I am just beginner, I just want to setup my mini data center and for testing i get a FTTH connection and /30 ip pool, ISP reply is something like that, "IP Pool 103.15.xx.xx/30 has been assigned and routed with 112.133.xx.xx, Now i have a 1U rack server and created 2 Virtual machine on that and want to assign 2 static ip into each VM, I have connected my Server via ethernet cable from ONT. Please help me as beginner.

Here is my ISP's Ticket Reply


  • A /30 network only has two usable IPv4 addresses: one for the ISP router, and one for your router. There are no other usable addresses for you in there.
    – Ron Maupin
    Commented May 18, 2022 at 21:58
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer.
    – Ron Maupin
    Commented Nov 19, 2022 at 23:01

3 Answers 3


103.15.xx.xx/30 can mean two different things:

  1. A subnet with four IP addresses, two of which usable, but one is used by the upstream router and one address is free for you to use. Likely, 103.15.xx.xx + 1 is the upstream router (that you use as default gateway) and 103.15.xx.xx + 2 is the address you can use.
  2. A pool of four addresses in a larger subnet. This requires you to be told that subnet's mask, larger than /30, and the default gateway within that subnet to use. Since that information is missing, this variant doesn't seem likely.

You are given only one public IP address by your Internet service provider, ISP. This is common. Assign that public IP address to the Internet facing interface on your router/firewall and use Port Address Translation, or NAT overload, to enable privately addressed devices on your local area network to share it for Internet connectivity.

You can probably pay your ISP extra money and they will assign you more public IP addresses if you need them.


I believe that you have link to the ISP using 2 address pool (103.x.x.x) and it s public IP address range one at your router interface and other at ISP interface. Other range 112.x.x.x is what is IP address that you bought ... and ISP set route for this subnet that redirect all traffic to your router. Now you have to do static NAT to configure your router when it receive packet with destination IP 112.x.x.x to redirect to your server inside your private network (192.168.x.x) steps for NAT (cisco router) are:

  1. Define static nat

    Router(config)# ip nat inside source static 192.168.x.x 112.x.x.x

  2. Define nat inside interface (probably gigabitethernet 0/0 or similar)

    Router(config-if)# ip nat inside

  3. Define nat outside interface (interface with link to ISP)

    Router(config-if)# ip nat outside

And that s it... or you could use port redirection ... using your public IP from interface faceing ISP in that case define interface (inside / outside like in previuose step) and this

Router(config)#ip nat inside source static (tcp or udp) 192.168.x.x (port ... if you redirect to web server then 80) 103.x.x.x 80

still you need to register domain and connect domain with appropriate IP address depending of scenario you choose.

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