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Re: IP aliasing question
On Thu, 7 Jan 1999, Jonathan Ben-Avraham wrote:
> > because there is only one entry in the arp table for that IP address...
> > why the HELL would you want two machines with the same IP?
> Hi,
> I have found that when you put two or more machines with the same IP
> address on the same network segment the behaviour can depend on the type
> of hub you are using. If your hub is smart it could bind to one interface
> and send all packets there. In this case you don't get DUPs.
basically, since IP (in this case ICMP/IP) doesn't broadcast pings. it
would try to resolve with arp the mac address of a particular IP address,
and then proceed to ping it implicitly through it's mac address. AFAIK arp
tables are 1-to-1 and would feature the last arp reply accepted (unless
you set and lock it manually, otherwise crackers on the LAN masquarade as
a trusted IP address and all hell breaks loose)
>
> To answer Ira's question, it can be instructive to put two machines with
> the same IP on a network to test how the network reacts. We then note the
> behaviour and show the client so that he will recognize the problem. As I
> said, the behaviour seems to differ if you are using a switching hub, but
> I don't really know why.
well, maybe if you use an IP-switch, but not a mac-address switch (the
common switch hub). and like Jonathan says, it's good for instructional
reasons, maybe to simulate a crack-in attempt as I mentioned, but not for
any practical reasons.
on a side note: the only time I ever needed to assign the same IP address
twice was when a Linux box was serving as a WAN router and the ISP was
using a private IP range for the link. pings through the router would go
fine, but not from the gateway itself (pinging through the NIC used the
right address, pinging through the FRIC gave the 10.0.0.1 address) so I
gave the FRIC interface the same IP as the NIC, and used aliasing
(the FRIC had a secondary IP 10.0.0.1). But two of the same IP addresses
on the same network don't make any sense.
--
Ira Abramov ; whois:IA58 ; www.scso.com ; all around Linux enthusiast
`When you say "I wrote a program that crashed Windows", people just stare
at you blankly and say "Hey, I got those with the system, *for free*".'
(Linus Torvalds)