Two routers

Kyle Waters unum at
Wed May 27 15:29:59 MDT 2009

Jones, Scott (GE Money, consultant) wrote:
> At the risk of sounding ignorant, what is the difference functionally
> between the uplink being plugged into the WAN port versus into one of
> the LAN ports? 

The main thing is the Network Address Translation(NAT).  If you can turn 
that and DHCP off I don't know that there would be any difference.  In 
order to communicate on the Internet each device needs to have an IP 
address.  In order to reduce the number of IP addresses required(and 
possibly for a small security benefit) many consumer grade routers pull 
a trick where they allow all the computers plugged into them to use one 
ip address.  This is what I'm referring to when I talk about NAT.  So 
the NAT device sees two worlds.  The WAN and the LAN, and it has two ip 
addresses.  When it talks to the internet it uses its WAN IP when it 
talks to your network it uses it's LAN IP.  If a computer on the LAN 
requests data from the internet, that request goes to the NAT device 
where it is repackaged to look like it is coming from the NAT 
device(with the WAN IP), but is specially marked so the NAT device can 
remember which computer made the request.  Then when the request in 
answered it is sent back to the NAT device that then repackages it and 
sends it back to the computer that asked for it.  If a computer on the 
LAN side asks for data from another computer on the LAN side then the 
device just sends the data between the two like a regular switch with 
out any repackaging. 

The other difference is DHCP.  Basically if you plug a cable into the 
WAN port the device will ask for an IP Address if you plug a cable into 
the LAN port the device is going to expect to give out an IP Address.  
So if you plug one NAT device's LAN port into an other NAT's device LAN 
port without disabling DHCP they will both be answering DHCP calls and 
trying to give out IP Addresses and routing information.  Which can 
cause all sorts of problems.


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