MySQL Standalone Server / Static IP question

EDIT

Never mind this one.

I found out for myself that it wasn't possible to access another "private ip" if I hadn't set it up for the other machines as well, which makes a lot of sense now in hindsight. :)

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I have 3 linodes.

1 for mysql

1 for cronjobs

1 for php-fpm/nginx

I'm accessing the mysql linode from the two others. never the other way around.

In the Db on the mysql linode I have granted access to the other two linodes using the hostname I have in the A-records on DNS for the other two linodes, to the tables I want the other two linodes to work with and be able to connect to remotely.

All linodes are in the same location (Newark).

If I configure a private IP for the mysql linode, is that enough or do I need to do that for the other two linodes as well?

If yes, please explain why.

I see it like so:

Since I have a private IP for the mysql server, I can access that by referring to it in the "db"-references I do from PHP and from the python cron jobs. So why would I need to fix the static IP for the other two who are never going to get accessed by eachother or the mysql linode. It's only the mysql linode that is going to be interally accessed by the other two.

The reason I'm asking is that in the walkthroughs found here in the library it said I should do that for all machines that were going to interact. Seems redundant to me.

Grateful for your input.

Also, do I need to "secure" the mysql linode in a special way? I use iptables for my php-fpm/nginx linode but since I have no web server on the mysql linode, do I need to bother any with a firewall on that one?

Thanks much!