To aloow everything, do not enter anything at all.
Leave the table blank, and absolutely anyone on the net could use the gateway.
To allow only portions, look a the examples below.
It is anormal mask for a router.
Ip to allow 188.8.131.52 mask 255.255.255.255 will allow only one I.P.
200.100.50.01 mask 255.255.255.0 will allow anyone that starts with 200.100.50.01 to 184.108.40.206
200.100.01.01 mask 255.255.0.0. will allow anyone that start with 200.100.xxx.xxx
220.127.116.11 mask 255.0.0.0 will allow anyone with an I.P. that start with 200.xxx.xxx.xxx
Hope that that helps.
On the allow table, if you use it, you also need to allow your own I.P., the I.P. of your gateway.