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Suggested prerequisite reading
»Cisco Forum FAQ »Things to expect when setup network for home or small business

When you are using ISP to connect to the Internet, most likely you will be dealing with DHCP, PPP, dynamic, or static IP address assignment (whether you are aware or unaware of it).

Let's say you have to configure Cisco router Ethernet 0 interface to have specific IP address. Then the following is illustration on how to configure the IP address.

1. Assign IP address by DHCP

interface Ethernet0
ip address dhcp

2. Assign IP address by PPP

interface Ethernet0
ip address negotiated

3. Assign IP address statically

interface Ethernet0
ip address xx.xx.xx.xx yy.yy.yy.yy

where xx.xx.xx.xx is the IP address and yy.yy.yy.yy is the subnet mask

In early days; DHCP and PPP were used to dynamically assign IP address to hosts. However with additional features, it is technically possible to assign "static IP address" via DHCP and PPP. By referring to specific MAC address of a host, the host is always receiving the same IP address via DHCP. By referring to specific username and password, a host is also always receiving the same IP address via PPP.

Why would your ISP use DHCP or PPP to "statically assign" IP addresses to their customer and not use the traditional way of statically assign IP addresses? Probably it is simpler from their network administration point of view. Whatever the reason is, you have to choose the most appropriate way to assign your ISP IP address and experience with the tip and tricks when you need to access the Internet using your ISP.

Assign Your Internet Gateway's IP Address

In term of configuring your Internet gateway's IP address, you need to consult with your ISP as to how exactly they assign IP address to your device.

When your ISP says the IP address would be assigned dynamically, you need to confirm the followings

* if they use DHCP or PPP (or PPPoE/PPPoA) technology to assign the IP address
* if they use PPP, confirm the username and password for the PPP authentication process
* if they use DHCP, confirm if the ISP lock down your IP address with specific MAC address
* if the IP address is always the same everytime or constantly changing
* assuming the IP address is changing, how frequent the change takes place and which event will trigger the change

When your ISP says the IP address would be static, you need to confirm the followings

* if they use DHCP or PPP technology to assign the IP address
* if the IP address might change
* assuming the IP address is changing, how frequent the change takes place and which event will trigger the change

Important Note:

Make sure that when you discuss this with your ISP representative, the representative is the technical person who knows what he or she is talking about. You don't want to get misinformed since you might not be able to access the Internet when you don't have the correct info.

Static IP without DHCP or PPP

If your ISP says "No DHCP, No PPP. It is static"; then it might mean that you have to statically configure your Internet gateway device with your assigned IP address. On Cisco router, you should then use the "ip address xx.xx.xx.xx yy.yy.yy.yy" command.

Check out this forum's FAQ for specific sample configuration of Cisco router with statically-assigned IP address
»Cisco Forum FAQ »How can I configure broadband router with cable/dsl using static IP address

Static IP with DHCP

When your ISP uses DCHP to "statically assign" your Internet gateway device, then from device perspective it is still DHCP (still somewhat dynamic IP address with "sticky IP" approach). To configure your Cisco router, you then still need to use the "ip address dhcp" command under the ISP-facing interface.

Check out this forum's FAQ for specific sample configuration of Cisco router as DHCP client.
»Cisco Forum FAQ »Configure router as DHCP client using external modem

Dynamic IP with DHCP

From DHCP client perspective, there is no difference between "static" and dynamic IP address assignment. As mentioned, "statically assigned" DHCP-based IP address is still dynamic process. Therefore you can use the same above FAQ for specific sample configuration of Cisco router as DHCP client when you only have dynamic IP address from your ISP.

As a note, the difference between DHCP-based static and dynamic IP address is probably the ISP requirement to lock down your Internet gateway device MAC address to a specific IP address. Although it is possible that the ISP administer MAC address lock down for both dynamic and static IP account customers due to network management simplicity. Check out the following thread for insight.

»[help] 851W and ISP DHCP

Dynamic IP with PPP

In general, your ISP usually supplies username and password for the PPP authentication process. Once your Internet gateway device successfully establishes PPP connection with your ISP (pass the Layer-2 process), then your device will deal with the IP address assignment issue (the Layer-3 process).

Under normal PPP-IP network environment, dynamic IP address assignment will require the "ip address negotiated" command under the ISP-facing interface on Cisco equipments. With static IP address, you need to use the "ip address xx.xx.xx.xx yy.yy.yy.yy" assignment on Cisco router. However there might be exception for certain ISP. If you have a static IP with PPP, read the next discussion.

Check out this forum's FAQ for specific sample configuration of Cisco router as PPP client
»Cisco Forum FAQ »Quick Guide of Configuring Cisco router for PPPoE using external modem

Static IP with PPP

When your ISP uses PPP to "statically assign" your Internet gateway device, then you may experience some unusual situation. To configure a Cisco router, you need to use the "ip address xx.xx.xx.xx yy.yy.yy.yy" command under the ISP-facing interface in normal static IP address environment. However for some ISP, you need to use the "ip address negotiated" command under the ISP-facing interface.

If you are in this situation, then you might try to use the 1st approach (the "ip address xx.xx.xx.xx yy.yy.yy.yy" command) and see if you are able to host public servers or establish VPN IPSec tunnel with remote end. If your public server is inaccessible from the Internet or you are unable to establish VPN tunnel, then try the 2nd approach ("ip address negotiated" command) and see if it makes any difference. When the 2nd approach works, then the 2nd approach is considered the most appropriate way to assign IP address to your ISP-facing interface.

Like the DHCP, static and dynamic IP address assignment in PPP-IP environment is using similar configuration. Therefore you can refer to the previous sample configuration of Cisco router as PPP client in static IP address assignment.

Additional Sample Configurations

For more sample configurations, check out the following FAQ
Various PPPoA/PPPoE/DHCP/Static Sample Configuration with Cisco

Feedback received on this FAQ entry:
  • Great information...and I don't even have a Cicso router! Thanks for posting it.

    2010-08-16 12:09:59



Expand got feedback?

by aryoba See Profile
last modified: 2008-01-04 09:18:58