Most probably, you are after a subnet mask of 255.255.255.0.
A subnet mask refers to the practice of subnetting, which is, from my understanding of it, a way of dividing one network into smaller logical networks. A subnet mask is used to tell the network devices (whether it be the computer or any routers, modems etc) what addresses are local (belongs to this network), and what addresses are remote (belongs to that network).
So, if a computer's IP address is 192.168.1.104, and it's subnet mask is 255.255.255.0, then the computer (and every other device attached to the same network) will assume that every IP on that computer's local network will be in the format 192.168.1.xxx, with'xxx being the only part that would vary. Likewise, if the subnet mask is 255.255.0.0, then the computer will assume that every IP address in it's local network will be in the format of 192.168.xxx.xxx
Subnetting a network is not really useful in a home network, such as yours. It is mainly used on larger networks (with 255+ computers) to reduce unnecessary network activity. MarkM provided this in the comments:
One of the biggest advantages of subnetting in a large network is to reduce the broadcast traffic. If you have thousands of hosts on a single subnet, your switches will be choking on ARP, DHCP, and other broadcasts. In a home network usually there isn't much of a need unless you want something like a guest wireless LAN that doesn't have any routing available to your main home network.
This explanation is very rough and sketchy, so please forgive me if I have made a mistake or two.