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Home > Archive > 70-216 > January 2002 > subnetting/supernetting
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subnetting/supernetting
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| 3pinhead 2002-01-27, 8:55 pm |
| Any good resources available on the web to explain ? having a bit of a problem with this one.
thanks | |
| 3pinhead 2002-01-27, 8:57 pm |
| well that'll teach me to read before posting!
I'm having a problem with supernetting, and am looking for a way to figure it out quickly.
Thanks | |
| wbafrank 2002-01-27, 9:12 pm |
| Basic Supernetting Example
In subnetting you create additional networks by moving some bits from the "host" portion of an address into the network portion of the address. Supernetting is just the opposite. You merge distinct networks by moving bits from the network portion of the address into the host portion of the address. To see how this works, let's just walk through a simple example.
Here are your 4 "class c" addresses:
192.168.0.0 = Sub1
192.168.1.0 = Sub2
192.168.2.0 = Sub3
192.168.3.0 = Sub4
You're goal is to make these networks all appear as one. Well, first ask yourself "what makes them appear distinct to the computer?" You probably already know that when one host wants to send a packet to another host it first compares the destination IP address with it's own, using the subnet mask to determine if the network is the same or different. It's the subnet mask that really makes the networks distince. The computer compares those bits in each address that correspond to "1"'s in the mask. If those bits are the same, then the network to which the addresses belong is the same. Take a look at those addresses above in binary. I'll also throw in the subnet mask for the comparison.
11000000 . 10101000 . 00000000 . 00000000 = Sub1
11000000 . 10101000 . 00000001 . 00000000 = Sub2
11000000 . 10101000 . 00000010 . 00000000 = Sub3
11000000 . 10101000 . 00000011 . 00000000 = Sub4
11111111 . 11111111 . 11111111 . 00000000 = Mask
Note that the subnet mask spans 24 bits. Now look at those 24 bits in each network (host bits have been greyed). Notice that they are each different and hence each network is different. But look closely at just how similar they actually are. The first 22 bits in each network is actually the same. So, how might you trick the computer into thinking these networks are actually the same? You change the subnet mask! Make it a bit shorter, so that it only spans those first 22 bits:
11000000 . 10101000 . 00000000 . 00000000 = Sub1
11000000 . 10101000 . 00000001 . 00000000 = Sub2
11000000 . 10101000 . 00000010 . 00000000 = Sub3
11000000 . 10101000 . 00000011 . 00000000 = Sub4
11111111 . 11111111 . 11111100 . 00000000 = Mask
Now compare the addresses against the new subnet mask. They are identical for bits set to "1" in the mask. When the hosts now do thier comparisons they will no longer see 4 networks but rather 1 large network. You've just "supernetted" these networks! Your "new" network address is 192.168.0.0 with a subnet mask of 255.255.252.0.
You will note that this technique requires the original network addresses to be contiguous. In other words, you couldn't simply take two class C addresses such as 192.168.32.0 and 192.168.63.0 and expect to supernet them without also supernetting all networks in between. You also don't have complete control over how many subnets your merge together. For instance, in the above example, what would happen if we decreased the subnet mask by yet another bit? Take a look:
11000000 . 10101000 . 00000000 . 00000000 = Sub1
11000000 . 10101000 . 00000001 . 00000000 = Sub2
11000000 . 10101000 . 00000010 . 00000000 = Sub3
11000000 . 10101000 . 00000011 . 00000000 = Sub4
11111111 . 11111111 . 11111000 . 00000000 = Mask
That additional bit would actually cause our hosts to also consider networks 192.168.4.0, 192.168.5.0, 192.168.6.0, and 192.168.7.0 to have been merged in with the others. So, when supernetting, the number of contiguous networks being merged must be a power of 2.
The idea aggregating blocks of addresses, by the way, is part of Classless Internet Domain Routing, or CIDR. It makes routing tables more efficient. For instance, consider you have 256 class C addresses. Traditionally this would mean 256 entries in a route table. But with CIDR this can be reduced to one single entry by having the router look at the entire block as one merged, supernetted network.
CIDR also means that the "class" of an address isn't really that important (hence the name). Rather the specific number of bits being masked is what realy matters. As such, you will commonly see a different notation for subnet masks being used when dealing with CIDR. The notation simply appends a "/" and the number of masked bits to the address. Our new network above, for instance, would be 192.168.0.0/22. It's a good idea to get familiar with this notation. While you won't see on the Microsoft exam, it is quickly becoming the "standard" method of writing IP addresses.
 Phew, this must be my biggest post!! | |
| fizz85 2002-01-27, 10:11 pm |
| Hey thanks alot Frank I did not write the question but you helped me alot. You bridged a gap for me. But could you be abit more specific in this part:
That additional bit would actually cause our hosts to also consider networks 192.168.4.0, 192.168.5.0, 192.168.6.0, and 192.168.7.0 to have been merged in with the others. So, when supernetting, ? The number of contiguous networks being merged must be a power of 2.
I am tring to hash this out in my head so please be patient. I hope I am not making too much out of this.
So are you saying you must use an even amount of networks. I understand that they must be contiguous. Power of 2? Divisible?, sqrt? exponential?
Thanks in advance | |
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| fizz85 2002-01-27, 11:16 pm |
| OK
So I think the trick is you can only use the # of bits that add up correctly (or over) as in your example since we only have to add up to 3 in binary you would only use the first two bits.
The number of of bits (added up in binary) must be greater then the number in the address being covered. Because if the mask is not large enough to cover the network ID it won't work.
Am I correct? | |
| 3pinhead 2002-01-28, 8:12 pm |
| Thanks wbafrank,
I found the article you posted that evening, and have been reading it through, very helpful.
Also found www.learntosubnet.com which was outstanding. Ordered the CD too, (couldn't beat the price)by the time I'm done, I'll be able to do this in my head.
thanks again! | |
| wbafrank 2002-01-28, 9:21 pm |
| quote:
Originally posted by 3pinhead
Thanks wbafrank,
I found the article you posted that evening, and have been reading it through, very helpful.
Also found www.learntosubnet.com which was outstanding. Ordered the CD too, (couldn't beat the price)by the time I'm done, I'll be able to do this in my head.
thanks again!
Sorry I didn't get back to your other post, and glad to see you found the answer saves me answering it!!
Learn to Subnet is a good product even if you don't want to buy the CD!! | |
| iceman2001 2002-01-29, 5:00 am |
| Thanks wbafrank,
Your post was very interesting,learnt something there in a few minutes.
thanks
iceman |
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