nyetworking in 2.4

Bob Edwards Robert.Edwards at anu.edu.au
Mon Aug 27 09:56:15 EST 2001

A couple of comments:
 1) are you sure that your switch has an IP address set that can be ping'd
	from any (switch) interface?
 2) about your point 2. (below), the first packet to be sent down the
	interface will be the ARP request (as evidenced in your tcpdump),
	not the ICMP request (ping) packet. This is if you don't have a
	static entry in your ARP table for your switch (unlikely).
 3) about your point 3. (below), if you are using a switch, then you no
	longer will be able to see all packets on the network - only
	broadcast packets. The switch filters all other packets to only
	the destination machine (that's why we use them).

Looks to me like your linux machine is set up correctly, but that your
switch is not configured for that IP address. What makes you think it is?
Can you ping the switch from any other machine?


Bob Edwards.

Mark Hummel wrote:
> I obviously don't understand networking. According to the doco all I have
> to do is such and such, (two lines) and it'll work. Well, it was working
> once.
> What am I trying to do? At the very least, I want two machines to be able
> to ping each other, across a hub or a switch.
> After doing:
> ifconfig eth0 netmask up,
> one machine (call it A) can't see the switch (address
> the routing table shows:
> Destination     Gateway         Genmask         Flags Metric Ref   Use Iface
>     *        U     0      0     0 eth0
> I just want to reiterate my understanding of how routing works, in case
> I've misunderstood something. Assume the routing table is represented by
> the output of the route command as above.
> Then, to connect to a machine with IP address, the following
> happens:
> 1. The destination address is ANDed with the network mask
> & =
> 2. This value is compared top down with all entries. In this case, there
> is a match. This means that the packet will be sent down interface eth0.
> 3. Because ethernet is a broadcast medium, all packets can be seen on the
> network, but only the destination should pay any attention to the packet.
> Now surely, all nodes are on the same class C network?
> Checking the output from tcpdump yields:
> 16:47:52.672874 arp who-has tell
> with ARP set in ifconfig, and with  NOARP set:
> (ICMP request) >
> The situation regarding blinking lights:
> 1. The activity light on the switch and hub blink furiously when there is
> one entry in the routing table
> 2. The lights are more sedate with more entries. I don't know if this is
> relevant.
> A question:
> Does the order in which the routing table is set up matter?
> Finally, (thankyou for your patience) I did once find the magic
> incantation to get this all working but I obviously don't know what,
> and further don't even understand why..
> Thankyou,
> Mark.

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