Could not get DHCP information when install os from ignite server

[tianchen_1]

Hi,
When I try to create an auto lanboot tools to install os from ignite server. I saw the error message like this for some of the some of the IA clients(not all).The client and the ignite server not in the same subnet.
error message like this.
********************************************
This could take up to 30 seconds if a DHCP server cannot be found. If
you wish to cancel the DHCP server search, you may press CTRL-C now.
* Could not get DHCP information. No host specific network defaults
will be supplied. (dhcpclient returned: 5)
ERROR: Hostname and/or IP address was not set with instl_adm, and not
obtainable via DHCP.
ERROR: Failed to Start networking.
***********************************************
I add the client information into /etc/bootptab. And it seems that the lanboot script do not read the information here.
so I try to setup DHCP on the ignite server as a temporary IP for usage.
I edit /etc/dhcptab, add these information:
dhcp_pool_group:\
pool-name=group1:\
addr-pool-start-address=15.1.162.66:\
addr-pool-last-address=15.1.162.69:\
reserved-for-other=15.1.162.66 15.1.162.69:\
lease-time=604800:\
lease-policy=accept-new-clients:\
allow-bootp-clients=FALSE:\
hn:\
subnet-mask=255.255.248.0
I also add these information via instl_adm
disable_dhcp=false
dhcp_class_id="IgniteDHCPDeviceGroup"
is_net_info_temporary=true
run_ui=false

But the error still exist without any change.
Any one who can help me to have a look?