My ISP has given me a router with the following information..

enter image description here

The router is provided by the ISP and the gateway/routerIP is already preset inside.

If I were to add a firewall behind the router as per above, and i would like to place my servers behind the firewall BUT

  • no NAT is to be set on the firewall
  • the servers are to have the public IPs allocated by the ISP

Does that means i would need to further subnet the allocate public IP range given by the ISP into 2 different network

1 - for network between firewall external interface and ISP router internal interface (

2 - for network between firewall internal interface and the rest of the servers.

and i will have a total of 4 ips (for network and broadcast) wasted

Is my understanding correct ?

Regards, Noob

  • This depends a lot on the firewall model (which you didn't list). Some firewalls allow you to bridge the interfaces. A firewall doesn't need to be a router, too; it just needs to inspect the traffic flowing through it.
    – Ron Maupin
    Feb 18, 2016 at 15:44
  • @RonMaupin thanks for the advice, i dont have any brand in mind yet, its still a concept yet.
    – tiongmaru
    Feb 19, 2016 at 11:45

1 Answer 1


You're understanding is correct. Note that this would require that the ISP change the subnet mask on its side.

However there's other alternatives :

You can ask you ISP to provide an interconnection network (likely a /31, possibly a /30).

If the ISP is not willing to spend additional public IP, the link between the ISP router and your firewall can be configured as unnumbered. Once again, it depends of what your ISP is willing to do...

Since on a point to point link, there's only one possible destination (the other equipment on the link), IP addresses are not necessary.

  • hi, thanks for the confirmation! by unnumbered, do you mean that there will be no ip address assigned ? and the route are specified by the exit interface instead of the next hop ip ?
    – tiongmaru
    Feb 18, 2016 at 15:12
  • Yes, this is exactly how unnumbered links work.
    – JFL
    Feb 18, 2016 at 15:23

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