router to assign the same IP address

Hi,

I have a router (compusa IP address at 192.168.2.1) and I'd like to be
able to test many computers on that router ONE computer at a time.
I have the test program and so I would like to have the router
assigning the SAME IP address everytime I connect ONE new computer.
I did limit the IP address in the DHCP from the start IP at
192.168.2.100 and the end IP address at: 192.168.2.101 (one for a host
PC and one for the PC under test).
It's sort of working, but everytime I have to power cycle (reset) the
router.
The router probably needs to refresh/clear/reassign the ARP cache
table really quick ? can we set/reset this ? how ?

Thank you in advance
Phil

ps: if you can e-mail me direct to (E-Mail Removed) that would be
nice thanks again