Cisco 1720 NAT need 3 segments with 2 interfaces

[shenmue2]

Greetings,

I have a Cisco 1720 which I would like to run a NAT - only thing is that I only have two interfaces. One serial interface that communicates with the ISP, and the ethernet interface with has the public ip range. I'm want to know how to make a private ip NAT, but is this possible with two interfaces without eliminating either current segments. I need the public range - since it is required to connect to the ISP's DNS server.

Can someone please help?

-Shenmue2
-------------------------------Here is my non NAT config----
hostname Cisco1720
!
enable password
!
!
!
ip subnet-zero
no ip domain-lookup
!
!
!
process-max-time 200
!
interface Serial0
description connected to Internet
ip address <IP REMOVED - ISP Serial IP>
no ip directed-broadcast
no keepalive
service-module t1 remote-alarm-enable
!
interface FastEthernet0
description connected to EthernetLAN
ip address <IP REMOVED - PUBLIC IPS>
no ip directed-broadcast
!
ip classless
ip route 0.0.0.0 0.0.0.0 Serial0
no ip http server
!
snmp-server engineID local <removed>
snmp-server community public <removed>
!
line con 0
exec-timeout 0 0
password 7 <removed>
login
transport input none
line aux 0
line vty 0 4
password 7 <removed>
login
!
no scheduler allocate
end

 

[Beggo]

Hi Shenmue2

you need to enable NAT on both interface

interface Serial0
ip nat outside

interface FastEthernet0
ip nat inside

ip nat inside source list 1 interface Serial0 overload

create access-list

access-list 1 permit any

this configuration should work fine..

Beggo

 

[fmonteiro]

With NAT you do not need to keep the public ip in the Ethernet segment. You could use a private one and NAT would translate to the public address when accessing the Internet. Everything else is just as eggo said.

 

[shenmue2]

Thanks Beggo - I tried your suggestions

Here's the config I ended up trying and it works to some degree - but then the clients intermittantly can not get out. Any ideas why?

-shenmue2
!
version 12.0
service timestamps debug uptime
service timestamps log uptime
service password-encryption
!
hostname Cisco1720
!
enable password 7 <omitted>
!
!
!
!
!
ip subnet-zero
no ip domain-lookup
!
!
!
process-max-time 200
!
interface Serial0
description connected to Internet
ip address <serial ip – ISP/WAN>
no ip directed-broadcast
ip nat outside
no keepalive
service-module t1 remote-alarm-enable
!
interface FastEthernet0
description connected to EthernetLAN
ip address 192.168.1.1 255.255.255.0 secondary
ip address <public ip>
no ip directed-broadcast
ip nat inside
!
ip nat pool ovrld <1 of the public ips> <1 of the public ips> <subnet>
ip nat inside source list 7 pool ovrld overload
ip classless
ip route 0.0.0.0 0.0.0.0 Serial0
no ip http server
!
access-list 7 permit 192.168.1.0 0.0.0.254
snmp-server engineID local <omitted>
snmp-server community public <omitted>
!
line con 0
exec-timeout 0 0
password 7 <omitted>
login
transport input none
line aux 0
line vty 0 4
password 7 <omitted>
login
!
no scheduler allocate
end