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uk.comp.home-networking (UK home networking) (uk.comp.home-networking) Discussion of all aspects of computer networking in the home, regardless of the platforms, software, topologies and protocols used. Examples of topics include recommendations for hardware or suppliers (e.g. NICs and cabling), protocols, servers, and specific network software. Advertising is not allowed.

Routing



 
 
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  #1  
Old December 2nd 09, 03:48 AM posted to uk.comp.home-networking
Dave J.
external usenet poster
 
Posts: 139
Default Routing


Would someone be kind enough to satisfy my curiousity here?


I thought that for any destination IP (possibly apart from private IPs
depending on configurations) where there isn't a permanent route and
there's nothing in the local routing table, that any packet sent there
would be automatically initially passed via the default exit interface (in
this case the *only* NIC) to the mac address of the default gateway?


So, for google (74.125.127.100) I do a ms tracert and the first TTL expiry
reply is from my local router at 10.32.32.128. This seems to be true for
many (most?) addresses that are outside my local network.

However, if I try it to 6.0.0.0/8 or 7.0.0.0/8 I don't even get the first
expiry packet. It's as if my machine has a route for these addresses via a
non existent interface.

Can anyone explain what's going on? The addresses are legitimately 'owned'
and there is nothing in my routing table that mentions them. I even
cleared the table and poked the default route back into place. Everything
stayed exactly the same.



Even if I 'owned' (leased) a 'proper' class B sized network and my router
exchanged routing information (perhaps someone would enlighten me on which
of the (two?) sorts of routable network I mean?) and even if there was no
legitimate route for those IPs, surely I would *still* get a TTL expired
from my default gateway? My machine certainly doesn't know anything about
global network routing, so why does it decide that there's nowhere
sensible to send the packets destined for these IPs?

Sorry, I'm thinking 'aloud' here, partially in the hope that someone will
correct my misconceptions and I might gain some more grasp of how things
work in the 'real' network world.

I guess part of the answer might be that my router's not returning its mac
address when the PC chucks it an ARP ? [Bloody hell, what does scribd USE
for its text display routine? it's snaffled 90% of my proccessor time!]


No, because the PC doesn't make an ARP request, cos it's not on a local
network. So, surely, in that case it sticks the router's mac onto the
packet, along with the desired IP address and chucks it out of the NIC
that's pointing at the router?

The router should then look at the IP and because it's not got any sort of
public routing table, it will just pass it up the ppp tunnel to the ISPs
default gateway? (I've even less idea how such things are labelled when
they're going via ppp for the ISP to handle) Or is there some sort of
additional decision making involving the internal link to the internal
modem's (presumable) invisible IP/mac addresses?


OK. Sorry all, this message is gonna end up long enough I'll be lucky if
anyone even digs through it to find the right bits to *begin* pointing out
how wrong I've got it :-( Maybe what I need to do is to telnet into the
router and have a look at its internal tables.

I shouldn't be doing this at 1/2 past three in the morning...

It's a Zyzel 643 running what I think is some peculiar mutation of 'nix.
And it's got a shell/Cli of sorts if I can remember how to get at it..

Hmm. No. Fergetit. There's an arp table, but it's got just one non LAN IP
entry, belonging to my ISP with 'proxy' as its mac address. That's it. I'm
lost. Anyone fancy dishing out some clue about the general appearance of
the right ends of the right bits of wood? What the hell's going on? Not
only with the main query, but what angles should I try researching to
improve my fractional understandings?


It's seriously the wrong end of the day to be this puzzled but I find it
*so* damn difficult to let go of perplexity. So, I'm afraid, this message
is gonna get posted. Sorry all.


Dave J.
--
Dave J.

Dark but neatly accurate. (Flash Anim)
http://www.markfiore.com/animation/gentle.html
  #2  
Old December 2nd 09, 07:20 PM posted to uk.comp.home-networking
Alex Fraser
external usenet poster
 
Posts: 553
Default Routing

Dave J. wrote:
Would someone be kind enough to satisfy my curiousity here?

I thought that for any destination IP (possibly apart from private IPs
depending on configurations) where there isn't a permanent route and
there's nothing in the local routing table, that any packet sent there
would be automatically initially passed via the default exit interface (in
this case the *only* NIC) to the mac address of the default gateway?


Correct. Basically what happens is this:
1. The routing table is consulted to make a routing decision based on
the destination IP address. Assuming no more-specific route is found,
the default route will be used. The route specifies the IP address of
the interface to use and the default gateway ("router").
2. The ARP table is consulted to find the MAC address of the router.
3. If the MAC address is not in the ARP table, an ARP "who-has" query is
broadcast, and the process continues once a reply is received.
4. The packet is forwarded; the layer 3 (IP) destination address will be
the original destination and the layer 2 (Ethernet) address will be that
of the router.

So, for google (74.125.127.100) I do a ms tracert and the first TTL expiry
reply is from my local router at 10.32.32.128. This seems to be true for
many (most?) addresses that are outside my local network.

However, if I try it to 6.0.0.0/8 or 7.0.0.0/8 I don't even get the first
expiry packet. It's as if my machine has a route for these addresses via a
non existent interface.

Can anyone explain what's going on? The addresses are legitimately 'owned'
and there is nothing in my routing table that mentions them. I even
cleared the table and poked the default route back into place. Everything
stayed exactly the same.


Something on the PC or router (eg firewall) is interfering. The former
is more likely based on the reported absence of ARP.

Even if I 'owned' (leased) a 'proper' class B sized network and my router
exchanged routing information (perhaps someone would enlighten me on which
of the (two?) sorts of routable network I mean?) and even if there was no
legitimate route for those IPs, surely I would *still* get a TTL expired
from my default gateway? My machine certainly doesn't know anything about
global network routing, so why does it decide that there's nowhere
sensible to send the packets destined for these IPs?


Even if it did, "no route to host" instead seems likely.

[big snip]

Alex
 




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