Re: Newbie question... Firewalls vs cisco routers - Proxy arp versus directly connected networks...

On 7/8/05, Mollemans, Bart <bart.mollemans@getronics.com> wrote:
>  
>
> All, 
> My question boils down to the following: 
> When is a host or network directly connected for a cisco router? 
>
> Is this when:   A the cisco sees a MAC/IP address on his interface? 
>                 or B when the host/network matches the network(s) defined on
> the interface of the cisco? 
>                 or C both above 
>                 or D none above... :) 
>
> Thanks for anny input you could give. 
>
> Regards, 
> Bart Mollemans 

Let's say everything was just turned on, no arp table exists yet.  Now
when the router attempts to talk to a host (192.168.0.10) it will
first send out an ARP request to the broadcast address of the network.
 If your network is 192.168.0.0/24 then the request is sent to
192.168.0.255 (the broadcast). The arp request is sent to all hosts on
the network segment asking who is responsible for talking for
192.168.0.10. The host with the IP address 192.168.0.10 responds back
to the router with it's MAC address saying that it is resonsibile for
that IP.   The router then knows the MAC address and can then talk
back and forth to that host.

  So, to answer your question.  The host and the router have to sit on
the same segment and have to have IP addresses within the same network
to be able to talk with each other.

In the case of Proxy arp a host between the router and the actual host
you are wanting to talk with plays middle man and passes traffic back
and forth.  The middleman host usually being a firewall of some sort.

--chip

-- 
Just my $.02, your mileage may vary,  batteries not included, etc....