Thank you!

Here is the output:

prodipas@ssydpapp007 /data/ipas/d2/ipsmk2 $ nslookup 10.68.129.57
prodipas@ssydpapp007 /data/ipas/d2/ipsmk2 $ nslookup 10.68.129.57
*** Can't find server name for address 10.68.128.1:No response from server
*** Can't find server name for address 10.72.128.1:No response from server
*** Default servers are not available
prodipas@ssydpapp007 /data/ipas/d2/ipsmk2 $

It took 75 seconds to display first line. and then 75 seconds again to display remaining output. Earlier it used to display different output , dont remember very quickly. The IP address for server , users generally connect or I used to connect is 10.68.129.57 I am not sure what is 10.68.128.1 ? Something looks to be changed as nslookup output is differnt now.

I dont see any changes made recently in this file too:

prodipas@ssydpapp007 /etc $ more resolv.conf
nameserver 10.68.128.1
nameserver 10.72.128.1
domain au.challenger.net
prodipas@ssydpapp007 /etc $ ls -lt resolv.conf
-rw-r--r-- 1 root system 71 Apr 17 2007 resolv.conf
prodipas@ssydpapp007 /etc $

Please advice ... whats going on?

Looks like the DNS servers are down.

Try using another DNS server, such as 208.67.222.222 of OpenDNS. You can add a "nameserver" line which points to that new IP at the top of /etc/resolv.conf (before the existing lines). See if there is any difference.

OpenDNS | Providing A Safer And Faster Internet

Hmmm... What exactly do we mean by DNS Server? What command is used to bring up DNS Servers?

As porter has said, it finds the IP address associated with a domain and host name such as 'www.unix.com', when you type the URL in the location bar for instance, because the Internet uses IP address for communication rather than names.

Domain name system - Wikipedia, the free encyclopedia

You don't need to do anything with DNS servers. Most of us simply use one of the many available. If a DNS server is down, the easiest thing you can do is to tell your computers to use another one, and see if it is ok again. OpenDNS is a reliable free DNS provider, and that can provide us with a more reliable basis for isolating DNS-related issues.

I have learnt that few days back the DNS team has removed the DNS Servers. And the IP mentioned in /etc/resolv.conf does not exist anymore.

Do you think, that has caused the issue?
Which all places do you suggest to modify DNS new IP on UNIX?
After modification, does a server reboot require or any command to be supplied after modifications to have new DNS activated on each UNIX box?

Of course that matters. You should update all hosts which use those DNS server IP addresses and change them to another provider.

Change them in /etc/resolv.conf as mentioned.

No, you don't need to reboot. Usually the change is instant, but for certain services you may need to restart service (such as mysql) to make sure the change really takes effect. If you can afford to, reboot is the easiest way to have everything reinitialized.