Lan Ip Addressing; Default Behavior - Cisco 800 Design Manual
Fast step software design guide
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Also See for 800:
- Hardware installation manual (292 pages) ,
- Release notes (36 pages) ,
- Configuring (32 pages)
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LAN IP Addressing
LAN IP Addressing
Setup configures the IP stack of the PC and the IP address of the router for the end user. You can modify
the IP addressing parameters by using the CFG flags.
Default Behavior
It is assumed that the end-user PC used to configure the router is the same PC as the one that will be
used to communicate over the WAN. Before running Cisco 800 Fast Step, the end-user PC can be in one
of three states:
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If there is no IP stack in Windows 95, Setup asks Windows to install an IP stack. The new IP stack
defaults to DHCP without a leased IP address.
Windows 98 and Windows 2000 use a modified algorithm that gives the PC a temporary, unique static
address when no DHCP server is found. Setup assumes that no leased IP address exists and treats all
operating systems the same.
The router requires the Ethernet interface to have an IP address to communicate with the PC over Telnet,
yet it has only the factory-default configuration at this stage. This creates a state where neither the PC
nor the router has an IP address. To overcome this, an algorithm temporarily assigns an address to the
Ethernet interface. (The PC must have a functioning IP stack for the algorithm to work.)
A PC running Setup sends a DNS request to new-router.cisco.com, a request that the router is listening
for. The router takes the originating IP address and uses the Address Resolution Protocol (ARP) to find
a free address, either above or below the originating address.
Where there is no IP stack or no DHCP address, Setup assigns a temporary static address to the PC and
later resets the PC to DHCP.
Unless you are using Windows 2000, changing the PC addressing requires that the PC be rebooted
twice, once when Setup configures the PC IP stack and once after router setup and testing are complete.
(If you are using Windows 2000, it is not necessary to reboot the PC.) If the PC has no IP stack, the
LAN has never had an IP addressing scheme, so Setup imposes the most logical configuration:
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Setup uses the same scheme for a PC that is set as a DHCP device with no IP address. We suggest that
you use this scheme for the majority of installations.
If the PC has DHCP with a valid address, Setup assumes there is a DHCP server on the LAN, and the
PC IP stack remains untouched. The router searches for a temporary IP address by using ARP, starting
at the PC IP address. The IP stack remains untouched. Because the DHCP server is local, it could assign
public IP addresses because the IP addressing scheme could be chosen at random. Setup uses (PAT) to
hide the DHCP server IP addresses from the ISDN interface.
Cisco 800 Fast Step Software Design Guide
2-8
Using dynamic host configuration protocol (DHCP) without a leased IP address
Using DHCP with a leased IP address
Statically addressed
PC is set to a DHCP client.
Router is set to a DHCP server and given a range of private address.
IP addresses in the DHCP pool are hidden by using Port Address Translation (PAT).
Chapter 2
CFG File Development
OL-10373-02
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