Netmask from subnet size

netmask.cpp

#include <cstdint>
#include <optional>
#include <string>

std::optional<std::string> getNetMaskFromSubnetSize(uint64_t subnetSize)
{
    auto isPowerOf2 = [](uint64_t num) { return num > 0 and (num & (num - 1)) == 0; };
    if (not isPowerOf2(subnetSize))
        return std::nullopt;

    // 64 bits because "0.0.0.0" subnet size is greater than numeric_limits<uint32_t>::max()
    uint64_t bits = ~(subnetSize - 1);
    return std::to_string((bits & 0xFF000000) >> 24) + "."
        + std::to_string((bits & 0x00FF0000) >> 16) + "."
        + std::to_string((bits & 0x0000FF00) >> 8) + "."
        + std::to_string((bits & 0x000000FF) >> 0);
}