Hello-am unsure whether this is valid If I use a class C address and use the first subnet with a /26 mask
You can use a /26, but it has to start at a multiple of 64, so the available ones are .128/26 and .192/26.
I understand-is it possible to start off with a /30 address and then work backwards ? example -- /29 /28 /27 /26
0-3 4-11 12-27 28-59 60-124 ?I am try> >
Yes, that's fine. But that's a /30.
That's not fine. That's a /29 (8 hosts) but it's aligned on a /30 (multiple of 4) boundary. Try 8-15 instead.
That's not fine. That's a /28 aligned on a /30 boundary. Use 16-31
And that's a /27 aligned on a /30 boundary. Use 32-63
That's a /26 aligned on a /30 boundary. Use 64-127
If your subnet size is 2^n then it has to start on a 2^n boundary.
You can carve the space up any way you please as long as you stick to that rule.
There are sixteen ways to carve a /25 up into a /26, a /27, a /28, a /29 and two /30's.
[Where do you want the /26 -- low half 0-63 or high half 64-127? Given that, where do you want the /27 -- low half or high half? Given that, where do you want the /28 -- low half or high half? Given that, where do you want the /29 -- low half or high half? Given that, the remaining space gets split into the two /30's]
Its just clicked now!!
If I start with a /30 then I must use at least 2 subnets (to avoid wastage) before going on to the /29.
Whereas if I start off big -/26 ,I can then "carve up "(nice description) the next subnet.
Many many thanks for clearing that up--I dont work in IT yet ,but in your experience how are subnets usually worked out? Is it Big to Small or vice versa.?
TIA
This may help (there are plenty of similar things around, just that this is the one I made myself).
Sam
Thanks alot for that-very easy to understand. Will be using it for along time to come !
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