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Home > Archive > CCNA > October 2003 > Question about VLSM
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Question about VLSM
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| cuestick 2003-10-30, 5:31 am |
| I'm having a difficult time understanding VLSM.
When using VLSM does 2^n-2 still hold true, or would (2^n-2)/2 be more of a realistic way of looking at subnetting the subnets?
Can someone please explain VLSM in a more clearer manor, and show how subnetting the subnets works in binary?
Thanks!! | |
| Demijohn 2003-10-30, 10:29 am |
| quote:
When using VLSM does 2^n-2 still hold true
Yes and No.
For a given subnet with n host bits, there are always 2^n-2 valid host addresses. Given n bits borrowed for subnetting, there are always 2^n-2 valid subnets.
With VLSM, n is a variable, not a constant. With VLSM, you can’t have more than 2^N-2 subnets, where N is the largest value of n = (number of bits borrowed for subnetting) you are using. Of course if you’re actually using VLSM, you’ll end up with fewer than 2^N-2 subnets.
quote:
would (2^n-2)/2 be more of a realistic way of looking at subnetting the subnets?
No. What value would you use for n? n is a variable with VLSM.
With VLSM you need to check if all n host/subnet bits are 1 or 0 for each value of n that you use.
Look at how you might actually use VLSM, given a single class C network 192.168.100.0.
You can start with /26 mask to get 2 subnets of 62 hosts each:
192.168.100.0/26 - invalid
192.168.100.64/26 – valid (engineering department)
192.168.100.128/26 – valid (sales department)
192.168.100.192/26 – invalid
Now take the wasted spaces and carve them up
192.168.100.0/27 – invalid
192.168.100.32/27 – valid (personnel department) 30 valid hosts.
192.168.100.192/27 – valid (reserved) 30 hosts
192.169.100.224/27 – invalid
Keep going. You might need some /30 subnets for serial connections, and a /28 for a server farm:
192.168.100.0/30 – invalid
192.168.100.4/30 – valid (WAN primary) 2 hosts
192.168.100.8/30 – valid (WAN secondary) 2 hosts
192.168.100.12/30 –valid (reserved) 2 hosts
192.168.100.16/28 – valid (server farm) 14 hosts
The 192.168.100.224 – 255 space can be divided up in a similar way if required, or reserved for future needs.
In this example we have 8 valid subnets, including reserved ones, and 2 invalid subnets. | |
| bloodshotx 2003-10-30, 1:06 pm |
| you can use ip subnet zero to utilize the 2 extra addresses. |
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