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configure Router to access ISP


Up untill now we use the ISP provided consumer grade Modem to connect to the ISP/Internet. But recently our small office has 2600 router and will like to use it instead of the modem.


1, So can some one advise me on the step by step needed to setup the ISP account on this router and have it connected to the internet?

2, Beside the Console port, this router also has 2 Ethernet ports, 1 serial port, and 1 Aux port. So in which of this port should we connect the ISP line?

We have Optical Fiber line by the way.

I'm novice here please, so clear/simple guide will be appreciated.

6 Replies 6



Based on your post i undertsand you are getting a ethernet drop from your ISP which you are currently connecting to a modem and want to connect to the router. if thats the case you can make use of one of the ethernet ports to this connectivity and the other port to the lan connectivity.

you have forgotten to mention the necessary info about your local lan setup how that is currently connected to your modem and how your are doing the ip addressing there for your pcs. do provide more info on those lines to get more sugggestions on how to do with your router.


- currently we have the isp modem directly connected to a proxy server (micorsoft ISA 2004) which has two NICs. One NIC for the external network and the other for the LAN.

- the 2600 router is essentially used to create a trunk channel for 3 vlans that exists in a 2950 switch in the LAN.

- two ethernet ports exists in the router, ether0 is used for the lan, while ether1 is directly connected to the ISA server.

- on the router ether0 ip address is

  while ether1 ip address is

- the LAN ip addresses is configured in 192.168.11.x

so as i mentioned earlier we want to turn things around to have the router interface the internet instead of the ISA, and then have the ISA behind the router (directly into the switch).

reason is that a plan is on the way to have another branch office whereby we need the router to do the WAN.

the router will server two purposes - trunk for the vlans and also internet access. (and also WAN later)

Ok..You can replace the ISA server with the router directly. We need to check what is the IP address you are getting on the NIC connected to the modem. Here are the steps that would help you :

1. Check if the ISA NIC connected to ISP device is getting a public IP or private IP.

2. If its a private IP which means the NAT is handled by the ISP device and we will just use the private IP addressing on the outside and inside enthernet interface of the router. We will then use static routing to make all the networks reachable.

3. If its a public IP which means that if you moving everything off to 2600 router, you have to do NAT/PAT on the router to have the private addresses reach the internet.

4. Once the connections are moved to 2600 router, we would use 802.1q trunking by using router on a stick model to configure the inter-vlan routing.

5. After configuring the inter-vlan routing, I would make sure that all the hosts in the respective vlans have the correct default gateway IP's of the 2600 router sub-interfaces.

6. Check that you have static routes on the 2600 to reach the outside world. May be a default route pointing towards the ISP device would suffice this. Also if you are getting a Private IP from the Modem and the router interface connected to the modem has a private IP. You might need to put another route on the ISP device towards the router outside interface to reach the internal networks behind the router.

Hope, I made myself clear.

Please put your question if ncase of any confusion.

HTH, Please rate if it does.


-amit singh

thanks a lot for the detailed answer.

but just curious on one thing, what if i conifgure this ISP settings on the serial port (s0) rather than the ethernet port, will it also work out?

or is there any difference?


It would not make any difference at all. If you are using the serial interface, just configure the same settings.You should get a serial connection from the Modem/ISP device to connect to your router.

Once done, we will configure the NAT an dother related features to get it work.


-amit singh

Thanks a lot and sorry for my long absence - just couldn't be able to respond earlier than this.

I will like to take these tasks step by step as you better outlined them. And first, the connection with the ISP.

Regarding your first question,

1. Check if the ISA NIC connected to ISP device is getting a public IP or private IP.

Its getting Private IP address (precisely from the DHCP service running in ISP's modem). The IP its getting is
So currently in the ISA server the NIC interfacing the modem (into the internet) is a DHCP client, while the NIC interfacing the LAN is static IP address.

And this is how I will like the internal and the external network interfaces of the router to be configured as well.

Based on these, can you guide me on how to configure the ISP provided username, password, NAT, Protocols, etc on the router, together with these information:


ISP's username - sksk123 (
ISP's password - password123

ISP's domain -

ISP's pop3 -

ISP's smtp -
ISPs smtp port - 555


Internal interface (ether0) -

External interface (serial0) -

By the way the connection line to the isp is fibre optics.

And I receive DHCP assigned IP address from the ISP, not static.

1, So, how do I configure things up?

2, Will the Serial0 interface be configured as a DHCP client? (I'm assuming so)
3, As NAT will be configured in the both interfaces, if Serial0 interface will be configured as DHCP client, how then do we configure NAT in it?


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