Chapter 8 - IP Subnetting, Troubleshooting and Introduction to NAT 9e

Chapter 8:
IP Subnetting,
Troubleshooting and
Introduction to NAT
CompTIA Network+
Study Guide: Exam N10-005, 2nd Edition
by Todd Lammle

2
Objectives
1.3 Explain the purpose and properties of IP
addressing Classless (CIDR) Subnetting
2.1 Given a scenario, install and configure
routers and switches.
● NAT
● PAT

3
Subnetting Basics
There are loads of reasons in favor of
subnetting, including the following benefits:
● Reduced network traffic
● Optimized network performance
● Simplified management
● Facilitated spanning of large geographical
distances

4
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 left available
for defining hosts.

5
How to Create Subnets (cont.)
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 previous 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

6
Exponents
21 = 2
22 = 4
23 = 8
24 = 16
25 = 32
26 = 64
27 = 128
28 = 256
29 = 512
210 = 1,024
211 = 2,048
212 = 4,096
213 = 8,192
214 = 16,384

7
Subnet Masks
For the subnet address scheme to work, every
machine on the network must know which part of the
host address will be used as the subnet address.
This is accomplished by assigning a subnet mask to
each machine. A subnet mask is a 32-bit value that
allows the recipient of IP packets to distinguish the
network ID portion of the IP address from the host ID
portion of the IP address.

8
Classless Inter-Domain Routing

9
Class C Subnets
There are many different ways to subnet a
network. The right way is the way that works
best for you. In a Class C address, only 8 bits
are available for defining the hosts.
Remember that subnet bits start at the left and
go to the right, without skipping bits.

10
Subnetting Class C
When you’ve chosen a possible subnet mask for your
network and need to determine the number of subnets,
valid hosts, and broadcast addresses of a subnet that
the mask provides, all you need to do is answer five
simple questions:
● How many subnets does the chosen subnet mask
produce?
● How many valid hosts per subnet are available?
● What are the valid subnets?
● What’s the broadcast address of each subnet?
● What are the valid hosts in each subnet?

11
Practice Example #1C:
255.255.255.128 (/25)
Because 128 is 10000000 in binary, there is only 1 bit for
subnetting, and there are 7 bits for hosts. We’re going to
subnet the Class C network address 192.168.10.0.
192.168.10.0 = Network address
255.255.255.128 = Subnet mask
Now, let’s answer the big five:
How many subnets? Because 128 is 1 bit on (10000000),
the answer is 21 = 2.
How many hosts per subnet? We have 7 host bits off
(10000000), so the equation is 27 – 2 = 126 hosts.
(continued….)

12
/25 continued
What are the valid subnets? 256 – 128 = 128. Remember,
we’ll start at zero and count in our block size, so our
subnets are 0, 128.
What’s the broadcast address for each subnet? The
number right before the value of the next subnet is all host
bits turned on and equals the broadcast address. For the 0
subnet, the next subnet is 128, so the broadcast of the 0
subnet is 127.
What are the valid hosts? These are the numbers between
the subnet and broadcast address. The easiest way to find
the hosts is to write out the subnet address and the
broadcast address. This way, the valid hosts are obvious.

13
/25 continued (cont.)
Before moving on to the next example, take a
look at the figure in the next slide.
Okay, looking at a Class C /25, it’s pretty clear
there are two subnets.
But so what—why is this significant? Well actually,
it’s not, but that’s not the right question. What
you really want to know is what you would do with
this information!

14
Subnet Example /25
There are two physical networks, so we’re going to
implement a logical addressing scheme that allows for
two logical networks. As always, it’s a really good idea to
look ahead and consider likely growth scenarios—both
short and long term, but for this example, a /25 will do
the trick.

15
255.255.255.192 (/26)
In this second example, we’re going to subnet the network
address 192.168.10.0 using the subnet mask 255.255.255.192.
● 192.168.10.0 = Network address
● 255.255.255.192 = Subnet mask
It’s time to answer the big five:
● How many subnets? Because 192 is 2 bits on (11000000), the
answer is 22 = 4 subnets.
● How many hosts per subnet? We have 6 host bits off
(11000000), so the equation is 26 – 2 = 62 hosts.
● What are the valid subnets? 256 – 192 = 64. Remember, we
start at zero and count in our block size, so our subnets are 0,
64, 128, and 192.

16
255.255.255.192 (/26) (cont.)
What’s the broadcast address for each subnet? The
number right before the value of the next subnet is all
host bits turned on and equals the broadcast address. For
the 0 subnet, the next subnet is 64, so the broadcast
address for the 0 subnet is 63.
What are the valid hosts? These are the numbers
between the subnet and broadcast address. The easiest
way to find the hosts is to write out the subnet address
and the broadcast address.
This way, the valid hosts are obvious. The following table
shows the 0, 64, 128, and 192 subnets, the valid host
ranges of each, and the broadcast address of each
subnet:

17
255.255.255.192 (/26) (cont.)
Okay, again, before getting into the next example,
you can see that we can now subnet a /26. And what
are you going to do with this fascinating information?
Implement it!
We’ll use the figure on the next slide to practice a /26
network implementation.

18
Implementing a Class C /26
The /26 mask provides four subnetworks, and we need a
subnet for each router interface. With this mask, in this
example, we actually have room to add another router
interface.

19
Practice Example #3C: /27
This time, we’ll subnet the network address 192.168.10.0 and
subnet mask 255.255.255.224.
● 255.255.255.224 = Subnet mask
How many subnets? 224 is 11100000, so our equation is 23 = 8.
How many hosts? 25 – 2 = 30.
What are the valid subnets? 256 – 224 = 32. We just start at zero
and count to the subnet mask value in blocks (increments) of 32:
0, 32, 64, 96, 128, 160, 192, and 224.
What’s the broadcast address for each subnet (always the
number right before the next subnet)?
What are the valid hosts (the numbers between the subnet
number and the broadcast address)?

20
/27 Continued
To answer the last two questions, first just write out
the subnets, and then write out the broadcast
addresses—the number right before the next
subnet.
Last, fill in the host addresses.
The following table gives you all the subnets for the
255.255.255.224 Class C subnet mask:

21
255.255.255.240 (/28)
Let’s practice on another one:
● 255.255.255.240 = Subnet mask
Subnets? 240 is 11110000 in binary. 24 = 16.
Hosts? 4 host bits, or 24 – 2 = 14.
Valid subnets? 256 – 240 = 16. Start at 0: 0 + 16 = 16. 16 + 16
= 32. 32 + 16 = 48. 48 + 16 = 64. 64 + 16 = 80. 80 + 16 = 96.
96 + 16 = 112. 112 + 16 = 128. 128 + 16 = 144. 144 + 16 =
160. 160 + 16 = 176. 176 + 16 = 192. 192 + 16 = 208. 208 +
16 = 224. 224 + 16 = 240.
Broadcast address for each subnet?
Valid hosts?

22
255.255.255.248 (/29)
Let’s keep practicing:
● 255.255.255.248 = Subnet mask
Subnets? 248 in binary = 11111000. 25 = 32.
Hosts? 23 – 2 = 6.
Valid subnets? 256 – 248 = 0, 8, 16, 24, 32, 40, 48, 56,
64, 72, 80, 88, 96, 104, 112, 120, 128, 136, 144, 152,
160, 168, 176, 184, 192, 200, 208, 216, 224, 232, 240,
and 248.
Broadcast address for each subnet?
Valid hosts?

23
255.255.255.252 (/30)
One more:
● 255.255.255.252 = Subnet mask
Subnets? 64.
Hosts? 2.
Valid subnets?0, 4, 8, 12, and so on, all the way to 252.
Broadcast address for each subnet (always the number
right before the next subnet)?
Valid hosts (the numbers between the subnet number
and the broadcast address)?

24
Troubleshooting IP Addresses
Troubleshooting IP addressing is obviously an important skill
because running into trouble somewhere along the way is pretty
much a sure thing, and it’s going to happen to you. Here are the
steps:
1. Open a DOS window, and ping 127.0.0.1.
2. From the DOS window, ping the IP address of the local host.
3. From the DOS window, ping the default gateway (router).
4. If steps 1 through 3 were successful, try to ping the remote
server.
5. If the user still can’t communicate with the server after steps 1
through 4 are successful, you probably have some type of
name resolution problem and need to check your Domain
Name System (DNS) settings.

25
Command Line Tools
Packet InterNet Groper (ping)
● Uses an Internet Control Message Protocol (ICMP) echo request and replies
to test if a host IP stack is initialized and alive on the network.
traceroute
● Displays the list of routers on a path to a network destination by using TTL
time-outs and ICMP error messages. This command will not work from a
DOS prompt.
tracert
● Same command as traceroute, but it’s a Microsoft Windows command and
will not work on other devices, like a Cisco router or Unix box.
arp -a
● Displays IP-to-MAC-address mappings on a Windows PC.
ipconfig /all
● Used only from a DOS prompt. Shows you the PC network configuration.

26
Troubleshooting Example
A user in the sales department calls and tells you that she can’t get to
Server A in the marketing department. You ask her if she can get to
Server B in the marketing department, but she doesn’t know because
she doesn’t have rights to log on to that server. What do you do?

27
Network Address Translation
The original intention for NAT was to slow the depletion of
available IP address space by allowing many private IP addresses
to be represented by some smaller number of public IP
addresses.
Since then, it’s been discovered that NAT is also a useful tool for
network migrations and mergers, server load sharing, and
creating “virtual servers.”
At times, NAT really decreases the overwhelming amount of
public IP addresses required in your networking environment.
And NAT comes in very handy when two companies that have
duplicate internal addressing schemes merge. NAT is also great
to have around when an organization changes its ISP and the
networking manager doesn’t want the hassle of changing the
internal address scheme.

28
Network Address Translation
Here’s a list of situations when it’s best to have
NAT on your side:
● You need to connect to the Internet, and your
hosts don’t have globally unique IP addresses.
● You change to a new ISP that requires you to
renumber your network.
● You need to merge two intranets with duplicate
addresses.
● You typically use NAT on a border router. For an
illustration of this, see the next slide.

29
Where to configure NAT?
The border router in this example, is the
Corporate router. This is where NAT would best
be implemented in this network.

30
Types of NAT
Static NAT (SNAT)
● This type of NAT is designed to allow one-to-one mapping between
local and global addresses.
Dynamic NAT
● This version gives you the ability to map an unregistered IP address to
a registered IP address from a pool of registered IP addresses.
Overloading
● This is the most popular type of NAT configuration. Understand that
overloading really is a form of dynamic NAT that maps multiple
unregistered IP addresses to a single registered IP address—many-to-
one—by using different ports. Now, why is this so special? Well,
because it’s also known as Port Address Translation (PAT). And by
using PAT (NAT Overload), you get to have thousands of users connect
to the Internet using only one real global IP address.

31
NAT Terms
The names we use to describe the addresses used with NAT are pretty simple.
Addresses used after NAT translations are called global addresses. These are
usually the public addresses used on the Internet; but remember, you don’t need
public addresses if you aren’t going on the Internet.
Local addresses are the ones we use before NAT translation. So, the inside local
address is actually the private address of the sending host that’s trying to get to
the Internet, while the outside local address is the address of the destination
host. The latter is usually a public address (web address, mail server, and so on)
and is how the packet begins its journey.

33
How PAT works (NAT Overloading)

Chapter 8 - IP Subnetting, Troubleshooting and Introduction to NAT 9e

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