In Enterprise Manager you right click the table  and choose Design Table. Then for the ID column  you choose set the Identity property to yes. It's better to use a integer (int) datatype  for your "autoincrement" column because it will never hold decimal values  just whole numbers.
The difference between varchar and nvarchar is that nvarchar will allow more international characters than varchar (not useful for an IP address).
You should watch out with the field length of the (n)varchar column as it is difficult to predict the maximum field length users will input. 50 characters is really small. You don't want your application to give an error  when it occurs. If you have no idea what is a good length for your field you can also use the (n)text field which autoincrements.