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ST_Wong's avatar
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Jul 12, 2023

Multiple IP addresses ranges on external network


We've LTM ( running for years, serving VLAN with addresses

Recently the same VLAN 'expanded' with another address range (

We tried to define virtual server in this address/netmask range but failed (invalid destination/mask setting).

Possible to serve this new IP address range on the same LTM ?

Thanks a lot.



9 Replies

  • Is this new subnet used only for Virtual servers?

    If you point this subnet via routing to the F5 it should work. 

    You can define this subnet as a blackhole type of network on the F5.,and then you should be able to create VS from this subnet.



  • You should be able to define the Virtual Server listener IP on any network, F5 will automatically create a Virtual Address object to match it.

    It's not required to have a self-ip on that network as long as routing points to F5, like mihaic said. 

    • JRahm's avatar
      Icon for Admin rankAdmin

      Good call, CA_Valli. In fact, early days of v9 when I was a customer, my SEs recommended as a best practice to avoid defining a self-IP on the public facing interface for DMZ-deployed boxes.

  • Hi all,

    Thanks for all your help.

    Inbound traffic now works, however, the default gateway on original subnet seems won't help routing traffic for    All outbound traffic to non-local network got timeout.   

    Can we forward outbound traffic for the new IP range to corresponding gateway ?

    Thanks and Best Rgds

    • JRahm's avatar
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      a diagram of what's local and what's remote and where you're wanting traffic to route would be helpful.

      Just note a couple things:

      1. auto-lasthop will return application traffic responses to where it came from unless you disable that behavior, so for response traffic anyway, you don't even need a default route from BIG-IP.
      2. An effective route is not enough to forward traffic through BIG-IP, you need to enable a network forwarding virtual server that is enabled on the ingress vlan in order for traffic initiating from that ingress network to egress the BIG-IP to another vlan.
  • first, the VS should be /32, usually if it is only one.

    Second I would split that /23 in 2  /24 for example. And use one for the VS if you plan to have many VS's.

    Then you could use some static routing for example on F5 and the router .

    If you let auto-lasthop enabled on F5 , you won't need a route on the F5.

  • Hi all,

    Thanks for your help.

    I tried to define a VS using the new subnet address before I did anything, with destination/mask setting to. say. or   But LTM complaints the value is invalid.    I can only provide a mask of or simply no mask at all.  

    Possible to specify the correct netmask (/23) in virtual server destination ?

    Thanks again.

    Best Regards

    • JRahm's avatar
      Icon for Admin rankAdmin

      Hi ST_Wong  the virtual server listens on a host or range of hosts as a CIDR block. So if you are trying to establish a /23 network for the virtual server to listen on, then it needs to be a valid network address to do so. If you are only wanting to listen to the host, then you do not need to specify the mask on the destination at all. 

      The virtual server is not an L3 routing table. Routes assure that traffic gets to/from the BIG-IP, the virtual server is how traffic flows through the BIG-IP.

  • Hi all,

    Here comes a simple diagram:

                   VLAN 123
       |                               |	   
    (gw            (gw
      LTM (self IP on
      VS (

    Some test results when ping from host (

    * To  OK, except the gateway

    * To all other networks: no response

    Inbound traffic from anywhere to the VS is okay.

    Since the outbound packet goes through gw, i wonder if the gw will route any reply to, where the packets will be discarded to prevevnt IP spoofing.  Just wild guess...

    Can I configure LTM to send outbound packets for through gw instead of using default gateway of LTM ?     Sorry for the newbie question.

    Thanks and Best Rgds