Is there some command in linux/unix by which i can find a URL(Domain name)
if i have the ip of that server
please help me urgently require 
You can get a PTR record by using dig -x <IP address>. For example:
$ dig -x 216.109.117.206
; <<>> DiG 8.3 <<>> -x
;; res options: init recurs defnam dnsrch
;; got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 405
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 5, ADDITIONAL: 5
;; QUERY SECTION:
;; 206.117.109.216.in-addr.arpa, type = ANY, class = IN
;; ANSWER SECTION:
206.117.109.216.in-addr.arpa. 20M IN PTR p19.www.dcn.yahoo.com.
;; AUTHORITY SECTION:
117.109.216.in-addr.arpa. 2D IN NS ns5.yahoo.com.
117.109.216.in-addr.arpa. 2D IN NS ns1.yahoo.com.
117.109.216.in-addr.arpa. 2D IN NS ns2.yahoo.com.
117.109.216.in-addr.arpa. 2D IN NS ns3.yahoo.com.
117.109.216.in-addr.arpa. 2D IN NS ns4.yahoo.com.
;; ADDITIONAL SECTION:
ns1.yahoo.com. 14h58m50s IN A 66.218.71.63
ns2.yahoo.com. 14h58m50s IN A 66.163.169.170
ns3.yahoo.com. 14h58m50s IN A 217.12.4.104
ns4.yahoo.com. 14h58m50s IN A 63.250.206.138
ns5.yahoo.com. 14h58m50s IN A 216.109.116.17
;; Total query time: 66 msec
;; FROM: smbigip1.str.nexteldata.net to SERVER: 167.20.228.102
;; WHEN: Sat Apr 9 08:29:04 2005
;; MSG SIZE sent: 46 rcvd: 251
Or you can cut out just the hostname as:
$ dig -x 216.109.117.206 | grep PTR | cut -f7 -d" "
p19.www.dcn.yahoo.com.
Cheers,
Keith