ISP gave us additional /29 but it needs to route through an IP of our existing primary /27

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ISP gave us additional /29 but it needs to route through an IP of our existing primary /27

L0 Member

We requested additional public IPs from our ISP. They gave us an additional  /29 to add to our /27 but we had to choose one IP in that original /27 to route this /29. I'm trying to determine how to do this properly. Let me know if more clarification is needed. 




L7 Applicator

Please provide more details to what you are trying to accomplish. 

Anything special as far as routing? Protocols? 

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Nothing special just static routing. Just trying to utilize this additional subnet for mostly outbound. I mainly just need to figure out how I can route this /29 subnet to using an IP in my already established /27 as the next hop. All public routes for this new subnet are already configured by ISP to route to our existing IP in the /27. 


Existing /27  - ISP gateway  - outbound general  - inbound service 1

......  - IP that we need to use as gateway for new /29


new /27 - Next hop needs to be



Not sure you need to? So the ISP is sending all IP's in your /29 and /27 to your firewall, so on your firewall you create your NAT's, etc. then on the firewall set the default route out to what it is now, i.e. to your /29 gateway since that is probably in the range you have your current external IP in.



Hi @jsafranek ,


You don't have to anything from your side. It all up to the ISP to properly route the new /29.

- Because your firewall have assigned IP in the /27 when ISP needs to route traffic to your ISP will ask who has the MAC address of <your-fw-ip> and send the traffic directly

- Now the new /29 is not assigned anywhere (and there is no need). So in ISP gateway needs to know how to reach this network. For which they need to create static route pointing to your firewall, an IP that will reply to the ARP request.


To put it simple:

- Your IP needs to create route on their end for the new /29 pointing to the IP assigned to your FW in the old /27

- You don't have to do any routing or IP configuration.

- You can simply start creating NAT rules assigning IP addresses from the new /29




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