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Route Aggregation and supernetting


Trying to figure out the differences between route aggregation and supernetting.

Say the network 172.16.10.0/24 is running out of space. So I change the mask to /23
172.16.10.0/23=510 hosts

The "changing" of the subnet mask to a smaller one, is called route aggregation..right?
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Now..................

172.16.10.0  with a 255.224.0.0 mask would be supernetting right? Since we are using *less* than the default class B mask?

Hope this makes sense.
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dissolved
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dissolved
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4 Solutions
 
gpriceeeCommented:
Hi.  Aggregation is the combining of addresses into less paths.  See the following Cisco article:
http://www.cisco.com/warp/public/459/aggregation.html

Supernetting does include dropping bits from the mask.  See the following link about supernetting:
http://www.gtoal.com/subnet.html
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CajunBillCommented:
gpricee is correct in the one-liner statements above.
Let me just add the clarification that supernetting has nothing to do with the "default" class masks.
In general, those default masks are not really used any more in the Internet.
For quite a few years now what has been used is CIDR (Classless Routing).
The only place that classes show up these days is in older, less capable routing protocols like RIP.
HTH
Bill
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ccomleyCommented:
It's two names for the same thing, really.

If you're managing routers, increasing the "chunk" size and combining two or more smaller networks into one large one means you can write fewer lines of config, and need to store fewer routes in the memory of your router. Often just done for convenience, but the world's biggest route aggregation exercise was the introduction of CIDR, and that was essential coz lots of people's routers were simply running out of memory for BGP routing tables. (Also because the "lost" IP space that was (potentially) hidden in all the badly-sized assignments was needed as 'net use grew).

If you're managing a LAN you increase the chunk size so you can assign enough IP addresses for all your computers without having to set up different subnets and route between them! :-)

 
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dissolvedAuthor Commented:
my cisco book defined it in an example, the example was something like:

" Let's say we have 172.16.10.0/24 and we need more host addresses.  Currently /24 is only giving us 254. So we change the subnet mask to something smaller. 255.255.254.0  172.16.10.0/23. This gives us 510 hosts. This is called route aggregation.  If we were to decrease the prefix of the default subnet mask, then this would be called supernetting."

That's where I got the idea from (sybex book by the way). Any ideas?
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CajunBillCommented:
OK, what Sybex is saying is this:
when they change the subnet mask from /24 to /23, the address range changes from
172.16.10.0 - 172.16.10.255
to
172.16.10.0 - 172.16.11.255

It is poorly worded but maybe they want supernetting to mean changing from
172.16.10.0
to
172.16.0.0

You other guys, any other ideas about this?
CajunBill
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harbor235Commented:
Aggregation is the process of summarizing your routes into the smallest prefix possible to reduce the number of prefixes you advertise.

For example lets say you have the block 110.110.0.0/16:  In your organization you can break that /16 into smaller networks for use as you see fit. Some /24 for LANs, /30s for point to points, whatever. However, you will only advertise the aggregate route of 110.1110.0.0/16 externally. This type of design takes into account sound addressing schemes to make your routing advertisements very efficent. It reduces the number of prefix's you advertise to a single prefix (if contigous)

Supernetting and CIDR (Classless inter domain routing) are the same. Networks are now not bound by their classful definitions, any block could be broken into smaller address blocks. Prefix's can be of any length (mask), this is supernetting.


Here is a doc:
http://www.cisco.com/en/US/about/ac123/ac147/ac174/ac183/about_cisco_ipj_archive_article09186a00800c83c5.html


harbor235
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dissolvedAuthor Commented:
I see, so aggregation is simply route summarization to the classful boundy.  You will always advertise the IP address with it's default subnet mask?????????

supernetting breaks this rule ?

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harbor235Commented:
Aggregation does not mean to the classful boundary, but to the smallest prefix possible.

harbor235
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CajunBillCommented:
Hold on there !
While harbor gave a good reference, it is quite old.
In current pratice it seems to me that these terms are used a little differently.
Real network professionals that I know do NOT think of supernetting and CIDR as the same thing.

In any case, you really shouldn't worry so much about the differences between these terms - you have the basic idea straight behind all of them.

If you are studying to take a test such as for CCNA - don't worry about it, you've got the idea well enough.
CajunBill
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harbor235Commented:
>Real network professionals that I know do NOT think of supernetting and CIDR as the same thing.

You cannot have supernetting without CIDR.

Here is another reference:
http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci854054,00.html
http://www.networkdictionary.com/networking/cidr.php
http://business.cisco.com/glossary/tree.taf-asset_id=92888&word=99303&public_view=true&kbns=2&DefMode=.htm

A CIDR block or a SUPERNET, they are the same thing,

harbor235
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CajunBillCommented:
Hi dissolved, I hope you got the point:
the differences between these terms are not worth worrying about at this point in your career.

Just remember that the key to CIDR is the "C" for "classless" - it removes the need to care about A, B, C, etc.
So the internet can handle a network specification like 172.16.10.0/30

To me, a Supernet implies that you had multiple subnetworks to clump together, such as
172.16.10.0/30 and 172.16.128.0/30
You could supernet those on a classful boundary, such as 172.16.0.0/16, even if you were using a routing protocol that cannot handle classless addressing.
Although I suppose you COULD say that 172.16.0.0/16 is a supernet of 172.16.128.0/30

Anyway, don't get hung up on it.
Good luck!
CajunBill
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dissolvedAuthor Commented:
hmmm  so are route summarization and route aggregation the same thing?  Except route summarization generally means summarizing to a classful boundary.  Where Route aggregation (aka supernetting?) does NOT have to be classful???
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harbor235Commented:
Sounds good!  :}

harbor235
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dissolvedAuthor Commented:
thanks guys
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dissolvedAuthor Commented:
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