Why are VLSM subnets allocated in a descending order by their block size?

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This is frustrating me to no end. Everybody and their mother on the internet says that when splitting up a network into subnets using VLSM, it is extremely important that they be allocated in a descending order based on the address block size, but nobody seems to be willing to explain **why.**

Get as technical with me as you want.

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Not a very ELI5 question.. but.. here you go.

Because it’s the most efficient way to allocate the IP range and not waste IPs. You read your bit mask from left to right. The higher order bits on the left can contain more IP addresses (specially for class B address spaces). So you allocate the biggest sections first.

It’s like filling a jar with rocks and sand. If you put the rocks in first you still have holes left for your sand to fit in later. If you put the sand in, it takes up too much space compacted at the bottom.

Additionally, having a standard for VLSM helps when you transition new IT staff to your network. There’s no guess work involved and the network layout is pretty easy to figure out on your own.