Task: Canonicalize CIDR
C++: #include <cstdint>
#include <iomanip>
#include <iostream>
#include <sstream>


class ipv4_cidr {
public:
    ipv4_cidr() {}
    ipv4_cidr(std::uint32_t address, unsigned int mask_length)
        : address_(address), mask_length_(mask_length) {}
    std::uint32_t address() const {
        return address_;
    }
    unsigned int mask_length() const {
        return mask_length_;
    }
    friend std::istream& operator>>(std::istream&, ipv4_cidr&);
private:
    std::uint32_t address_ = 0;
    unsigned int mask_length_ = 0;
};


std::istream& operator>>(std::istream& in, ipv4_cidr& cidr) {
    int a, b, c, d, m;
    char ch;
    if (!(in >> a >> ch) || a < 0 || a > UINT8_MAX || ch != '.'
        || !(in >> b >> ch) || b < 0 || b > UINT8_MAX || ch != '.'
        || !(in >> c >> ch) || c < 0 || c > UINT8_MAX || ch != '.'
        || !(in >> d >> ch) || d < 0 || d > UINT8_MAX || ch != '/'
        || !(in >> m) || m < 1 || m > 32) {
        in.setstate(std::ios_base::failbit);
        return in;
    }
    uint32_t mask = ~((1 << (32 - m)) - 1);
    uint32_t address = (a << 24) + (b << 16) + (c << 8) + d;
    address &= mask;
    cidr.address_ = address;
    cidr.mask_length_ = m;
    return in;
}


std::ostream& operator<<(std::ostream& out, const ipv4_cidr& cidr) {
    uint32_t address = cidr.address();
    unsigned int d = address & UINT8_MAX;
    address >>= 8;
    unsigned int c = address & UINT8_MAX;
    address >>= 8;
    unsigned int b = address & UINT8_MAX;
    address >>= 8;
    unsigned int a = address & UINT8_MAX;
    out << a << '.' << b << '.' << c << '.' << d << '/'
        << cidr.mask_length();
    return out;
}

int main(int argc, char** argv) {
    const char* tests[] = {
        "87.70.141.1/22",
        "36.18.154.103/12",
        "62.62.197.11/29",
        "67.137.119.181/4",
        "161.214.74.21/24",
        "184.232.176.184/18"
    };
    for (auto test : tests) {
        std::istringstream in(test);
        ipv4_cidr cidr;
        if (in >> cidr)
            std::cout << std::setw(18) << std::left << test << " -> "
                << cidr << '\n';
        else
            std::cerr << test << ": invalid CIDR\n";
    }
    return 0;
}

Java: import java.text.MessageFormat;
import java.text.ParseException;

public class CanonicalizeCIDR {
    public static void main(String[] args) {
        for (String test : TESTS) {
            try {
                CIDR cidr = new CIDR(test);
                System.out.printf("%-18s -> %s\n", test, cidr.toString());
            } catch (Exception ex) {
                System.err.printf("Error parsing '%s': %s\n", test, ex.getLocalizedMessage());
            }
        }
    }

    private static class CIDR {
        private CIDR(int address, int maskLength) {
            this.address = address;
            this.maskLength = maskLength;
        }

        private CIDR(String str) throws Exception {
            Object[] args = new MessageFormat(FORMAT).parse(str);
            int address = 0;
            for (int i = 0; i < 4; ++i) {
                int a = ((Number)args[i]).intValue();
                if (a < 0 || a > 255)
                    throw new Exception("Invalid IP address");
                address <<= 8;
                address += a;
            }
            int maskLength = ((Number)args[4]).intValue();
            if (maskLength < 1 || maskLength > 32)
                throw new Exception("Invalid mask length");
            int mask = ~((1 << (32 - maskLength)) - 1);
            this.address = address & mask;
            this.maskLength = maskLength;
        }

        public String toString() {
            int address = this.address;
            int d = address & 0xFF;
            address >>= 8;
            int c = address & 0xFF;
            address >>= 8;
            int b = address & 0xFF;
            address >>= 8;
            int a = address & 0xFF;
            Object[] args = { a, b, c, d, maskLength };
            return new MessageFormat(FORMAT).format(args);
        }

        private int address;
        private int maskLength;
        private static final String FORMAT = "{0,number,integer}.{1,number,integer}.{2,number,integer}.{3,number,integer}/{4,number,integer}";
    };

    private static final String[] TESTS = {
        "87.70.141.1/22",
        "36.18.154.103/12",
        "62.62.197.11/29",
        "67.137.119.181/4",
        "161.214.74.21/24",
        "184.232.176.184/18"
    };
}