NXDOMAIN ici aussi, et oui, ac est un TLD c'est pour ça que dig le remonte.

par contre c'est chelou que dig sorte un enregistrement 'A' pour ce nom, non?

edit: marrant:

[squalyl@host ~]$ dig ac.

; <<>> DiG 9.5.0-P1 <<>> ac.
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 24963
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 7, ADDITIONAL: 5

;; QUESTION SECTION:
;ac.                            IN      A

;; ANSWER SECTION:
ac.                     86400   IN      A       80.249.100.35

;; AUTHORITY SECTION:
ac.                     86400   IN      NS      b.nic.ac.
ac.                     86400   IN      NS      ns1.communitydns.net.
ac.                     86400   IN      NS      a.nic.ac.
ac.                     86400   IN      NS      b.ns13.net.
ac.                     86400   IN      NS      a.ns13.net.
ac.                     86400   IN      NS      b.nic.io.
ac.                     86400   IN      NS      ns3.icb.co.uk.

;; ADDITIONAL SECTION:
b.nic.io.               3471    IN      A       194.0.2.1
ns1.communitydns.net.   430     IN      AAAA    2001:678:4::1
a.ns13.net.             137746  IN      A       202.181.97.168
ns3.icb.co.uk.          16331   IN      A       217.199.188.61
b.ns13.net.             730     IN      A       202.181.96.60

;; Query time: 10 msec
;; SERVER: 88.191.254.60#53(88.191.254.60)
;; WHEN: Mon Nov 24 09:04:24 2008
;; MSG SIZE  rcvd: 284

[squalyl@host ~]$ ping ac.
PING ac (80.249.100.35) 56(84) bytes of data.
64 bytes from ip100-35.thdo.ncuk.net (80.249.100.35): icmp_seq=1 ttl=55 time=8.70 ms
64 bytes from ip100-35.thdo.ncuk.net (80.249.100.35): icmp_seq=2 ttl=55 time=8.68 ms
^C
--- ac ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1000ms
rtt min/avg/max/mdev = 8.683/8.694/8.705/0.011 ms
[squalyl@host ~]$ nslookup ac
Server:         88.191.254.60
Address:        88.191.254.60#53

Non-authoritative answer:
Name:   ac
Address: 80.249.100.35