Routing with gateway on a different subnet

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Jun 20th, 2009
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I'm trying to configure a router but ISP provided address doesn't match up.

range: to 128

subnet mask:


If I use any IP from the range above on Windows XP, I can connect to the internet.

The ISP said it should work. And most of their clients are configured that way. And they don't have problems.

It just doesn't make sense.

Anybody have any idea why its working? Can someone provide more info why it works on XP but not on my router?

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Giuseppe Larosa Sat, 06/20/2009 - 09:04
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Hello Riann,

what if you configure on the router a default route and you specify only the outgoing interface

ip route fas0/0

in this way your device relies on proxy-arp on ISP device

I guess that is only a fake GW and PCs use proxy-ARP concept to find their way to the internet.

Hope to help


r.banez Sun, 06/21/2009 - 01:18
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After I've added the following:

ip route fa0/0

no ip route

I can now ping the gateway ( but still no route to the internet. I cannot also ping any other IP outside/past the

I was told by ISP that the IP range they gave and the gateway is on the same vlan. And that IP is on a Router with Firewall.

My guess is that his equipment checks all packets and see next hop is his IP, it picks up the data and route it. But wouldn't it be noisy on his network?

Giuseppe Larosa Sun, 06/21/2009 - 03:29
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Hello Riann,

until the provider device doesn't build a NAT entry for your router ip address you cannot reach the internet.

Also you need a DNS server.

Be aware that some provider device can be configured with a limit in max. number of MAC addresses to be served that is usually the reason to place a router there to overcome this limitation.

However, you also need to NAT on your router to use a PC that is connected to an internal interface in another subnet: you should nat using the "WAN" interface name: that IP subnet is not known to the service provider device.

int f0/0

ip nat outside

int f0/1

ip nat inside

ip addr

access-list 11 permit

ip nat inside source list 11 int f0/0 overload

Hope to help


r.banez Sun, 06/21/2009 - 09:06
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I just reviewed my original post. I forgot to mention that the IP were changed to private IP numbers for security reason, but keeping it close to the original IP as much as possible.

The IPs that was provided by the ISP are all public routable IPs.

NAT is already in place in my router (for internal network PCs).

The XP workstation is connected in parallel with the router. That one works but router doesn't. The router can only ping the ISP gateway but I can't route to it.


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