Tab Advanced
At the advanced section, you can specify a comma separated black- and whitelist of IP adresses, checked on showing the login page.
So you can restrict/allow access for specific adresses or adress-ranges.
Each IP should be separated by comma.
The shema used is Classless Inter-Domain Routing (CIDR)
See https://wikipedia.org/wiki/Classless_Inter-Domain_Routing for a detailed description.