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Community Member

Using two public IP blocks


- There is a rack of servers for our clients who are all being served by our ISP through an Cisco 3560.

- Our ISP provided us with a block of two public IP subnets.

- Our ISP provided a gateway for both of those blocks which is not on either subnet.

Normally, I would setup one IP as a routed port with an IP in the subnet to use as my interface with the WAN connection coming in from the provider. In this case, I'm a bit confused as to how to best set this up to get proper communication since the gateway is outside of both subnets. I tried setting up a routed port with an IP address in one of the subnet blocks and forcing IP Route to point to the default gatway given, but it doesn't seem to be able to communicate (find a route) with the gateway.

Example blocks:

Block 1:




Block 2:




Any help is appreciated.


Hall of Fame Super Blue

Re: Using two public IP blocks

So there is no NAT going on ie. you are assigning the public IPs to actual devices ?

The gateway address of that is their end is it ? Did they not assign you an IP from the same subnet ie. usually they use a /30 so it would be -  where they use .145 and you would use .146.

Obviously i'm not suggesting you just do this but this is normally how it works so it is perhaps worth checking with them.


Community Member

Using two public IP blocks

No nat. That IP is their device.

That is my thinking as well. But no they did not assign an IP in that subnet for our usage. It's strange to me, but I thought maybe I'm missing though since I'm not too familiar with datacenter setups.

Hall of Fame Super Blue

Using two public IP blocks

With point to point links using PPP etc. you can have the two ends in different subnets and it works because on a point to point link there is only one place to send packets.

However with ethernet even if the default route was pointed out the interface it would still arp out for every unknown address and i'm pretty sure your ISP does not want this ie. arping out for every single internet address.

I would talk to them and see if they were meant to send you an address for your end.

This is not DC specific, it is more to do with basic routing.


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