IPRangeExpander.java

import java.util.Objects;
import java.util.regex.Pattern;

class IPRangeExpander {
    private static final Pattern IPV4_PATTERN = Pattern.compile("^(([1-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\\.){1}(([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\\.){2}([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])$");

    public void expand(String from, String to) {
        if (from == null || to == null) {
            throw new IllegalArgumentException(
                    "Invalid ip range: from " + from + " to " + to);
        }

        long floor = inet_aton(from);
        long ceil = inet_aton(to);
        if (floor > ceil) {
            throw new IllegalArgumentException(
                    "Invalid ip range: from " + from + " to " + to);
        }

        long count = ceil - floor + 1;
        long step, tmp;
        int prefix;
        while (count > 0) {
            step = floor & -floor;
            while (step > count) {
                step >>= 1;
            }

            tmp = step;
            prefix = 33;
            while (tmp > 0) {
                prefix--;
                tmp >>= 1;
            }

            System.out.println(inet_ntoa(floor) + "/" + prefix);

            count -= step;
            floor += step;
        }
    }

    public static long inet_aton(String ipv4) {
        Objects.requireNonNull(ipv4, "ip");
        if (!IPV4_PATTERN.matcher(ipv4).matches()) {
            throw new IllegalArgumentException("Only ipv4 address supported");
        }

        String[] parts = ipv4.split("\\.");
        return (Integer.parseInt(parts[0]) & 0XFFL) << 24
                | (Integer.parseInt(parts[1]) & 0XFFL) << 16
                | (Integer.parseInt(parts[2]) & 0XFFL) << 8
                | Integer.parseInt(parts[3]) & 0XFFL;
    }

    public static String inet_ntoa(long numeric) {
        return ((numeric >>> 24) & 0XFF) + "." +
                ((numeric >>> 16) & 0XFF) + "." +
                ((numeric >>> 8) & 0XFF) + "." +
                (numeric & 0XFF);
    }
}