Postgres configuration, incoming connections

Question

Hello I'm trying to figure out the best way to allow connection from certain ips/hosts to my db instance.

I've got an api(s) pointing to the database and right now we do something like this (pg_hba.conf) :

host    prod_db   prod_user         xxx.xxx.77.xx/32        password

And since the api are getting deployed in cloud and with new instances, having new ips, we kind of change this config every time we deploy, which is not very good, cause we have to restart the db server and therefore have to put the whole eco system in the maintenance mode.

I'm just reading this :

http://www.postgresql.org/docs/9.1/static/auth-pg-hba-conf.html

This bit looks appealing to me:

# Allow any user from hosts in the example.com domain to connect to
# any database if the user's password is correctly supplied.
#
# TYPE  DATABASE        USER            ADDRESS                 METHOD
host    all             all             .example.com            md5

So this means any incoming connection from api.example.com or api1.example.com or any from this domain will be accepted if the password is correct.

But my question is how does the db server know that it's coming from the something.example.com.

Does this mean that the server is doing some kind of reverse dns lookup before allowing connection?

Does this assume that example address 123.456.22.11 which points to api.example.com, by point I mean A record will be allowed connection from or how this works?

What do I need to do to make this work?

My ultimate goal is not to have to edit the pg_hba.conf every time there is a new api server instance booted up to add that new ip. And not having to restart postgres db instance