Re: Routing Tables Mysteries by Herb
Herb
Sun Feb 08 21:49:53 CST 2004
> Hi Experts,
> I am confused about the concept of routing tables. I am a newbie in
> networking. Knows a bit about IP Addressing schemes and how they work.
They are just a tad bit archane but once you know the three or four "idioms"
or "conventions" they are pretty easy to decode.
> I know a default gateway is provided for every hosts TCP/IP properties.
Not every, but most.
> I have seen sometimes people create routing tables . What is the need
> of it. How they are created ?
Usually the act of filling out the dialog properties on the NIC creates the
(basic) table automatically, including the normal "default gateway" entries.
One a "normal" station you seldom need to create any manual entries;
creating manual entries on a "router" is much more common, but even there
isn't always necessary.
Principle: Every machine routes for "itself". Routers "forward packets"
for
other stations.
If a router or even a machine is directly connected to only ONE (other)
router,
then manual entries aren't needed.
The default gateway (route) entry causes either a machine or a router to
send all
remote traffic to the default gateway so with one (other) gateway/router
there is no
need for additional routes.
When a router (or perhaps even a regular) station is BETWEEN two (or more)
routers then it needs to have routes added. The default gateway points the
the
"larger" net (like the Internet) usually, and the additional route(s) point
to the net(s)
behing any additional router(s).
> I have just dumped the output of "route
> Print" .Please help me to understand this . I have asked the same
> question in one of other forums also. But i m sorry i m still not very
> clear about the concept of routing tables and in what case we need to
> modify them.
Seldom on a station; more frequently on a router, but only if the machine in
question is "between" two or more routers. Count your Internet ISP default
gateway as "one" so if you have no other routers you won't need to add any.
Count the ISP as "one" so if you have two or more routers the "intermediate"
routers will need to "know" about the nets "behind" the other router(s).
ISP------Router1----net2----Router2----net2
Net2 machines use Router2 as the default gateway; Router2 is connected to
both net1 and net2 so it doesn't need (explicit) routes to find these --
they are
built-in to the routing table automatically.
Router2 uses Router1 as "default gateway" so it can reach the Internet since
Router1 uses the ISP as its default gateway, BUT....
Router1 cannot see "net2" directly and since its default gateway is to the
left, the
ISP, net2 is unreachable without an EXPLICIT route so you must add a route
on Router1 TO net2 by way of Router2
Conceptually it looks like this:
route add -p Net2 MASK 255.what.ever.0 Left-NIC.Route.2
Substitute the appropriate mask, net address for net2, and IP address for
Left-NIC.
> > route PRINT
> C:\>route print
>
===========================================================================
> Interface List
> 0x1 ........................... MS TCP Loopback interface
> 0x1000003 ...00 0d 56 50 bd 98 ...... Broadcom 440x 10/100 Integrated
> Controller
>
>
===========================================================================
>
===========================================================================
> Active Routes:
> Network Destination Netmask Gateway Interface Metric
> 0.0.0.0 0.0.0.0 172.25.4.254 172.25.4.141 1
Send out interface 172.25.4.141 (the machine's address) to the default
gateway
at address 172.25.4.254
The one above is a Default route. The "idiom" is Destination=All Zeros, and
Mask
all Zeros means: default gateway.
(Mask all zeros means "check no bits" and any more specific route will be
preferred,
since the rules is LONGEST match is preferred.)
> 127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1
This is the route (automatic) that makes "ping 127.0.0.1" always test the
local machine;
along with the one 2-down.
> 172.25.4.0 255.255.255.0 172.25.4.141 172.25.4.141 1
This route (automatic) is the one for the local net. Out the machine's own
interface to
its own "address" is another idiom for "drop the packets" on the local
wire -- or rather,
the local subnet can be reached directly with no (true) gateway/router
required to help.
> 172.25.4.141 255.255.255.255 127.0.0.1 127.0.0.1 1
This routes the local machine address back to the same machine on 127 so
with the
other it means this address is LOCAL to THIS MACHINE.
The convention is that ALL 255's means EXACTLY one machine; it's the
opposite
of the "all 0's" mask.
> 172.25.255.255 255.255.255.255 172.25.4.141 172.25.4.141 1
Broadcast address.
> 224.0.0.0 224.0.0.0 172.25.4.141 172.25.4.141 1
Multicast (most people don't use it.)
> 255.255.255.255 255.255.255.255 172.25.4.141 172.25.4.141 1
Another (mostly irrelevant) broadcast entry.
> Default Gateway: 172.25.4.254
>
===========================================================================
> Persistent Routes:
> None
>
> What is a persistent route??
One that survives a reboot -- it's "persistent" across boots.
> I will appreciate every bit of information from you .
Try that and ask any questions you have.
--
Herb Martin