External IPs on OPT1

  • Hello,

    I have a vexing issue. I have two WAN subnets being routed to me by my ISP.

    x.x.246.224 / 30
    x.x.248.0 /26

    x.x.246.226 is my WAN IP Address with x.x.246.225 as its gateway

    My LAN has / 16 subnet and works fine.

    My OPT1 interface is the problem. I took a small piece of the x.x.248.0 subnet and allocated it to OPT1.

    OPT1's IP address x.x.248.48 / 29 (no gateway, no bridge, and no outbound NAT)

    I have rules to allow all traffic back and forth.

    If I take a laptop and set it's IP address to x.x.248.50 /29 and connect it to the OPT1 network, it works perfectly. I can hit the outside world and they see me as x.x.248.50

    My problem is that I need to hook a Cisco router up to that little subnet and it doesn't work. I set the Ethernet interface on the Cisco to x.x.248.51 /29 connect it to the OPT1 network alongside the laptop and it can't get anywhere. It sees a connected route of x.x.248.48 (right, I think), but it can't ping x.x.248.48

    It can ping the laptop at x.x.248.50 (and vice-versa) no problem. I can ping from the firewall to x.x.248.50, but cannot to the Cisco at x.x.248.51

    Anyone have any ideas? Thanks a million!

  • Laptops are a bit more forgiving than Cisco. Cisco sees the 248.48 as the subnet ID for the /29 you are breaking off. The first usable address in 248.48/29 is .49. Could you try setting your OPT1 interface to that address and see if it works. Otherwise, set your subnet mask to /26 to match the range given by the ISP and see if that works.

  • Fixed! Thank you, podilarius! Sometimes it's the little things… I gave OPT1 The 248.49 address, changed the default route to that in the Cisco and voila! works like a charm.

    A million thanks!

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