if you want to use a linux command to provide a reverse lookup of an IP address you can use the dig command for example you can dig the yahoo ip address

forward lookup maps the hostname to IP address, and reverse lookup maps the IP address into hostname. dig is a DNS lookup utility shich sends the query to DNS server



so lets say for example i send this command

# dig -x 72.30.2.43

this is what the output of the command would look like from the shell 



; <<>> DiG 9.3.6-P1-RedHat-9.3.6-4.P1.el5_4.1 <<>> -x 72.30.2.43
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 51079
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 5, ADDITIONAL: 0

;; QUESTION SECTION:
;43.2.30.72.in-addr.arpa.       IN      PTR

;; ANSWER SECTION:
43.2.30.72.in-addr.arpa. 1200   IN      PTR     ir1.fp.vip.sk1.yahoo.com.

;; AUTHORITY SECTION:
2.30.72.in-addr.arpa.   172800  IN      NS      ns4.yahoo.com.
2.30.72.in-addr.arpa.   172800  IN      NS      ns1.yahoo.com.
2.30.72.in-addr.arpa.   172800  IN      NS      ns2.yahoo.com.
2.30.72.in-addr.arpa.   172800  IN      NS      ns5.yahoo.com.
2.30.72.in-addr.arpa.   172800  IN      NS      ns3.yahoo.com.

;; Query time: 201 msec
;; SERVER: 208.74.8.101#53(208.74.8.101)
;; WHEN: Wed Mar 31 21:12:17 2010
;; MSG SIZE  rcvd: 169