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This chapter will pick up right where we left off in the last chapter.
We will continue our discussion of IP addressing.
We’ll start with subnetting an IP network. You’re going to
have to really apply yourself, because subnetting takes time and practice in order to nail it. So
be patient. Do whatever it takes to get this stuff dialed in. This chapter truly is important—
possibly the most important chapter in this book for you to understand.
I’ll thoroughly cover IP subnetting from the very beginning. I know this might sound weird
to you, but I think you’ll be much better off if you can try to forget everything you’ve learned
about subnetting before reading this chapter—especially if you’ve been to a Microsoft class!
After our discussion of IP subnetting, I’m going to tell you all about Variable Length Subnet
Masks (VLSMs), as well as show you how to design and implement a network using VLSM
networks.
The topics IP addressing and VLSMs move smoothly into IP address troubleshooting and
takes you through the steps Cisco recommends when troubleshooting an IP network.
I’ll wrap up the chapter by going over an introduction to network address translation
(NAT), which has the great benefit of helping us conserve address space. You’ll learn the
basics of the various types of NAT.
So get psyched—you’re about to go for quite a ride! This chapter will truly help you understand
IP addressing and networking, so don’t get discouraged or give up. If you stick with it,
I promise that one day you’ll look back on this and you’ll be really glad you decided to hang
on. It’s one of those things that after you understand it, you’ll wonder why you once thought
it was so hard. Ready? Let’s go!
For up-to-the-minute updates for this chapter, please see
www.lammle.com
and/or
www.sybex.com/go/ccent
.
Subnetting Basics
In Chapter 2, you learned how to define and find the valid host ranges used in a Class A, Class B,
and Class C network address by turning the host bits all off and then all on. This is very good, but
here’s the catch: You were defining only one network. What happens if you wanted to take one
network address and create six networks from it? You would have to do something called
subnetting
, because that’s what allows you to take one larger network and break it into a bunch of smaller
networks.
Subnetting Basics
117
There are loads of reasons in favor of subnetting, including the following benefits:
Reduced network traffic
We all appreciate less traffic of any kind. Networks are no different.
Without trusty routers, packet traffic could grind the entire network down to a near
standstill. With routers, most traffic will stay on the local network; only packets destined for
other networks will pass through the router. Routers create broadcast domains. The more
broadcast domains you create, the smaller the broadcast domains and the less network traffic
on each network segment.
Optimized network performance
This is a result of reduced network traffic.
Simplified management
It’s easier to identify and isolate network problems in a group of
smaller connected networks than it is within one gigantic network.
Facilitated spanning of large geographical distances
Because WAN links are considerably
slower and more expensive than LAN links, a single large network that spans long distances
can create problems in every area previously listed. Connecting multiple smaller networks
makes the system more efficient.
In the following sections, I am going to move to subnetting a network address. This is the
good part—ready?
IP Subnet-Zero
IP subnet-zero
is not a new command, but in the past, Cisco courseware and Cisco exam
objectives didn’t cover it—but they certainly do now! This command allows you to use the
first and last subnet in your network design. For example, the Class C mask of 192 provides
subnets 64 and 128 (discussed thoroughly later in this chapter), but with the ip subnet-zero
command, you now get to use subnets 0, 64, 128, and 192. That is two more subnets for every
subnet mask we use.
Even though we don’t discuss the command line interface (CLI) until the next chapter, “Introduction
to the Cisco IOS and SDM,” it’s important for you to be familiar with this command:
P1R1#
sh running-config
Building configuration...
Current configuration : 827 bytes
!
hostname Pod1R1
!
ip subnet-zero
!
This router output shows that the command
ip subnet-zero
is enabled on the router.
Cisco has turned this command on by default starting with Cisco IOS version 12.
x
.
When studying for your Cisco exams, make sure you read very carefully
and understand if Cisco is asking you
not
to use
ip subnet-zero
. There are
instances where this may happen.
118
Chapter 3
IP Subnetting, Variable Length Subnet Masks (VLSMs),
How to Create Subnets
To create subnetworks, you take bits from the host portion of the IP address and reserve them
to define the subnet address. This means fewer bits for hosts, so the more subnets, the fewer
bits available for defining hosts.
Later in this chapter, you’ll learn how to create subnets, starting with Class C addresses.
But before you actually implement subnetting, you need to determine your current requirements,
as well as plan for future conditions.
Before we move on to designing and creating a subnet mask, you need to
understand that in this first section, we will be discussing classful routing,
which means that all hosts (all nodes) in the network use the exact same subnet
mask. When we move on to Variable Length Subnet Masks (VLSMs), I’ll
discuss classless routing, which means that each network segment
can
use a
different subnet mask.
To create a subnet, follow these steps:
1.
Determine the number of required network IDs:
One for each subnet
One for each wide area network connection
2.
Determine the number of required host IDs per subnet:
One for each TCP/IP host
One for each router interface
3.
Based on the above requirements, create the following:
One subnet mask for your entire network
A unique subnet ID for each physical segment
A range of host IDs for each subnet
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