IP mask numbers are used to divide internet addresses into blocks called subnets. The mask number represents the number of 1s in the binary of the address that is 'masked" against the address so that it ignores the last bits which are for the group of IP addresses in the masked address.
The first address of a subnet block (all 0s) is called the network address or network ID. The last address (all 1s) is the broadcast address of the network. Typically the network address +1 or the broadcast address -1 is the gateway to the internet. The 'slash' notation (ie /24) is known as CIDR format, while the more conventional 255.255.255.0 notation is considered a subnet mask.
The first address of a subnet block (all 0s) is called the network address or network ID. The last address (all 1s) is the broadcast address of the network. Typically the network address +1 or the broadcast address -1 is the gateway to the internet. This leaves us with total number of IP numbers -3 left over for host address with in a sub net block. That's why you either get 1 IP (4-3= 1) or if you ask for one more you get 5 (8-3=5). Here is an example:
192.168.1.0/25 would include all address between 192.168.1.0 and 192.168.1.127
while 192.168.1.128/25 would include 192.168.1.128 and 192.168.1.255
Below is a mask table that makes it easy to look up the mask for a group of IP addresses.