The subnet mask is something you can set depending on how large you want the network to be. Basically a 255.255.255.0 translates to 11111111.11111111.11111111.00000000
Theres 24 1s and 8 0s. To figure out how many hosts you have raise 2 to the number of hosts. Then you subtract 2 for the network and broadcast address of that network. SO in this case, 2^8 = 256. 256-2 = 254. There you have 254 IPs to hand to devices. The more 0s you have in the mask, the more hosts per network. The more 1s you have, the more networks you can create.
Your subnet mask should have been provided by your hosting server service provider. Basically what it does is determine how many IP addresses are on your local segment of the network so the system knows what addresses it must go out through the gateway to access. i.e. If you were assigned 8 Ip addresses (say 184.108.40.206 through 220.127.116.11) your subnet mask would be 255.255.255.248 or 255.255.255.255 - 0.0.0.8