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Paul JP


[Config] Cisco 877 with public static IP

Presently I have a VOIP server (vps) running on Win 2003R2 with home based ADSL2+ modem router which can allows me to assign public static IP to this server. Rest of the computers on this network are on LAN IP with NAT. ISP here only allow one static IP per ADSL connection.

Now I have Cisco 877 to replace the above modem router. Normal configurations are done on this cisco router. I need help with assign my static IP to the server. The server needs to send/receive this public ip to remote server (provider)/client for call termination

I would appreciate any advise

London, UK
It sounds like all you really need is a port forward.

Outbound, the server will be NATed like all the other PCs, so its requests will appear to come from your static IP.

On the way in, you just forward the SIP port 5060 to the server, allowing incoming calls to be received by the server.

Thats about the best you're going to get.

Check out this FAQ for configuration samples:
»Cisco Forum FAQ »Router configuration to run server (with and without port forwarding)

Paul JP

Thanks for the response
I tried that idea of port forwarding before. It does not work for the operation on the whole. The server does not send the static public ip to the remote server which is required for authentication. DMZ also not work.

At present, the static public ip is reflecting on server in DHCP

London, UK

1 recommendation

The problem with that setup is that the modem owns the static IP. Any incoming packets will reach the modem, which will take them and process them, because it believes they are destined for itself.

The port forward will cause the router to forward on packets for specific protocols and ports, effectively allowing you to farm out portions of it to other devices.

Unless you can get a small subnet of IPs, like a /30, you are going to have to find a way to make it work with port forwards, or make the server do PPPoE and share the Internet to your LAN so that it has the static IP directly on it via the PPP session to your ISP.

Welcome to the evils of NAT, and why IPv6 will be so awesome, when every device can have its own publicly routable IP.


3 edits
reply to Paul JP
If you read through the FAQ TomS_ See Profile suggested, it was describing several approaches of network designing and router configuration. As such, you need to choose which approach that is most suitable to your requirement and to your-ISP-dictated specification and come up with a solution.

said by Paul JP :

ISP here only allow one static IP per ADSL connection.

I need help with assign my static IP to the server.

Basically what you were intended was using the non-port-forwarding approach (refer to the FAQ for details), which is not applicable to your situation. As TomS_ See Profile mentioned, you have to use NAT/PAT (port forwarding) approach; unless your ISP is able to provide you with a block of static Public IP addresses as additional services.