Supernetting/CIDR
Introduction
Introduction
Supernetting,
also known as Classless
InterDomain Routing (CIDR),
is another awesome subject. It exists thanks to the
wide adoption of the Internet, which lead to the
exhaustion of the available IP Addresses. More
specifically, supernetting was invented in 1993 with
the purpose of extending the 32 bit IP address
lifetime until the adoption of IPv6 was complete.
Putting it as simply as possible, supernets are used
to combine multiple Class C networks into groups,
which the router, in turn, treats as one big
network. It might not seem like a smart thing to do,
but if you look at the picture on a larger scale you
will notice some of the really awesome advantages
this offers.
The creation of
Supernets
is also known as
Address Aggregation.
The Big Picture
Consider this realistic
example: You work for a large ISP with a few hundred
networks to which it provides services like Internet
access, e-mail etc. These networks, which basically
are your ISP's clients, consist of 254 host IPs each
(One full Class C network for each client), and they
each have a permanent connection to your
headquarters via ISDN (represented by the
yellow lines)
and from there your ISP has a
direct connection
to the Internet Backbone.
This diagram shows the example
network we're talking about. Our main focus is the
two routers the ISP has,
Router No.1
and
Router No.2,
because these will be affected
when we supernet the networks.
Routers
No.1
&
No.2
exchange information with each
other and update their tables, which contain the
networks they know about.
Router 2
connects
directly
to 10 networks
and needs to let
Router 1
know about each one of them.
Router 1
in turn will also advertise
these networks to the
Internet Backbone Router
so it too will know about
these networks.
The above setup requires that
Router No.1
and the
Internet Backbone Router
each have more than 13 separate entries in their
routing tables to make sure that each network is
accessible from them. This is not so bad for this
example, but try to imagine the problems and the
complexity of a similar setup where you have
thousands of networks, where the routing tables
would be enormous ! Also, you should keep in mind
that the larger the routing table, the more work the
router needs to do because it has a huge table of
routes to maintain and look through all the time.
By using
Supernetting,
we could supernet the whole network so it appears to
the Internet as follows:
You can clearly see that all
the clients' networks have been combined into one
big network. Even though
Router No.1
and the
Internet Backbone router
see only one big
network, Router No.2
knows all about the smaller Class C networks since
it is the one "hiding" them from the rest of the
world and makes sure it sends the correct data to
each network.
We are going to look at a more detailed example
later on so we can understand exactly how
supernetting works.
NOTE
There are some limitations with Supernetting - this
is why there is a rule which we must follow so we
don't bump into big routing problems and upset the
network. We will have a closer look at the rule on
the next page.
The reason for evolution
Supernetting has become very popular and there are a
lot of reasons why:
-
Class B network address space has nearly been
exhausted
-
A small percentage of class C network addresses
have been
assigned to networks
-
Routing tables in Internet routers have grown to
a size beyond
the ability of software and people to
effectively manage
-
The 32-bit IP address space will eventually be
exhausted
How
Supernets work
If you understand how
Subnetting
works, then you will surely
understand Supernetting.
Supernets are the opposite of Subnets in that they
combine multiple Class C networks into blocks rather
than dividing them into segments.
When
Subnetting,
we borrow bits from the
Host ID
portion, which increases the
number of bits used for the
Network ID
portion. With
Supernetting
we do exactly the opposite,
meaning we take the bits from the
Network ID
portion and give them to the
Host ID
portion, as illustrated in the picture below:
The next page deals with a
detailed example to give you an in-depth analysis of
Supernetting.
The main concept you need to understand is that
Supernetting
is all about combining
multiple Class C networks into one or more groups
and it does this by taking bits from the
Network ID
portion and, by doing so, the
bits assigned to the
Host ID portion
increase.
I think that's a pretty good
introduction to
Supernetting :)
Let's take a look at an
example to see and understand how exactly
Supernetting
works in practice.
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