Yes your Gateway by default should be 255.255.255.0 Unless you configured this different then you wont have to worry about this. But it brings for a good thing to look in to. Make sure that your IP on your computer wasn't set to Static and somehow you ended up changing or using a different Gateway, such as 255.255.255.254 or 255.255.0.0 ... Make sure it matches the router... Again I would allow DHCP to handle all that, unless you have a Server in which case all you need to worry with is setting a static IP.
I also found out that you are not allowed to access and configure a Router/Modem settings via a Wireless Connection. So you have to be connected directly to the Router/Modem. (in most cases)