How to ensure one user login in only one machine?

First Create one Dictionary collection in your application startup and add it to Application not Session.

Then Get the Ip Address of users pc:

string ipaddress;
ipaddress = Request.ServerVariables["HTTP_X_FORWARDED_FOR"];
if (ipaddress == "" || ipaddress == null)
    ipaddress = Request.ServerVariables["REMOTE_ADDR"];

When User Login Check for the username in your Applications Dictionary Collection. if Username and Ipaddress is same as exists then proceed with Login. if not exists then add that pair and Login. if username exist but ip address is diffrent than present then popup the message that user already logged in other pc.