Re: Two ip's
From: ps (fsck_at_smicker.com)
Date: 05/24/03
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Date: Sat, 24 May 2003 12:15:06 -0700
> I guess my network knowledge is pre-VLAN. In those days (mid-1990's),
> the "multi-homed" router connected to various hubs. Each of those hubs
> was a specific network. For example:
>
> hub A: 192.168.0.x hub B: 192.168.1.x hub C: 192.168.2.x
>
> SUN system host_a connected to hub A must have an IP address in the
> range 192.168.0.[1-255] for a class C network with netmask 255.255.255.0
>
> If you attempt to define a virtual IP address on host_a with a different
> network like 172.16.0.1, isn't it the router's job to segregate all the
> packets destined for the "0-net" (192.168.0.x) from the rest of the
> network. Will it do anything with all the 172.16.0.1 traffic?
If each subnet has its own hub, then yes, the router interface on each hub
will ignore packets from other subnets for the simple reason that this
design dictates one subnet per "wire" and the router interface on each
subnet is configured to listen for packets on that subnet only. However
consider if all three hubs were combined into one. Each of the subnets now
share a common medium and each of the router interfaces will see traffic
from all three subnets. Extend this further to using just a single interface
on the router and assign the other two subnets as secondary addresses on
that single router interface and you have three subnets operating within two
interfaces and a shared medium
> What if
> this was an intelligent switch that segregated the traffic down to the
> port on the hub based on MAC address? SUN, I believe, puts the same MAC
> address on all packets coming out of that hme0 interface. What would a
> switch make of two nets coming out of the same MAC address?
A switch doesn't know what an IP address is, it happily forwards a packet
based on its MAC address to the correct port regardless of the contents of
the packet. It's up to the host to sort out what to do with the packet after
that. Specifically extracting the IP packet from the ethernet frame and
sending its contents up the OSI model.
A MAC address is unique to each physical interface, so packets coming out of
virtual interfaces will share the same MAC address. I think that by default
SUN even sets all physical interfaces via Open Boot to share the same MAC
address so you would need to change this if you have more than one physical
interface on the same switch.
> Take it to the VLAN level (as far as my meger understanding). host_a is
> plugged directly into a switch or router that creates virtual "hubs" and
> does all the switching and sorting. Then, I don't think it would care
> that a port was producing traffic for two diffent nets.
>
A VLAN simply segregates broadcast traffic, so I think you're right--this
shouldn't affect multiple subnets sharing the same interface.
--Paul
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