How to configure an ISP with a gateway on a different subnet?

thebard — Fri, 18 Aug 2023 14:26:22 GMT

I got myself a weird one, for me at least. Got a remote office I'm not much involved with that got a new internet line and I'm supposed to configure it, but the info I got is just this:

Client IP: 50.150.200.6 / subnet mask 255.255.255.255

50.150.201.136-139 / subnet mask 255.255.255.252

Gateway: 170.140.30.1

DNS: 1.1.1.1/9.9.9.9

(IPs are obviously fake, but the difference between them, their subnets and positions inside them are correct and all)

And that's it. No DHCP enabled, ISP router is a Microtik with 4x 10Gb ports that we have no access to. No additional info has been provided, local colleague tried asking for more info but they said basically "that's it, good luck"

I'm not a huge expert but I did configure quite a good number of internet lines around and I never ever seen anything like that, with the gateway outside of the subnet of the IPs. Not much hope in contacting their assistance because they don't speak any language I know.

Any idea on how should I configure the interface/virtual router/nat rules for this ... thing?

I am sure it's something simple and dumb to configure and I'm just missing it but I'm swamped and confused.

Re: How to configure an ISP with a gateway on a different subnet?

TomYoung — Sat, 19 Aug 2023 13:30:11 GMT

Hi @thebard ,

The solution to this puzzle is not simple and dumb. However, I believe this is your solution:

Create a default route to 170.140.30.1.
Create a route to 170.140.30.1/32 pointing to your interface only. The NGFW will consider it connected and ARP for the MAC address. https://knowledgebase.paloaltonetworks.com/KCSArticleDetail?id=kA10g000000Clp5CAC&lang=en_US
If that does not work, verify their configuration with a laptop configured with your parameters above since a laptop allows a default gateway to be assigned to the interface. If the laptop does not work, the ISP is jacked.

Thanks,

Tom

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How to configure an ISP with a gateway on a different subnet?

Re: How to configure an ISP with a gateway on a different subnet?