If you are connecting to the internet get rid of the 1751 or it will have to do NAT on it. Have the users get their default route pointing to the PIX (learned via DHCP) and have the PIXs default route pointing to the internet (your ISP router).
If this connects to your private network, keep the 1751 and your addressing the same. The users point to the PIX again and the PIXs default route points to the 1751. The 1751 doesn't need a static route to your network as the NATed IP will be on the same subnet as the router/PIX (seen as directly connected network).
See sample config below:
pixfirewall# sh config
: Saved
PIX Version 6.1(4)
nameif ethernet0 outside security0
nameif ethernet1 inside security100
hostname pixfirewall
domain-name xxxx
fixup protocol ftp 21
fixup protocol http 80
fixup protocol h323 1720
fixup protocol rsh 514
fixup protocol rtsp 554
fixup protocol smtp 25
fixup protocol sqlnet 1521
fixup protocol sip 5060
fixup protocol skinny 2000
names
pager lines 24
logging on
logging buffered informational
logging timestamp
logging trap informational
logging host inside 192.168.0.x
no logging message 106015
no logging message 106007
no logging message 105003
no logging message 105004
no logging message 309002
no logging message 305012
no logging message 305011
no logging message 303002
no logging message 111008
no logging message 302015
no logging message 302014
no logging message 302013
no logging message 304001
no logging message 111005
no logging message 609002
no logging message 609001
no logging message 302016
interface ethernet0 10baset
interface ethernet1 10full
mtu outside 1500
mtu inside 1500
ip address outside 216.39.x.x 255.255.255.248 (or 172.16.1.2 if not connected to internet)
ip address inside 192.168.0.1 255.255.255.0
ip audit info action alarm
ip audit attack action alarm
pdm logging informational 100
pdm history enable
arp timeout 14400
global (outside) 1 interface
nat (inside) 1 0.0.0.0 0.0.0.0 0 0
route outside 0.0.0.0 0.0.0.0 216.39.0.1 (or 172.16.1.1 if not connected to internet)
timeout xlate 0:05:00
timeout conn 1:00:00 half-closed 0:10:00 udp 0:02:00 rpc 0:10:00 h323 0:05:00 sip 0:30:00 sip_media
0:02:00
timeout uauth 0:05:00 absolute
aaa-server TACACS+ protocol tacacs+
aaa-server RADIUS protocol radius
no snmp-server location
no snmp-server contact
snmp-server community test
no snmp-server enable traps
floodguard enable
sysopt connection permit-ipsec
no sysopt route dnat
telnet 192.168.0.0 255.255.255.0 inside
telnet timeout 5
ssh x.x.x.x 255.255.255.255 outside
ssh timeout 5
dhcpd address 192.168.0.2-192.168.0.254 inside
dhcpd dns x.x.x.x
dhcpd lease 3600
dhcpd ping_timeout 750
dhcpd domain inside
dhcpd auto_config outside
dhcpd enable inside
terminal width 80
Hope it helps.
Steve
