Re: TCP/IP gurus, please help! Can this be a valid gateway address?

From: Tyler Larson (tylerl_at_localhost.localdomain)
Date: 09/14/03 

Date: Sat, 13 Sep 2003 22:27:16 GMT

In article <epjvjb.io2.ln@corncob.inetport.com>, Clifford Kite wrote:
> ff <ff@here.us> wrote:
>> HisNameWasRobertPaulson wrote:
>>> I am stumped by this one so, I will get right to it:
>>>
>>> Ip address: 68.2.20.180
>>> Netmask: 255.255.252.0
>>> Gateway: 68.14.232.1
>>>
>>> Can this be a valid gateway? I say no, simply because this gateway
>>> (host) does not reside on this network. But, my buddy has cable and
>>> says that this is his current working configuration... how can this
>>> be?
>
>> I can see this working if their gateway is proxy arping. The clients
>> will ask, "Who has 68.14.232.1?" and the gateway will respond with the
>> correct ethernet address. It has to put it on the wire, so it has to arp
>> for it, no?
>
> No, a host with traffic to send will only send arp requests for traffic
> that is bound for an IP address belonging to it's own network.
>

Absolutely correct. If you want to use proxy arp, set your netmask
to 0.0.0.0. To clear things up for everyone else, it works like this:

The netmask tells your computer what IP addresses are on your local
network. The netmask is simply a bitmask. Using bitwise AND operations,
If (my_ip AND netmask) == (remote_ip AND bitmask) then the remote
host is (supposed to be) on your local network. So a netmask of
255.255.255.255 directs ALL traffic through the gateway, whereas
0.0.0.0 never uses the gateway.

When your computer wants to connect to another host on the
same network, it sends out an ARP request to determine the remote
host's MAC address. If the remote host is NOT on the same network, then
instead it uses the gateway's MAC address. The gateway receives a packet
with its own MAC but a different IP, and it knows how to forward the
traffic onward.

Proxy ARPing means that the gateway responds (with its own MAC) to ARP
requests for hosts not on the local network. This then creates the
illusion that the whole Internet is on your local network. It's useful
in situations where you don't want to configure a default gateway for
hosts on your network, or you've changed your network topography and
(for some crazy reason) can't reconfigure the network.

Proxy ARP isn't very widely used (at least partially because the standard
method is more efficient), so you're probably not ever going to run into
it. 

-- 
Tyler Larson

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