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Chapter 5

IP Routing

have a route over to the destination network, the packet will be dis-
carded immediately. (This is the source point of confusion for a lot of
administrators--when a ping fails, most people think the packet never
reached the destination host. But as we see here, that's not

always

the

case! All it takes is for just one of the remote routers to be lacking a
route back to the originating host's network and POOF! The packet is
dropped on the

return trip

, not on its way to the host.)

31.

But the router does know how to get to network 172.16.10.0--the
exit interface is Ethernet 0--so the packet is switched to interface
Ethernet 0.

32.

The router checks the ARP cache to determine if the hardware address
for 172.16.10.2 has already been resolved.

33.

Since the hardware address to 172.16.10.2 is already cached from
the originating trip to Host B, the hardware address and packet are
handed to the Data Link layer.

34.

The Data Link layer builds a frame with the destination hardware
address and source hardware address, then puts IP in the Ether-Type
field. A CRC is run on the frame, and the answer is placed in the
FCS field.

35.

The frame is then handed to the Physical layer to be sent out onto the
local network one bit at a time.

36.

The destination host receives the frame, runs a CRC, checks the des-
tination hardware address, and looks in the Ether-Type field to find
who to hand the packet to.

37.

IP is the designated receiver, and after the packet is handed to IP at the
Network layer, it checks the protocol field for further direction. IP
finds instructions to give the payload to ICMP, and ICMP determines
the packet to be an ICMP echo reply.

38.

ICMP acknowledges it has received the reply by sending an exclama-
tion point (!) to the user interface. ICMP then attempts to send four
more echo requests to the destination host.

You just experienced 38 easy steps to understanding IP routing. The key

point to understand here is that if you had a much larger network, the pro-
cess would be the same, with the packet simply going through more hops
before it finds the destination host.

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