Do you have your master server name in the configuration of the clients? Check to make sure that the client name configured on the clients is the same as you have configured for it on the master. Do you have multiple NICs on the clients? If so you might need a required interface or something like that.

ping    both ways by ip & hostname
nslookup    both ways by ip & hostname (do this twice in case of duplicates)
bpclntcmd    this T/N explains in depth:

DOCUMENTATION: Explanation of bpclntcmd options, the system calls being used, and recommended troubleshooting when the commands return errors.

telnet into the mix

can you telnet to 13724 to the master from the client and vice versa?

Tell us what "can't scan netbackup client" means.
cannot connect to clients during backup?
Or, can not be found in bpplclients(or Host properties on GUI)?
 

Thanks all .

Now I can connect to clients run OS : Solaris 10 and Redhat 4 enterprise .
But I can't connect to clients run OS : CentOS 4.8 .  Have Netbackup 6.5 support client run OS CentOS 4.8 ? 
Please Help me !

What did you do to resolve your connection issue with the Solaris client.  There is a good possibility that whatever afflicted your Solaris client is also causing connection issues with your CentOS client.

As for compatibility, officially it looks like NBU supports only CentOS versions 5.2 and beyond, but maybe there's someone on this forum that has been able to get older versions to work.  Below is the link to the compatibility matrix:

ftp://exftpp.symantec.com/pub/support/products/NetBackup_Enterprise_Server/325328.pdf

CentOS is listed on pg. 10.

Below is a technote from the compatibility matrix regarding CentOS considerations- it might be worth a ready as well:

http://seer.entsupport.symantec.com/docs/301525.htm

It is possible that xinetd/inetd is not running or may need to be refreshed.

telnet localhost bpcd [if you can successfully connect] then bpcd is listenting.

you can also try this.

netstat -a | grep bp
tcp        0      0 *:bpcd                  *:*                     LISTEN
tcp        0      0 *:bpjava-msvc           *:*                     LISTEN

if the above 2 commands fail then it means most likely it is xinetd/inetd problem.
you can start bpcd in standalone mode and see if you can make connection.
/usr/openv/netbackup/bin/bpcd -standalone