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Chapter 5
IP Routing
The IP Routing Process
T
he IP routing process is fairly simple and doesn't change, regardless
of the size of network you have. For an example, we'll use Figure 5.1 to
describe step by step what happens when Host A wants to communicate with
Host B on a different network.
F I G U R E 5 . 1
IP routing example using two hosts and one router
In our example, a user on Host A pings Host B's IP address. Routing will
not get simpler than this, but it still involves a lot of steps. Let's work
through them:
1.
Internet Control Message Protocol (ICMP) creates an echo request
payload (which is just the alphabet in the data field).
2.
ICMP hands that payload to Internet Protocol (IP), which then creates
a packet. At a minimum, this packet contains an IP source address, IP
destination address, and a protocol field with 01h. All that tells the
receiving host to whom it should hand the payload when the destina-
tion is reached--in this example, ICMP.
3.
Once the packet is created, IP works with Address Resolution Proto-
col (ARP) to determine whether the destination IP address is on the
local network or a remote one.
4.
Since ARP and IP determine this is a remote request, the packet needs
to be sent to the default gateway so the packet can be routed to the
remote network. The Registry in Windows is parsed to find the con-
figured default gateway.
5.
The default gateway of host 172.16.10.2 is configured to 172.16.10.1.
To be able to send this packet to the default gateway, the hardware
Ethernet0
172.16.10.1
Ethernet1
172.16.20.1
Host A
172.16.10.2
Host B
172.16.20.2
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