Ethernet, Router & ARP

Archived from groups: comp.dcom.lans.ethernet (More info?)

In article <3080cb7f.0406080409.7c7fb352@posting.google.com>,
Sally <womanontheinside@yahoo.com> wrote:
>On the ethernet network, if a machine doesn't know the MAC of an IP it
>wishes to communicate, it sends a broadcast ARP packet to find out.
>
>When the IP is on another segment, the machine sends the request to
>the router. My question is, how does a machine which has just been
>added to a network, know the router's MAC address so it can talk to
>it? Is there some broadcast packet for this?

Typically, the machine is either configured with a static IP address for
itself and it's router, or it uses DHCP to broadcast a request asking
for (among other things) an IP address for it to use, the local network's
subnet mask, and the IP address of the router.

(DHCP = Dynamic Host Configuration Protocol)

========= For LAN/WAN Protocol Analysis, check out PacketView Pro! =========
Patrick Klos Email: patrick@klos.com
Klos Technologies, Inc. Web: http://www.klos.com/
==================== What goes around, comes around... =====================

Archived from groups: comp.dcom.lans.ethernet (More info?)

On 2004-06-08, Sally <womanontheinside@yahoo.com> wrote:
> On the ethernet network, if a machine doesn't know the MAC of an IP it
> wishes to communicate, it sends a broadcast ARP packet to find out.
>
> When the IP is on another segment, the machine sends the request to
> the router. My question is, how does a machine which has just been
> added to a network, know the router's MAC address so it can talk to
> it? Is there some broadcast packet for this?

Yes, but let's first review what is happening, as you seem to be a bit
confused:

The mechanism for talking to machines on another segment is almost the
same as talking to ``local'' machines: If the machine knows an IP but
not the MAC it sends out an ARP request for the MAC belonging to an IP.
You already understood this. The difference between the two scenarios is
that the ARP request will either contrain a request for the MAC of the
local, or a request for the MAC of the router. So, no, it does not know
any MAC, if it needs to know it'll ask for it.

The reason why you need to tell a machine the subnet and -mask is so
that it can decide if it has to ask for the MAC of a local directly, or
for the MAC of the router (which thus also must be local), to then send
out packets for a non-local IP with the MAC of the router.

You need to tell that machine the IP of the router exactly so it can ask
for the router MAC. In a ``pure router'' case[1], you won't see much IP
traffic with the router's IP address in the destination field, only ARP
requests for its MAC address.

As ObOtherPoster pointed out, you can configure those parameters
``statically'' or ``dynamically'', using an automation mechanism like
DHCP[2]. In addition, there does exist such a thing as ICMP router
discovery, in which one sends out an ICMP message asking for the nearest
router. But you'll get an IP address for an answer, and after that
you'll have to resolve that into a MAC using ARP again. It is not very
often used, however, so if that ICMP type is not supported, you will not
get an answer at all. According to my _limited_ experience, that is.

On top of that, it would also be possible to enter a ``static'' MAC
entry for the router in the client machine's ARP table, but one
generally doesn't do that because configuring IP is annoying enough,
having to also deal with ethernet MACs just increases the pain, and
usually needlessly so. However, there are of course cases where it
might be useful. (Very hostile local networks, for example.)


[1] I mean a real router or a machine that really only does routing,
not ten other things as well, as some home routers do.
[2] Which is very useful in helping administering networks, as it
also allows you to set a host of other parameters. Unfortunately,
not all OSen pick up all the parameters. But that is another issue.

--
j p d (at) d s b (dot) t u d e l f t (dot) n l .

Archived from groups: comp.dcom.lans.ethernet (More info?)

patrick@klos.com wrote in message news:<ca4avi$sap$1@pyrite.mv.net>...
> Sally <womanontheinside@yahoo.com> wrote:
> >On the ethernet network, if a machine doesn't know the MAC of an IP it
> >wishes to communicate, it sends a broadcast ARP packet to find out.
> >
> >When the IP is on another segment, the machine sends the request to
> >the router. My question is, how does a machine which has just been
> >added to a network, know the router's MAC address so it can talk to
> >it? Is there some broadcast packet for this?
>
> Typically, the machine is either configured with a static IP address for
> itself and it's router, or it uses DHCP to broadcast a request asking
> for (among other things) an IP address for it to use, the local network's
> subnet mask, and the IP address of the router.

OK. Does the machine need to know the address of a DHCP server or
does it just send out a broadcast packet with no destination?

Archived from groups: comp.dcom.lans.ethernet (More info?)

In article <3080cb7f.0406081231.639e7286@posting.google.com>,
Sally <womanontheinside@yahoo.com> wrote:
>patrick@klos.com wrote in message news:<ca4avi$sap$1@pyrite.mv.net>...
>> Sally <womanontheinside@yahoo.com> wrote:
>> >On the ethernet network, if a machine doesn't know the MAC of an IP it
>> >wishes to communicate, it sends a broadcast ARP packet to find out.
>> >
>> >When the IP is on another segment, the machine sends the request to
>> >the router. My question is, how does a machine which has just been
>> >added to a network, know the router's MAC address so it can talk to
>> >it? Is there some broadcast packet for this?
>>
>> Typically, the machine is either configured with a static IP address for
>> itself and it's router, or it uses DHCP to broadcast a request asking
>> for (among other things) an IP address for it to use, the local network's
>> subnet mask, and the IP address of the router.
>
>OK. Does the machine need to know the address of a DHCP server or
>does it just send out a broadcast packet with no destination?

It just broadcasts the DHCP request(s).

========= For LAN/WAN Protocol Analysis, check out PacketView Pro! =========
Patrick Klos Email: patrick@klos.com
Klos Technologies, Inc. Web: http://www.klos.com/
==================== What goes around, comes around... =====================

Archived from groups: comp.dcom.lans.ethernet (More info?)

Albert Manfredi <albert.e.manfredi@nospam.com> wrote:

> This scheme was originally developed for
> BOOTP, the precusor to DHCP, used for the same purpose. And BOOTP in
> turn was an elaboration of "inverse ARP," used primarily by diskless
> workstations back when.

I think you mean RARP (Reverse ARP). I believe inverse ARP is used with
IP over ATM, but could be mistaken.

Tony