to use an address with a subnet mask
of 255.255.0.0 then the first 2
segments would make up the network address and the last two
would be the host address. This isthe kind of effect the subnet
mask has on your IP address. It determines how big your
networkgroup is.
With that said, to go into any deeper detail we must look at the
addressing in binary. Since we
are just dealing with 32 binary bits we can stop grouping them
in sets of 8 bits per segment anddeal with them as a 32 bit string.
Now when you create a mask, it does not have to be limited to
groups of 8 bits (and it can have man more values than 255 or
0). These are called VariableLength Subnet Masks (VLSMs). By
calculating out your VLSM you can create any block of IP
addresses in sizes of powers of 2 minus 2.
For example, We can create a mask that only allows for 14 hosts
(remember 14 hosts is 2 to thepower of 4 then we subtract 2).
Knowing that it is 2 to the power of 4, we know that we have
anaddress block (subnet) with 4 bits for the hosts. That leaves a
remaining 28 bits for the network
address, also known as a /28. Now that we know the size of the
network block we want (14hosts) let's calculate the subnet mask
that we would use for a /28 network.
CLUE: A block of IP addresses is referred to as a subnet.
Because of this that is how subnet
masks got their name, they are key into declaring how large a
subnet actually is.
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First map out your binary numbers again and keep them in groups
of 8 bits each. That makes iteasy to get the decimal number for
each segment. Since our network size is 28 bits long we
represent that with a string of 28 1s and the remainings 4 bits
as 0s. So the first 3 segments (of 8bits each) are all 1s. The
decimal equivalent of an 8 bit segment of all 1s is 255. So the
first 3
segments are 255. That leaves us with 4 bits left in the mask
for the last segment. It will look
something like this:
128 64 32 16 8 4 2 1
1 1 1 1 0 0 0 0
To make it easy, we have included a subnet mask calculator. This
calculator converts decimal IP
addresses and Subnet Masks. You can also optionally enter a
destination IP address to see theresults.
Add up the bits that are flagged with a one and we see that the
last segment of the subnet maskwith 14 hosts ( 16 - 2 ) is 240. For
a subnet mask of 255.255.255.240; That is pretty much it with
creating subnet masks, but now you need to learn a new rule
about addressing.
IP Addresses with VLSMs
When you use a subnet mask of 255.255.255.192 what you have
essentially done is divided up
your last segment into 4 blocks (subnets). Using the binary of
the last segment we can see oursubnet mask only uses the first 2
most significant bits. Recall that the network address is made
up from all the bits that line up with the 1s in the subnet
mask. So in this case the networkaddress 'overflows' into the last
segment because it has two bits available in the last segment.
The
host address has the last 6 bits in the last segment. Let's see
what this subnet masks looks like:
128 64 32 16 8 4 2 1
1 1 0 0 0 0 0 0
First lets say we have a network subnet of 10.10.10.X and we are
using the same subnet mask
255.255.255.192 Our network host (HOST1) is currently using the
address 10.10.10.75 Nowlets display the binary of the last segment
for both the subnet mask and the address of our host.
|
128 64|32 16 8 4 2 1
SM 1 1| 0 0 0 0 0 0
HOST1 0 1| 0 0 1 0 1 1
|
Network Host
Notice the bar I drew in that divides the bits of the last
segment. The last 6 bits on right are the
host bits, we know this because it matches up with all the 0s in
the subnet masks. From the same
logic we know about all the bits for the network masks because
of the same reason, the networkaddress matches up with all the 1s
in the subnet mask.
