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TOPIC: doubt on supernetting

doubt on supernetting 9 years 10 months ago #18385

  • leopold
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i need to clear a doubt that i have regarding supernetting.

if we supernet 2 consecutive subnets for example

213.61.218.0 and 213.61.219.0

the network and broadcast ip address for each of these before supernetting would be

213.61.218.0
213.61.218.255

213.61.219.0
213.61.219.255

after supernetting the network and broadcasting ip would be

213.61.218.0
213.61.219.255

hence making 213.61.218.255 and 213.61.219.0 valid and usuable ip addresses

my question is how can we assign these ip addresses to systems on this network especially the 213.61.218.255 would not this be considered as a broadcast ip by the systems on the network and be processed by all or will the datalink layer filter this with the mac address in all systems except the one that is assigned this ip?

hope you have understood my query!
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Re: doubt on supernetting 9 years 10 months ago #18392

  • Smurf
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Hi there,

Its an interesting question and i can see where its come from. The simple answer is the Subnet Mask. The subnet mask is used to determine the whole addressing space for the network. With the example that you have used, this has a subnet mask of 255.255.254.0. You are using the last bit in the 3rd octet to define part of the network range. For this reason you are looking at a network range of 213.61.218.0
213.61.219.255.

The network ID is 213.61.218.0 and the broadcast address for that network is 213.61.219.255. This is all because you have used the next bit to increase the network.

to calculate the number of usable addresses its 2^9 - 2. Therefore you are looking at 510 usable addresses in that network range which uses 213.61.218.255 & 213.61.219.0. (N.B you get the 9 from the subnet, in the 255.255.254.0 we have 9 zeros at the end. 11111111 11111111 11111110 00000000)

The Network ID's are when all the zeros in the subnet mask, match the zeros in the ip address.

e.g.

11010101 00111101 11011010 00000000 = 213.61.218.0
11111111 11111111 11111110 00000000 = 255.255.254.0

As you can see, where the zeros are in the subnet, there are zeros in the ip address

11010101 00111101 11011011 00000000 = 213.61.219.0
11111111 11111111 11111110 00000000 = 255.255.254.0

As you can see, the zeros don't match therefore this is an addressible address space.

11010101 00111101 11011010 11111111 = 213.61.218.255
11111111 11111111 11111110 00000000 = 255.255.254.0

Here you can see that the zeros in the subnet do not correspond to all 1's in the ip address. Therefore this is addressible.

11010101 00111101 11011011 11111111 = 213.61.219.255
11111111 11111111 11111110 00000000 = 255.255.254.0

Here, all the zeros in the subnet do match the 1's in the ip address, therefore this is the broadcast address for 213.61.218.0/23

Hope this makes sense to ya

Cheers

Wayne
Wayne Murphy
Firewall.cx Team Member
www.firewall.cx

Now working for a Security Company called Sec-1 Ltd in the UK, for any
Penetration Testing work visit www.sec-1.com or PM me for details.
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Re: doubt on supernetting 9 years 9 months ago #18404

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hi wayne,

thanks for the reply.

with the example that you gave i understood how the subnet plays a vital part in determining the network and broadcast addresses in regards to supernetting and how the 218.255 and the 219.0 becomes useful ip addresses.

however, since the packet would be first read by the datalink layer would that make any difference in the detection of the packet or is it all left for the network layer.

thanks,
Leens.
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Re: doubt on supernetting 9 years 9 months ago #18407

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Hi there,

Its another good question which you have answered already. Its left to the transport layer (IP - Layer 3). The datalink layer is the layer responsible for communicating using the MAC addresses. Once Leyer 3 determines if its local (utilising the subnet mask) or it it needs routing, the machine knows if it needs to talk to a local machine or to the default gateway. Once thats determined, the host will either ARP (if its not already in its ARP Cache) for the router, then forward the packet at layer 2 using the MAC address thats returned through the ARP (direct to the host or the router).

The ARP process is done using an IP Broadcast packet on the network.

Hope it makes sense for ya.

Cheers

Wayne
Wayne Murphy
Firewall.cx Team Member
www.firewall.cx

Now working for a Security Company called Sec-1 Ltd in the UK, for any
Penetration Testing work visit www.sec-1.com or PM me for details.
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Re: doubt on supernetting 9 years 9 months ago #18410

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Just incase you were wondering :wink: there is also a broadcast MAC address which is FF:FF:FF:FF:FF:FF.

Cheers

Wayne
Wayne Murphy
Firewall.cx Team Member
www.firewall.cx

Now working for a Security Company called Sec-1 Ltd in the UK, for any
Penetration Testing work visit www.sec-1.com or PM me for details.
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