 So, basically the subnet mask determines I mean which IPs belong to one network. For example, we saw 255, 255, 255, 0 as the subnet mask for the IP address 192, 16801. So, how do you get to know which are the other IP addresses in the same network? As we know that 256 are the total number of numbers that can be represented in this one or in this one octet as we call it. So, if your network mask is 255, so the number of representations you can use for that particular network is 256 minus 255 that is 1. So, you could use only one representation which means 192. Similarly, if we have 255 in three places that means all the IP addresses will begin with 192, 168 and 0 belong to the same network, but here what we have we have a 0, 0 means we can have 256 representations that would be 0 to 255 in the same network. So, what does that mean? That means any address with 192, 1680 which were the single numbers allowed and the last octet from 0 to 255 belong to the same network. So, whenever a network is created by default two IP addresses are reserved which is the first and the last IP address. So, here the first IP address is 192, 168, .0, .0 and the last one .255. What we see here is the first one which is called the network address is used to identify the network and the last is called the broadcast address. The number of total number of IP addresses that you could use are all the IP addresses between 0 to 255 with the net mask of 255, 255, 255, 0. What you can say is all the IP addresses between 192, .168, .0, .0 and 192, .168, .0, .255 belong to the same network. So, what are the numbers that the subnet mask can take? As we saw in a in case of IP addresses this could take anything between 0 to 255, but in case of subnet mask it is different. So, a subnet mask can take any of the values between 0, 128, 192, so on to 252, 254 and 255.