Can an ADSL WAN interface share the same IP range as the LAN Interface

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Aug 30th, 2007
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Can anyone tell me if the following is possible?

I have an 1801 router connected via ADSL to my service provider. My ISP has provided me with 8 IP addresses ( The LAN interface connects to a Watchguard Firebox firewall. The LAN interface is using the IP range xx.x.x.168/29 and I'm having to use NAT on the router to pass the external IPs to the internal IPs. What I'd like to know is if its possible to bind (if thats the correct phrase) the WAN and LAN interfaces together so the router and the Firebox are both on the external IP range and I don't have to use NAT on the router.

I hope this makes sense to someone, and any help would be greatly appreciated.


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mohammedmahmoud Thu, 08/30/2007 - 23:45
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Hi Vincent,

No this is not possible, a router can't have 2 interfaces using the same IP address range, you'll get the "% x.x.x.x overlaps with ".


Mohammed Mahmoud.

d.bigerstaff Fri, 08/31/2007 - 00:43
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I have a similar setup using ADSL and here is my relevent configs.

interface ATM0

no ip address

atm ilmi-keepalive

pvc 0/38

encapsulation aal5mux ppp dialer

dialer pool-member 1


dsl operating-mode auto


interface Dialer0

description ADSL to Griffin

ip unnumbered Ethernet0

no ip redirects

no ip unreachables

no ip proxy-arp

encapsulation ppp

dialer pool 1

dialer-group 1

ppp authentication chap callin

ppp chap hostname [email protected]

ppp chap password 0 xxxx

ppp ipcp dns request


interface Ethernet0

description LAN Ethernet server connection

ip address 85.x.x.145

hold-queue 100 out


This is an 800 series router so it has a 4port switch intergrated. With ethernet0 as the LAN interface.

With this setup I can plug any device into that 4port switch and assign it a public ip within the range of 85.x.x.144/29

mohammedmahmoud Fri, 08/31/2007 - 02:02
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The use of ip unnumbered as illustrated by David can be used as a work around.


Mohammed Mahmoud.


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