Farhan,
Well, every page in Moodle has a web address, so theoretically this is possible. But practically it is not possible because you need to know the database IDs of the course, assignment and student. The URL to grade a particular assignment for a given student would look something like this: "http://yourmoodlesite.org/mod/assign/view.php?id=50927&rownum=3&action=grade" or even "http://yourmoodlesite.org/grade/edit/tree/grade.php?courseid=562&id=82902&gpr_type=report&gpr_plugin=grader&gpr_courseid=562"
I highly recommend grading through the assignment activity grader and completely avoiding direct grading in the grade book. The reason is that directly entering grades in the grade book overrides grades entered in the activity grader. You cannot then go back and change the grade via the activity grader. Plus when grading directly in the grade book, you cannot use features such as rubrics or marking guides. Another benefit of grading with the activity grader is that you do not need to scroll around through the grade book to find the assignment you want to grade, and you can sort the columns by name, status, grade, last modified, etc. Carefully read through the assignment module documentation, especially the latter parts of the page where grading is discussed.
Floyd