private function hostCalculation($format, $separator = '')
    {
        $network_quads = [
            sprintf("$format", $this->quads[0] & ~($this->subnet_mask >> 24)),
            sprintf("$format", $this->quads[1] & ~($this->subnet_mask >> 16)),
            sprintf("$format", $this->quads[2] & ~($this->subnet_mask >>  8)),
            sprintf("$format", $this->quads[3] & ~($this->subnet_mask >>  0)),
        ];

        return implode($separator, $network_quads);
    }