I don't think that's going to result in what you're actually after.
Try this:
var portdetails = lines.Where(acc => lines.IndexOf(acc) >= 200 && lines.IndexOf(acc) <= 400 && acc.Trim().StartsWith("PORTDTLS ").ToList();
After you get the lines, THEN split them on the comma separator.
Another point - is the index range ALWAYS going to be 200-400? How do you know this? How can you guarantee it. I think you need to step back and look at the problem again.