[exim] [Newbie] Configure exim for a LAN

Hi everyone,

i'm pretty new to exim and mail servers in general.
I try to set a lan mail server at my home and by the way learn more about
domain name and mail delivery.

I'm under Linux Debian.

>>At first, i'd like to understand well what exactly a domain is. I see that
pretty like a network name.
And each machine on this network has a FQDN = machine_name.domain_name. To
this FQDN we can match up IP adresses.
First of all, do machines of the same domain need to have same class of IP
adress?
For example, can i gather under the same domain (domain.org) two machines
with IP: 10.0.0.2 and 192.168.1.1 ?

>>About domain name resolution. Do i need to configure something special
other than /etc/hosts for exim to find a local FQDN?
Is there something special about some MX to do on the server?
How can i say to exim to look up on the local network first?

>>I'd like to send mail thru my local network.
First machine (client) is atom.domain.org 10.0.0.1
Second machine (server) is serveur.domain.org 10.0.0.2
I want to send mail from client to server.
What kind of configuration must i put in /etc/hosts, /etc/resolv.conf or
others...?

When i perform a dpkg-reconfigure exim4-reconfigure, i have serveral choice
of configuration template:
1) internet site; mail is sent and received directly using SMTP
2) mail sent by smarthost; received via SMTP or fetchmail
3) mail sent by smarthost; no local mail
4) local delivery only; not on a network

which configuration for my server, which one for my client?

>>I have added an iptable line to allow smtp connexion on my server, do I
need something more on the server or on the client?

>>Maybe something i forgot?

Thanks for all the answers you could give to me.