VLSM
VLSM - A Very Brief History
You read in the subnetting section that we were
about to run out of IP addresses. One way to preserve IP
addresses was to allocate certain ranges of IP address which can be
used for free on your network so long as you don't use them to get
out to the internet. If you want to get out to the internet
using them then your router can just swap it for a routable address
using a technique called Network Address Translation (NAT).
Free addresses to use are:
10.0.0.0 - anything starting with a 10
172.16.0.0 to 172.31.0.0 - inclusive
192.168.0.0 - any address starting with these two
numbers
Using VLSM
Say you are using the following number:
192.168.1.0 which if you have read the subnetting
section you know lets you use 254 hosts on your network. If
you only have 20 hosts on your network it means you have wasted 204
addresses. Now imagine if you are paying for each block of
addresses, you are going to waste a huge amount of money as well.
VLSM lets you cut your network into smaller chunks
which are called subnets. All you have to do it add some
numbers to the end of your subnet mask and your network will see
that you are subnetting it down. Please see the easy way to
subnet section for more information on how to actually do it.
How VLSM Works
If you change your subnet mask from 192.168.1.0
255.255.255.0 to 255.255.255.224 your network device will see the
change and let you break down your network into smaller parts.
That actual number tells the network to chunk your network down into
subnet blocks of 32. If you use the subnetting cheat chart you
will see how this actually works.
The subnet mask 255.255.255.224 gives you 8
subnets with 32 available addresses. Each subnet must start
with a subnet number and end with a broadcast number so technically
that leaves you with 30 host addresses per subnet. This is
still plenty if you only needed 20 per subnet.
We have gone from one subnet with 255 addresses:
To 8 subnets with 32 IP addresses per subnet.
Click
here to enlarge
A RFC (request for comment) was submitted which
suggested that we could allocate numbers to hosts on networks
instead of hostnames. The current implementation of this
scheme is IP version 4 although IPv6 is now replacing this.
The idea is that every address would be made up
from four groups of eight binary numbers. Each group of eight
binary numbers is known as an octet. Because we struggle to
write numbers out in binary we usually convert them into decimal but
computers and network devices still see all numbers in binary
because they can only recognise on and off signals so either a 0 or
a 1.
If you want to see how to apply this to a network
then please read the easy way to subnet. There is a lot more
in Subnetting Secrets. More than we could fit on one web site
in face.
Or call us on the toll free numbers at the top.
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