Quote from src/core/ip2c/ip2c.h

inline bool isLocalhostAddress(unsigned ipv4Address)
{
        const static unsigned LOCALHOST_BEGIN = QHostAddress("127.0.0.0").toIPv4Address();
        
const static unsigned LOCALHOST_END = QHostAddress("127.255.255.255").toIPv4Address();

        return ipv4Address >= LOCALHOST_BEGIN && ipv4Address <= LOCALHOST_END;
}