I am given a list of IPs, some of them have a netmask, other no. I got the idea of how to create a mask but i'm not understanding why there are cases where an IP doesn't have a mask.

Like the following:

• 121.34.56.64–121.34.56.128 > Don't have mask
• 128.131.9.0–128.131.9.192 > Don't have mask
• 93.20.10.0–93.20.11.0 > Don't have mask

• 67.56.34.64-67.56.34.79 > Prefix notation: 67.56.34.64/28 | Address/Mask: 67.56.34.64/255.255.255.240 and so on.

For example if I look at the third one of the addresses without mask i do the following:

93.20.10.0–93.20.11.0

93.20. 0000 101|0 . 0000 0000
93.20. 0000 101|1 . 0000 0000

NetMask: 255.255.254.0, that is not correct.

Can somebody explain me why and how to understand when a mask exists?

The reason for a mask is to divide an address into two parts: the network part and the host part. You can have a list of addresses which do not fit into one network, but they will fit into a larger network. Your example: `93.20.10.0–93.20.11.0` will not fit into the `93.20.10.0/24` network, but will fit into the `93.20.10.0/23` network. The catch is that there are more addresses in the `93.20.10.0/23` network than are in your list. That means that your list is a subset of the addresses in the `93.20.10.0/23` network.