'Authoritative' means that for clients on the network 192.168.1.0/24 (your local LAN) the DNS server on the OS X server machine says I am the authoritative / the final word, source for all DNS records and information for the domain 'iwantaserver.com'. Of course we know that on the Internet (unless you specific that your home office DNS server also serves DNS for your domain — which I wouldn't do) there is another DNS server machine which is official for your domain. The reason to do it this way (which yes is double work) is to control what clients in your LAN do.
To make your server authoritative you simply say in the Server | Services | DNS section that for the zone iwantaserver.com the nameserver is server.iwantaserver.com and the machine record for server.iwantaserver.com is 192.168.1.2 (in the System Preferences | Network section you will see that the DNS server is 127.0.0.1 that's OK, that is the localhost IP. Again have to say that the book I mentioned has all this information in it in much more detail + screenshots, I strongly advise you to get a copy, the Kindle price is cheaper than the printed book.
To check this on the server you can run the Network Utility program and in the 'Lookup' tab type in (again we are using the pseudo domain 'iwantaserver.com' your real one is obviously different, and assuming that server.iwantaserver.com IP is 192.168.1.2) 'iwantaserver.com'. You should see something like this if you have setup DNS correctly. Basically the OS X server has taken over serving DNS queries for the domain — they no longer go to the Internet.
Lookup has started…
Trying "iwantaserver.com"
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 38381
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 4, AUTHORITY: 0, ADDITIONAL: 1
;; QUESTION SECTION:
;iwantaserver.com. IN ANY
;; ANSWER SECTION:
iwantaserver.com. 10800 IN SOA iwantaserver.com. admin.iwantaserver.com. 2012122801 3600 900 1209600 86400
iwantaserver.com. 10800 IN NS server.iwantaserver.com.
iwantaserver.com. 10800 IN A 192.168.1.2
iwantaserver.com. 10800 IN MX 10 server.iwantaserver.com.
;; ADDITIONAL SECTION:
server.iwantaserver.com. 10800 IN A 192.168.1.2
Received 145 bytes from 127.0.0.1#53 in 3 ms
Then in Terminal you should issue this command:
sudo changeip -checkhostname
If it does not say something like this (using our examples still) then you have a problem.
Primary address = 192.168.1.2
Current HostName = server.iwantaserver.com
DNS HostName = server.iwantaserver.com
The names match. There is nothing to change.
dirserv:success = "success"
I would make the OS X server to be the DHCP server for your network, you just configure server DHCP put in address range, router & DNS IP address of the server and the search domain and turn it on.