Subnet is only a way to be within or outside of a number range. They don't secure the physical lan. If for example your two computer connect via the IP address and associated subnet then OK, they should do that all day long. If you want to access addresses outside of subnet then you need to route it out and tell it how to get out.
The combination of subnet and ip is separate from gateway/router. Many separate subnets could all contact a single server/gateway/router.
Subnets don't really secure a lan.
Almost no one would use a standard subnet either. Most folks would consider a classless based subnet.
Youtube isn't the place to go for a subnet calculator. It may have a web video on how to calculate it. Not sure anyone wants to do a manual calculation for than once.
Put your data into this calculator if you really need to make a classless subnet.
http://www.subnet-calculator.com/