ARP necessary?

Hi
We have two subnets that are connected with a Cisco router. One of the
machines on subnet A is not visible from the subnet B (doesn't respond
to ping)

I needed to add this command (as root) to my server (192.168.5.152):

arp -s 192.168.5.58 000:BA:2D:CE:61

If the machine is rebooted, this will probably get cleared. So I added
the line to /etc/rc2.d/S99local so it should automatically happen on a
reboot.

I'm wondering if a new server is added to Subnet A, we have to repeat
this process which is not a very good thing to do.

Is there a better way to do this?
Thanks