You need to use Global Security Events Handler when users attempt to sign in on the website. In the handler, you can save IPs to your custom table/compare etc using Custom Table API.
You can probably use GeoLocation API to resolve IP to Location
using AnalyticsContext.CurrentGeoLocation class
GeoLocation currentGeoLocation = GeoIPHelper.GetCurrentGeoLocation();
if (currentGeoLocation != null)
{
var countryAndState = ValidationHelper.GetString(currentGeoLocation.CountryName, "");
var ipCountryCode = ValidationHelper.GetString(currentGeoLocation.CountryCode, "");
var ipRegion = ValidationHelper.GetString(currentGeoLocation.RegionName, "");
var ipCity = ValidationHelper.GetString(currentGeoLocation.City, "");
}
And you do your analysis. If user logins every day from different country... :)