I just finished a Cisco CCNA video course, so I know just enough about networking to be dangerous. I have a pretty good idea of how IP's and subnets work, but one thing I haven't been able to find out is why, exactly, 255.255.255.0 is the default subnet mask in most of the routers I've seen (admittedly, nearly all consumer-grade and SOHO routers).
Granted, there's not all that much of a reason for a consumer-grade router to use anything else – unless you need more than 254 IP's, or want to do something complex like organizing your network (servers on 192.168.1.X, desktops on 192.168.2.x, etc.)
That got me thinking, are those the only reasons to use 255.255.255.0? What exactly would change for people if the default mask was 255.255.0.0 instead? Is there some downside to using the latter?
Best Answer
Well, sometimes you find some consumer hardware with that mask hard coded here and there. But that's relatively rare and not a deciding factor. Also you want to stick within the predefined private ip ranges, that's the bigger consideration.
Usually common practice for choosing defaults (in my neck of the woods at least) is to use the tightest mask possible and that one is an easy one for humans to deal with even if it isn't the absolute tightest.
You can use whatever you want though just be careful to not open up a range that starts to conflict with internet IPs. There's a list of reserved addresses at https://en.m.wikipedia.org/wiki/Reserved_IP_addresses. E.g. 192.x.x.x/255.0.0.0 is not acceptable for private LAN use.
I know this doesn't answer your question fully. Barring other answers I'll do some research on more concrete reasons why the default is /24 when I'm not on my phone. I strongly suspect it's either tradition, or the safest bet given that unwise users may choose nonprivate ranges.