internet sharing in a complex network

[purashuri]

hi all, i am an admin of a complex network.
i have a problem in internet sharing in my network. plz see my description of the scenario that i have in my network :
i have a cisco 6500 core switch with ip address : 172.16.52.1 with 12 zone switches (2950) connected to it. this core switch is also my default gateway
for all of my clients.
in addition i have to server with os win 2000 ad server connected to my core swithc with optic fiber NICs too.
on of server is DC for lan (DNS , DHCP , ...), that connects to core switch with optic fiber nic.
and another server is for internet sharing with two
nic's, one is optic fiber that connects to Core switch and another is 100 mbps that connects to my internet line.
see the ip addresses that i gave to my nics' and machines :
-core switch : 172.16.52.1 (default gateway for all machines)
-DC optic fiber nic : ip address : 172.16.0.25 (connects to core switch)
Default Gateway : 172.16.52.1
DNS server : 172.16.0.25
WINS server : 172.16.0.25
-Internet Sharing Server : 1- internal nic :
ip address : 172.16.0.30
default gateway : 172.16.52.1 ***
(*** i should set this to my core switch ip, if i didn't i couldn't see the lan ***)
Dns server and wins server : empty
2- External nic :
ip address : 2XX.X.X.X (from my ISP)
default gateway : 2XX.X.X.X
Dns Server : 2xx.X.X.X
wins server : empty
ok, now my problem for internet sharing : i couldn't use NAT! cause if i do , then the clients should have the ip address gateway of my ISP(2XX.X.X,X) for their default gateway ip address, but i couldn't give this to my clients, because if i did this, my clients couldn't see each other on lan. the only default gateway that i should gave to them is 172.16.52.1 (my core switch ip address).
so i decided to use proxy server : i tested winroute, winproxy,wingate, ezproxy and ...
but all of them have one problem : sometime to sometime my clients internet connections be disconnects!!!
i found the problem but i can't solve it : i should left the internal nic default gateway ip address for my internet server , empty. but i can't , because if i did this , my internet server can not see my lan ! and if i didn't, then my internet connection become sometime to sometime disconnects for clients!
(sometime to some time : i mean that for exapmle my clients work well with internet for some minutes
and have not any problesm, but suddenly become disconnect from internet , such as they couldn't see the internet server, and this scenario repeats ) can any body say me how i could solve my probelm ????
some persond said to use ISA server , but i think it can't solve my problem , another person said that you can configure the core switch somehow that it do the
internet sharing job. but i am not an expert in IOS.
and anothers said another things.
what i should do ? i asked many admin's , but nobody could help me !
don't forget that i couldn't use router at this time.
i am so confused plz help me !

 

[Parcival21]

so you are using a class B private subnet.
I did not quite understand why you cannot use NAT because this should be a normal scenario for this.
Your internet sharing server could be configured as a router and then you can configure NAT on the server.
Why can't you use a router.
Another suggestion: Post you post in cisco switch forum because the guys in that forum know everything about cisco switches. so if your problem can be solved there, they will help you.
good luck,
busche