def is_ipv4_address(addr):
    """Return True if addr is a valid IPv4 address, False otherwise."""
    if (not isinstance(addr, str)) or (not addr) or addr.isspace():
        return False
    octets = addr.split('.')
    if len(octets) != 4:
        return False
    for octet in octets:
        if not octet.isdigit():
            return False
        octet_value = int(octet)
        if octet_value < 0 or octet_value > 255:
            return False
    return True