Set the specific IP addresses from which user can log in Salesforce.
Setup > Manager Users > Profiles (Select a profile) Custom Sales Profile
System > Login IP Ranges
Login IP Ranges > Add IP Ranges
For each profile you can specify IP addresses. User can log in from specified IP address but when they try to log in from a different IP address it is denied.
Enter the IP Start and End Address. The start and end addresses define the range of allowable IP addresses from which users can log in.