Re: gateway IP address

I see. Is there any practical way to look at layer 2 data?

It seems Ethereal only captures higher layers.

Thanks.

kieranmilne@xxxxxxxxx wrote:
Hi,

You're not wrong - it is just the code used in the product. However,
that 'code' is in every IP-enabled device, it's just the mechanics of
how IP works. And the number of interfaces on the device doesn't
affect how IP does its thing.

An Ethernet (Layer 2) network is a 'local' environment, and it's very
good at communicating with other nodes in the local area. But Ethernet
itself doesn't know how to - and isn't designed to - get to nodes on
remote networks. To help solve this problem, a gateway IP address can
be used. Although it's an IP address that you configure on the node,
the gateway address is actually used by Ethernet, basically to help
Ethernet when it says 'the place I'm trying to get to isn't local, now
what?' The gateway address provides a pointer for Ethernet to use in
situations where the destination isn't local.

BTW, it's not mandatory to have a gateway address configured on a
device. If a device is on a LAN, and it will never need to talk to any
other nodes besides those on that LAN, the gateway address is never
used.

Kieran


Chris Mason wrote:
I have now seen many nice answers and the OP has emphasised his
justifiable mystification to no effect. I stand by my original
assertion that it's just the code used in the product. Does anyone have
the *real* answer?

Chris Mason wrote:
A nice answer but I rather think not to the question as asked. I
suspect that the true answer is that the logic used is in the device is
general and hence applies to an IP node with multiple interfaces.

Chris Mason

thrill5 wrote:
Networking is all about encapsulation. Ethernet is Layer 2, IP is Layer 3.
Layer 2 uses MAC address, and Layer 3 uses IP address. IP packets are
encapsulated into Ethernet Frames. ARP is used to obtain the MAC address of
a device on YOUR IP subnet. If you have two segments connected via a
router, and you want to talk to a device on the other segment, you need to
send the Ethernet frame to the router, so that it can pass it to the device
on the other segment. You need to configure the default gateway (the
routers IP address) so that you device can figure out the MAC address of the
router. Example.

Device A on Subnet 1, needs to talk to Device B on Subnet 2. Subnet 1 is
connected to Subnet 2 via router R. Device A has a packet that it needs to
Device B. It knows that the Device B is on a different subnet (IP address
and subnet mask are used for this purpose), it knows that it needs to send
the packet to the router (default gateway). When the packet leaves Device
A, the source IP is itself, the source MAC is itself. The destination IP is
Device B, the destination MAC is the router. The router gets the frame
(because the destination MAC of the Ethernet frame is addressed to it),
looks at the destination IP and determines where to send it, (in this case,
a directly connected subnet.) When the frame leaves the router, the source
IP is Device A, the source MAC is router, the destination IP is Device B,
and destination MAC is Device B.

This is a very simple example.

For more information on the "Layers" (There are seven of them) Google "OSI
Model" This will give you a more complete explanation of the workings of
networking.

Scott


<mike7411@xxxxxxxxx> wrote in message
news:1152043416.445618.84050@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Could someone refer me to a good explanation of how the gateway IP
address is used in
networking?

I have a Cisco ATA186 analog telephone adapter, and it requires me to
configure gateway IP address. It only has one RJ45 Ethernet port, so
shouldn't it just be sending all the packets out of the port? Why
would it need a gateway IP address?


.

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