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There's this Cisco 2500 with one V.35 port going to an HDSL DCE, and the ethernet port to a Cisco PIX; there are also other IPs (subnet 80.something) in the configuration on the serial side that are used to route traffic to the DSL provider.

I'm wondering if it is possible to remove the PIX and move everything on the Cisco 2500 - I can usually manage to do this when the public IP is on the wan-facing port and the private IP on the lan one, but I don't know how could I add a private IP to the 2500 and use the public one for nat when they're both on the same internal interface.

The current config for the 2500 looks like:

ip subnet-zero
interface Ethernet0
 description connected to PIX
 ip address 217.x.x.1
 ip nat inside
 no ip directed-broadcast
 no ip mroute-cache
 no cdp enable
interface Serial0
 no ip address
 no ip directed-broadcast
 no ip mroute-cache
 encapsulation frame-relay
 bandwidth 1024
 no cdp enable
interface Serial0.1 point-to-point
 ip address 80.x.x.x
 ip nat outside
 frame-relay interface-dlci xxx IETF
 bandwidth 1024
 no ip directed-broadcast
 no cdp enable
ip classless
ip route Serial0.1

The PIX has 217.x.x.2 on its WAN interface and uses the 2500's 217.x.x.1 as default gateway.

I would like to remove the PIX and connect the LAN directly to the Cisco 2500, which would have on its Ethernet port and NAT everything out from one of the 217.x.x.X IPs. After that I will probably want to also add other static IPs to the Cisco from the 217.x.x.X/29 subnet and use them to portforward some services to internal servers.

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up vote 0 down vote accepted

There's lots of documentation on about NAT services.

Try starting at:

As a general guide, you would want 'ip nat outside' on s0/0.1, 'ip nat inside' on e0.

You would make a NAT pool using your 217.x.x.1/29 addresses, that things on the inside could use. They would not appear on an interface anymore.

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