In IPv4 addressing, each of the four octets can legally contain zeros (0-255 range). The testers' concern about disallowing zeros appears to stem from common misconceptions about special IP addresses:

• 0.0.0.0 - Represents "this host on this network" (but won't appear in remote addresses)
• X.X.0.0 - Valid network address in classful addressing
• X.X.X.0 - Valid host address in CIDR notation

For IP-based authentication storing FROM_IP and TO_IP ranges, zeros are perfectly valid. Here's a proper validation approach:

public boolean isValidIPv4(String ip) {
    String[] octets = ip.split("\\.");
    if (octets.length != 4) return false;
    
    try {
        for (String octet : octets) {
            int value = Integer.parseInt(octet);
            if (value < 0 || value > 255) return false;
        }
    } catch (NumberFormatException e) {
        return false;
    }
    return true;
}

When storing IP ranges for authentication, consider these approaches:

// Convert IP to integer for efficient range queries
public long ipToLong(String ip) {
    long result = 0;
    String[] octets = ip.split("\\.");
    for (int i = 0; i < 4; i++) {
        result |= Long.parseLong(octets[i]) << (8 * (3 - i));
    }
    return result;
}

// Example SQL for range checking
String sql = "SELECT user_id FROM ip_ranges " +
             "WHERE ? BETWEEN ip_from_num AND ip_to_num";

Common valid ranges containing zeros:

While zeros are technically valid, consider these security aspects:

• Private IP ranges (like 10.0.0.0/8) shouldn't appear in public-facing auth systems
• Implement CIDR notation support for more flexible range definitions
• Combine IP restrictions with other auth factors for better security

For more robust validation, use Java's built-in InetAddress:

public boolean isValidPublicIPv4(String ip) {
    try {
        InetAddress address = InetAddress.getByName(ip);
        if (!(address instanceof Inet4Address)) return false;
        if (address.isAnyLocalAddress() || 
            address.isLoopbackAddress() ||
            address.isLinkLocalAddress() ||
            address.isSiteLocalAddress()) {
            return false;
        }
        return true;
    } catch (UnknownHostException e) {
        return false;
    }
}

In IPv4 addressing, each of the four octets (the numbers between dots) can legally contain zeros. Addresses like 192.168.0.1 or 10.0.0.1 are perfectly valid and commonly used in both private and public networks. The testers' suggestion to prohibit zeros entirely in IP range validation is technically unnecessary and may actually cause operational issues.

Let's examine why zeros are valid:

// Examples of valid IP addresses containing zeros
String[] validIPs = {
    "167.23.0.1",      // Valid public IP with zero in third octet
    "203.0.113.0",     // Valid network address
    "198.51.100.255",  // Valid broadcast address
    "0.0.0.0"          // Special address (but still valid format)
};

The only special case is 0.0.0.0 which has specific meanings in different contexts, but even this is a properly formatted IP address.

Here's how to correctly implement IP range validation in Java:

import java.net.InetAddress;
import java.net.UnknownHostException;

public class IPRangeValidator {
    public static boolean isInRange(String ipToCheck, String fromIP, String toIP) 
            throws UnknownHostException {
        long ipLo = ipToLong(InetAddress.getByName(fromIP));
        long ipHi = ipToLong(InetAddress.getByName(toIP));
        long ip = ipToLong(InetAddress.getByName(ipToCheck));
        return ip >= ipLo && ip <= ipHi;
    }

    private static long ipToLong(InetAddress address) {
        byte[] octets = address.getAddress();
        long result = 0;
        for (byte octet : octets) {
            result <<= 8;
            result |= octet & 0xff;
        }
        return result;
    }
}

Some developers mistakenly believe:

• Zeros indicate broadcast addresses (only true when all host bits are set to 1)
• Zeros make the address invalid (completely false - all octets can be 0-255 except special cases)
• Zeros are only for private networks (public IP ranges can contain zeros too)

When storing IP ranges in a database:

CREATE TABLE ip_ranges (
    id INT PRIMARY KEY,
    from_ip VARCHAR(15) NOT NULL CHECK (from_ip ~ '^((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$'),
    to_ip VARCHAR(15) NOT NULL CHECK (to_ip ~ '^((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$'),
    CONSTRAINT valid_range CHECK (ip_to_long(from_ip) <= ip_to_long(to_ip))
);

While zeros are valid, you should still:

1. Validate complete IP format (four octets, proper values)
2. Handle special addresses appropriately (0.0.0.0, 255.255.255.255)
3. Consider using CIDR notation for range definitions when possible

For high-volume applications, consider storing IPs as integers:

// Alternative storage approach
CREATE TABLE optimized_ip_ranges (
    from_ip BIGINT NOT NULL,
    to_ip BIGINT NOT NULL,
    INDEX (from_ip, to_ip)
);

// Then query using:
SELECT * FROM optimized_ip_ranges 
WHERE ? BETWEEN from_ip AND to_ip;