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Lesson 2
Planning an IP Routing Solution
2-15
a list of destination addresses, with the information needed to forward
traffic to those
destinations. Using the information in its routing table, the router
determines where to
send the packet next. The router might be able to transmit the packet
directly to its des-
tination (if the router has an interface on the destination network), or it
might send the
packet to another router, where the entire process begins again. On a
private network,
packets might travel through several routers on the way to a given
destination. On the
Internet, packets commonly pass through a dozen routers or more.
Tip
To see a list of the routers between your computer and a specific
destination address,
you can use the traceroute utility that is provided with most TCP/IP
implementations. On com-
puters running the Microsoft Windows operating systems, the traceroute
utility is called Trac-
ert.exe. To use it, display a Command Prompt window and type tracert
address, where
address is the IP address of a destination computer.
Routers obtain the information in their routing tables in one of two ways.
Either an
administrator manually enters the information, which is called static
routing, or the
router receives the information automatically from another router using a
specialized
routing protocol. This is called dynamic routing. On Internet routers, the
routing tables
can be long and complex, but the tables on private network routers are
simple.
Creating LANs
Ethernet LANs are typically defined in terms of broadcast domains and
collision
domains.
■
A broadcast domain is a group of computers, all of which receive broadcasts
transmitted by any one of the computers in a group. For example, when you
con-
nect 100 computers using only Ethernet hubs, any one of those computers can
generate a broadcast and all the other computers will receive it.
■
A collision domain is a group of computers that are connected in such a way
that
when any two computers transmit packets at exactly the same time a collision
occurs.
The collision destroys both packets and forces the computers to retransmit
them.
When you create two LANs and join them using a router, you are creating two
separate
broadcast domains, because routers do not forward broadcast transmissions
from one
network to another, and two separate collision domains, because packets
transmitted
on the same network may collide, but packets on different networks do not.
Planning
The reason to split a private network into multiple LANs is to create
different
broadcast domains and collision domains.
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