Add an optional variable to specify a netnum offset

[robgonnella]

Describe the Feature

Allow users to specify an offset for the netnum value when calculating subnet cidr blocks.
e.g.

subnet_names = ["one", "two"]
cidr_block = 10.10.0.0/16
netnum_offset = 1

Which would result in 2 subnets with cidr blocks, 10.10.16.0/20 and 10.10.32.0/20

Expected Behavior

Specifying a netnum offset would offset the call to cidrsubnet by the given value
e.g.

cidrsubnet(var.cidr_block, ceil(log(var.max_subnets, 2)), count.index + var.netnum_offset)

Use Case

This valuable in cases where you want to use available addresses in a cidr block before needing to add an additional cidr block to the vpc.

Describe Ideal Solution

An optional variable (type = number) is added to the module that allows specification of a netnum offset. This offset is then used to offset the netnum value when calculating the subnet cidr block via the call to the cidrsubnet function.

Alternatives Considered

I could just add a new cidr block to the vpc, forego any netnum offset, and calling it a day. This is a totally valid and easy solution, but the addition of an offset doesn't seem too difficult either, and maybe someone else would also find value in this option.

[robgonnella]

After working with this approach a bit more, I realized that in order for the netnum_offset to work as described in the above example, max_subnets will need to be more than what is actually needed. The example above works because the default value for max_subnets is 16, which translates to 4 newbits log(16, 2) = 4.
e.g.

subnet_names = ["one", "two"]
cidr_block = "10.10.0.0/18"
max_subnets = 3 # will cause a newbits value of 2 - ceil(log(3, 2)) = 2
netnum_offset = 1

Should result in 10.10.16.0/20 and 10.10.32.0/20
or...

subnet_names = ["one", "two", "three"]
cidr_block = "10.10.0.0/17"
max_subnets = 5 # will cause a newbits value of 3 - ceil(log(5, 2)) = 3

Should result in 10.10.16.0/20, 10.10.32.0/20, and 10.10.64.0/20

I realize this is a bit ugly so maybe there is a better solution? Maybe whenever netnum_offset is specified the cidr calculation becomes cidrsubnet(var.cidr_block, local.<public_count | private_count>, count.index + var.netnum_offset) instead of using ceil(log(var.max_subnets, 2)) ?