First of all, where do those subnets sits? Do you used dedicated PIX525 interface to host them? Are you running PIX6.3(x) or PIX7.x?
If yes, do static nat for the subnets, see summarize example below:
PIX 6.3(x):
!
interface e1 inside security100
interface e2 dmz security50
!
ip address inside 172.x.x.1 255.255.255.0
ip address dmz 192.x.x.1 255.255.255.0
!
static (inside,dmz) 172.x.x.x 172.x.x.x
The above static command allows you to use original IP from both segmen to access each other.
For latest PIX7.x, and if both of these segments are with same level of security (i.e security 100) located on different interfaces, you can use the same-security-traffic permit inter-interface" command.
static (inside,dmz) 10.1.1.0 10.1.1.0 netmask 255.255.255.0
static (dmz,inside) 10.1.2.0 10.1.2.0 netmask 255.255.255.0
http://www.cisco.com/en/US/products/ps6120/products_configuration_guide_chapter09186a008045247c.html#wp1009571
HTH
AK
