In
many ways, the task of a router is very simple. It
must look at the destination address of an IP packet and then forward
the packet to another router or to a directly connected host. However,
if you've ever attempted to get two devices to communicate on an IP
network, you know that it doesn't always work.
In this Network Design Manual article we'll show
you the steps involved in getting a packet from Point A to Point
B on an IP network. We'll talk about the settings that are necessary
on a host to communicate on a routed network and what they really
mean. Once you understand this, you'll be able to ask the right
questions when things don't go as expected and even do a little
bit of your own troubleshooting.
Anatomy of an IP Address
When one host has data to send to another host, it
sends it to its local IP process, which builds an IP packet for transmission
on the network. The packet has a header, which includes the destination
IP address as well as the source address of the local host. The destination
and source IP addresses stay with the data until it arrives at the
host so that all the routers it traverses know in what direction to
send it. The source address provides the destination host with an
address to which it can send its response.
To understand how this happens requires a deeper
knowledge of the IP address. It's obvious that an IP address consists
of four separate numbers, separated by dots. Each number represents
one byte worth of data, limiting it to a range of 256 possible values,
commonly represented as decimal values from 0 to 255 or binary values
from 00000000-11111111.
One thing that is not so obvious is that each IP
address is always divided into two sections. The first section
always delineates the network address, or subnet. This is the part
routers are primarily concerned with. The second section, combined
with the first section forms a unique node address. The subnet mask
is configured to tell the host or client which part is the network
address and which part is the node address.
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