IP restriction are enabled in our org. when i try to access workbench i get a invalid_IP login error.What range of IP addresses has to be white listed and how safe can white-listing those IP addresses be?
This is mentioned in KB 000214031:
Security concern about Workbench's source IP addresses
Workbench is NOT a supported product of or supported by Salesforce.com, inc. This application is hosted on Heroku and as a result it's IP address will be that of Amazon Web Services.
workbench.developerforce.com is hosted on Heroku and thereby on AWS (Amazon Web Services), which uses the IP ranges documented at https://ip-ranges.amazonaws.com/ip-ranges.json, instead of the IP addresses used by Salesforce servers.
If this is a concern, you may consider configuring workbench within a firewall server in your company's network and use the source code and instructions found in this GitHub repository: https://github.com/ryanbrainard/forceworkbench. Alternatively, if you are an existing Heroku customer, you may run this app on your own Heroku instance.
Please note that Workbench is NOT a supported product of or supported by Salesforce.