2 subnets on the same interface

Patrick_Jessett — Fri, 26 Nov 2021 16:43:06 GMT

We current have 1 subnet linked to an layer 3 interface which is supplied by our isp. We have run out of ip addresses and our isp want to present another subnet but on a completely different range. (too many services to move to a new range)

Is it just a case of adding an ip address from the new subnet to the current interface so there will be 2 gateways ?

Re: 2 subnets on the same interface

TomYoung — Fri, 26 Nov 2021 18:19:09 GMT

Hi @Patrick_Jessett ,

You do not need to add another subnet to your interface. As long as the ISP has a route for the new subnet to your existing IP address, it will work fine. Notice in the 3rd paragraph of this article -> https://knowledgebase.paloaltonetworks.com/KCSArticleDetail?id=kA10g000000ClSDCA0 that for "NAT configuration, the additional IP addresses do not necessarily need to be configured on the interface". Configure NAT for IP addresses in the new subnet, and the NGFW will translate them.

This practice of NATing a different subnet than on the interface is quite common and works well for exactly this scenario. All firewalls do it, and you do not need to burn an IP address on the interface.

Thanks,

Tom

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2 subnets on the same interface

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