Comment 4 for bug 1439870

I guess you are saying - why isn't neutron allocating the ports again of the VMs you are deleting?
When you create a port in neutron - the table neutron.ipavailabilityranges is updated.
Here the range of IPs for your subnet (from which your port would be created) - is updated with new range.
For example - Let's say you had a initial subnet pool = 192.168.1.102 - 192.168.1.110
Now if you create a port out of this pool - the ranges would be updated to: 192.168.1.103 - 192.168.1.110

Your VM ports that have been used but deleted would again be available for reallocation once you have exhausted this pool range. That is - once your available pool with increments for every allocation becomes 0.
This is the time, the pool would be reset and your previously deleted ports would be used.