https://login.salesforce.com/services/Soap/c/api_version is the new recommended endpoint for
API login requests, where api_version specifies the API version, such as 17.0.

If you send non-login requests to https://login.salesforce.com/services/Soap/c/api_version, an error is returned.The less secure version of the URL—http://login.salesforce.com/services/Soap/c/api_version—is also supported, but not recommended. It is helpful for debugging through proxy servers.