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Allow splitting an existing subnet into smaller subnets

We often split up an existing /24 subnet into smaller subnets to assign different VLANS.  I'd like a way to split an existing subnet into smaller subnets (longer CIDR), with a warning if the reserved broadcast addresses needed for the new subnets are used. 

Example:  Start with a 10.10.10.0/24 and divide into 10.10.10.0/25 and 10.10.10.128/25, verifying that 10.10.10.127 and 10.10.10.128 are unused.