There is no such thing as "non reserved shinkansen", Shinkansens usually have two sections "Unreserved" Section and "reserved section" (plus green cars), if you need a reservation then you sit in the "reserved" section. If you want to use the "unreserved" section, then you do not need to make a reservation.

"NEX" not "LEX".

I think you might be using some incorrect terminology. It is impossible to make a "reservation" for a "unreserved" section of a train.

You can only make a reservation for seats in the "reserved" seat sections on trains that have reserved seating. That's why its called "reserved".

If you have a JR pass you just show the JR Pass to sit in the unreserved seating section.

I've been confused by different answers in this site. Some answers said seat reservation outside of Japan is not possible.

But what I've read from below link is:
passengers who plan on using the JAPAN RAIL PASS to travel on Shinkansen and some Limited Express services can use this site to reserve seats before arrival in Japan.

http://jreast-shinkansen-reservation.eki-net.com/PC/Eng...

I hope someone can clear my doubts here.

I'm using JR-Pass on my journey Tokyo-Hokkaido-Tokyo. Can I make seat reservation through online one month before I depart to Japan?

The website says you can.
"Reservations can be made from one month before your departure date up until 5:00 p.m. (17:00, Japan Standard Time) 3 days prior to your departure date. "

But note the tickets collection is to be in Japan and a day before your travel.So if you are planning to travel on the day you arrive in Japan, this will not work for you.

Liem, thanks for your answer. It seems like online seat reservation is still possible as long as I am able to collect the tickets a day before.

I will be in Tokyo on 25/Dec/09. And I would like to make reservation for below trips:

Shinkansen Hayate (Tokyo to Hachinohe) on 28/Dec morning.
Ltd.Exp Super Hakucho (Hachinohe to Hakodate) on 28/Dec afternoon.
Ltd.Exp Hakuto (Hakodate to Noboribetsu) on 29/Dec morning or afternoon.
Ltd Exp Super Hakuto (Noboribetsu to Sapporo) on 30/Dec morning or afternoon.
Necessary trains from Sapporo to Tokyo on 2/Jan.

For my case, I am able to collect the tickets once I reach Narita airport on 25/Dec. If reservation can be made online, then it will be a good news for me!

Up to here, I got another question. After I bought the JR-Pass, it is just a voucher to be exchanged into an actual pass in Japan later. In this case, how am I going to make online reservation using the voucher (before travel to Japan)? Do I still need a credit card or just using some kind of serial number on that voucher for making the reservation?