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TOPIC: CIDR superneting problem

CIDR superneting problem 10 years 1 week ago #26801

Hello

I was just reading on CIDR and superneting and there is something i dont understand.

The point with superneting is to that several smaller networks kan be addvertised as a "big" network.

But if a packet i headed for one of those networks then that means that the router hiding them will know that the big network is accualy several smaller ones and so all those networks have their own broadcast id and gateway id.

But according to the article thats not the case! Anyone who can explaine.


This is under Networkin-->General-->Superneting-CIDR-->In-Depth Analysis.

At the bottom.


Let me also point out to you (in case you didn't ask yourself :> ) that IP Addresses 203.31.218.255 and 203.31.219.0, which would have been used as the broadcast address for our first old network and the network address of our old second network, are now usuable addresses!

Yes, you can actually assign them to hosts, because we have a Supernet. Now, even though you can use these addresses, I would probably not use them unless I really needed to. Not that it makes a difference, but I always tend to reserve these types of addresses, it's just a habit of mine :)



sorry for the bad english

Re: CIDR superneting problem 10 years 1 week ago #26815

Supernetting is not anything more then increasing your subnetmask, increasing the available hosts on a subnet.

So for:

192.168.0.0 /24 would become for instance
192.168.0.0 /23 (we are 'stealing' 1 bit to increase the ammount of hosts for this subnet)

This would mean the 192.168.0.255 address is not the broadcast address anymore because you have a subnet of :

192.168.1.0 with mask : 255.255.252.0 (11111111.11111111.11111110.00000000)

So your range would be :

192.168.0.0 = Network ID
192.168.1.255 = Broadcast ID

With the /24 subnetmask you would have the 192.168.0.255 as broadcast address which is now a host instead.

[edit mode]
The thing you refer to "having multiple subnets bound to one for your routing" is called summarization.

this means for example :

You have a company with the following subnets :

192.168.0.0/24
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24

You can summarize these subnets into 1 big subnet to make your routing table smaller --> thus faster

The route becomes : 192.168.0.0 /22

Hope this is a bit understandable as my english isn't that good as well :)

Re: CIDR superneting problem 10 years 6 days ago #26846

...
The thing you refer to "having multiple subnets bound to one for your routing" is called summarization.

this means for example :

You have a company with the following subnets :

192.168.0.0/24
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24

You can summarize these subnets into 1 big subnet to make your routing table smaller --> thus faster

The route becomes : 192.168.0.0 /22

Hope this is a bit understandable as my english isn't that good as well :)

Thats what i mean, the router connected to these networks knows that there are 4 diffrent /24 networks but to the out side world it looks like a /22 network. but these network are compeletly diffrent so there are still 4 network adresses and 4 broadcast adresses.



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.

This is what he writes first.

but then he says that the broadcast adress and network adresses in the middle can be assigned to hosts but that would make no sense to "router 2".

Let me also point out to you (in case you didn't ask yourself :> ) that IP Addresses 203.31.218.255 and 203.31.219.0, which would have been used as the broadcast address for our first old network and the network address of our old second network, are now usuable addresses!

Re: CIDR superneting problem 10 years 6 days ago #26848

Note there is a big difference between a real subnet, and your routing table.

For example in our company we have lets say the first 2 octets to define a country.

We use the A subnet, so thats the 10.0.0.0

For example

Germany = 10.49.x.x
England = 10.44.x.x

In germany we have 10.49.1.0/24, 10.49.2.0/24, etc etc etc.

For our main router in England we can have a routing towards 10.49.0.0 /16 instead of having all 10.49.1.0 /24 and 10.49.2.0 /24
Thats what they call 'Summarize'.

It's the same our post company's do.
They first sort mail on country. in the country they will sort on region, city, street, number.

Note that in routing you only define the next hop.
So.

which country? oh.. Germany, than we have to send it to router Germany.

Router Germany accepts the package and looks into the package.

Which region? ah..the south.. then we send this package to south-Germany. etc etce

The thing about Supernetting :

Normally you have for instance a 192.168.0.0/24 SUBNET (not a route.. but a subnet).

Lets say you need 300 hosts on your subnet.
With 192.168.0.0 /24 you CANNOT have 300 hosts... you need to have a different subnet mask for that.

So, the 192.168.0.0 /24, it becomse the 192.168.0.0 /23.

This means you have 1 bit MORE for your hosts.
What would change this :

Previous situation :

192.168.0.0 /24 :

Subnet ID = 192.168.0.0
Available Hosts = 192.168.0.1 - 192.168.0.254
Broadcast Address = 192.168.0.255

Available Hostst = 254

New Situation

192.168.0.0 /23

Subnet ID = 192.168.0.0
Available Hosts = 192.168.0.1 - 192.168.1.254
Broadcast Address = 192.168.1.255

This means, your previous broadcast address of 192.168.0.255 has become a HOST now.

Re: CIDR superneting problem 10 years 6 days ago #26854

  • SteveP
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Nice analogy Chojin. The Summarize bit never really sunk in, but it has now!
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