IP address, how to find

[bogstad at pobox.com]

Seth wrote:
>On Thu, 2003-09-11 at 09:06, heidi wrote:
>> (I'll be a newbie for ever.  Thanks for help at last fall's InstallFest.)
>> How do I find my own computer's ip address.  If I type ifconfig I get 
>> the standard:
>> 
>> inet addr:192.168.0.101  Bcast:192.168.0.255  Mask:255.255.255.0
>
>A subtle point worth keeping in mind: Strictly speaking, computers don't
>have IP addresses; *network interfaces* have IP address.  "Your
>computer's IP address" is a kind of shorthand for "the IP address of the
>interface that can be used to do stuff with your computer and nobody
>else's", or something like that.

PEDANTIC MODE ON

That is certainly the way that I (and it seems most people) think about
this subject.  However, I'ld argue that's not the way that the people
who actual write the networking code for Linux think, As evidence, I
give the long running argument about how Linux's ARP code should
operate.  (see http://lwn.net/Articles/45373/ ).  To summarize, Linux
responds to ARP requests for any address that it has configured on any
Ethernet interface to a request that arrived on any Ethernet interface.
One of the reasons seems to be because they believe that IP addresses
are associated with machines not interfaces.  This usually comes up when
somebody wants to run two seperate IP 'networks' on the same Ethernet
(but with two physical interfaces in their Linux machine).  For whatever
reason, they want the traffic for the different IP 'networks' to use
different physical interfaces.  This is possible with most other IP
implementations, but Linux's ARP behavior makes this close to
impossible.  Apparently the actual standards on the subject make Linux's
behavior legal, albeit very much in the minority.

Bill Bogstad
bogstad at pobox.com