code: func (r *Range) Release(ip net.IP) error {
  	ok, offset := r.contains(ip)
  	if !ok {
  		return nil
  	}
  
  	return r.alloc.Release(offset)
  }
docs: // Release releases the IP back to the pool. Releasing an
  // unallocated IP or an IP out of the range is a no-op and
  // returns no error.