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| vschristopher 2002-11-12, 8:53 am |
| can anyone show me an easy method of calculating VLSM, the Cisco Press book for Diane and Catherine lists the cumbersome method. | |
| ssimpson53 2002-11-12, 5:51 pm |
| There is no easy way to calculate VLSMs. By calculate I guess you want to know the number of hosts and network IDs. You really need to understand the binary conversion process. In any given octet bit 1 = 128 bit 2=64 bit 3=32 bit 4=16 bit 5=8 bit 6=4 bit 7=2 bit 8=1 and also remember that two of the possible hosts are taken by the broadcast and network ID. So if you need to know how many hosts are in a 255.255.255.248 subnet or /29, you know that 248 = 11111000, so all you need to look at are the last three bits. How many possibilities are there with three bits?
000 (through this one out for Network ID)
001
010
011
100
101
110
111 (through this one out for Broadcast)
You are left with 6 possible Host IDs.
Hope this helps. It is much easier once you figure out the binary conversion. | |
| pseudocyber 2002-11-13, 8:48 am |
| Writing down the binary possibilities works ... but if you get beyond 2^3 it really gets to be a pain.
You can get the same answer with 2^N-2 where N is the number of bits left for hosts, or the number of bits borrowed from the hosts to subnet - if you're trying to figure out subnets.
Therefore, in the above example, you have 3 bits left for hosts - so 2^3-2=6.
HTH! | |
| vschristopher 2002-11-14, 9:44 am |
| thank you friends for helping me out, i now fully understand the VLSM part, hey one more question, shall we go for 2^-2 or just 2^ in CCNP because of IP subnet zero , what do u say ? | |
| pseudocyber 2002-11-21, 8:33 am |
| This is one of great debate. Most people agree that in CCNA exams, forget about IP subnet zero. In CCNP exams, it's something to take into account - especially if it's mentioned on the particular exam.
It depends.  | |
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| darthfeces 2002-11-25, 12:28 pm |
| suresh did you get your # ? | |
| SureshHomepage 2002-12-02, 1:56 am |
| Well man I got it a while ago....on October last month. I managed to pass on my 2nd go!
when is your turn darthfeces? | |
| darthfeces 2002-12-02, 9:53 am |
| good job
i'm going in may or june 03. | |
| dbowen 2002-12-02, 8:33 pm |
| This Cisco page is helpful.
http://www.cisco.com/en/US/tech/tk6...
80093f18.shtml
However, unless a question mentions 'ip subnet-zero' or the IOS release, I'd stick with (2^N - 2).
Also, VLSM (or supernetting) is just pattern matching subnet bits so you can address/advertise a bunch of subnets at one time.
Here's an easy example:
Say you have 200.200.200.NNNHHHHH/27
Here's your possible subnet id's:
(I'll use the all-zeros and all-ones subnets)
sub 0 -> 000
sub 1 -> 001
sub 2 -> 010
sub 3 -> 011
sub 4 -> 100
sub 5 -> 101
sub 6 -> 110
sub 7 -> 111
I could advertise only subnets 0 thru 3 as 200.200.200.0/25 by shrinking back the subnet mask two places (because the first subnet bit is the same in all four subnet id's).
I could advertise only subnets 0 and 1 as 200.200.200.0/26 by shrinking back the subnet mask one place (because the first two subnet bits are the same in both subnet id's).
I could advertise only subnets 2 and 3 as 200.200.200.64/26 by shrinking back the subnet mask one place (because the first two subnet bits are the same in both subnet id's).
I could advertise only subnets 4 and 5 as 200.200.200.128/26 by shrinking back the subnet mask one place (because the first two subnets bits are the same in both subnet id's).
I could advertise only subnets 6 and 7 as 200.200.200.192/26 by shrinking back the subnet mask one place (because the first two subnet bits are the same in both subnet id's).
I could advertise only subnets 4 thru 7 as 200.200.200.128/25 by shrinking back the subnet mask two places (because the first subnet bit is the same in all four subnet id's).
Basically, shrink back the subnet mask until you define the group of subnets you want to supernet. Don't forget to decrease your total network mask by this same number. |
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