def cidr_to_common(cidr_mask):
    """Function that returns a common mask (Ex: 255.255.255.0)
       for a given input CIDR mask (Ex: 24)"""
    cidrtocommon = {
        1: "128.0.0.0",
        2: "192.0.0.0",
        3: "224.0.0.0",
        4: "240.0.0.0",
        5: "248.0.0.0",
        6: "252.0.0.0",
        7: "254.0.0.0",
        8: "255.0.0.0",
        9: "255.128.0.0",
        10: "255.192.0.0",
        11: "255.224.0.0",
        12: "255.240.0.0",
        13: "255.248.0.0",
        14: "255.252.0.0",
        15: "255.254.0.0",
        16: "255.255.0.0",
        17: "255.255.128.0",
        18: "255.255.192.0",
        19: "255.255.224.0",
        20: "255.255.240.0",
        21: "255.255.248.0",
        22: "255.255.252.0",
        23: "255.255.254.0",
        24: "255.255.255.0",
        25: "255.255.255.128",
        26: "255.255.255.192",
        27: "255.255.255.224",
        28: "255.255.255.240",
        29: "255.255.255.248",
        30: "255.255.255.252",
        31: "255.255.255.254",
        32: "255.255.255.255",
    }
    if int(cidr_mask) >= 0 and int(cidr_mask) <= 32:
        return cidrtocommon[int(cidr_mask)]
    else:
        raise ValueError("Incorrect CIDR mask entered")