Continue the series of Abstract CCNA study guide book .
Practice Example #5B:
255.255.255.0 (/24)
Subnetting Basics
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.
There are loads of reasons in favor of subnetting,
including the following benefits:
Reduced network traffic : 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 within one gigantic network.
Facilitated spanning
of large geographical distances: Because WAN links are slower and expensive
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.
IP Subnet-Zero
in the past, Cisco
courseware and exam, didn’t cover it—but it certainly does 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, but with the ip
subnet-zero command, you now get to use subnets 0, 64, 128, and 192.
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.
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.
Understanding the Power of 2
The power of 2 should
commit to memory
2^1 =2 , 2^2=4 ,2^3 =8 ,2^4 =16 ,2^5 =32 ,2^6 =64 ,2^7 =128 ,2^8 =256
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.
The network administrator
creates a 32-bit subnet mask composed of 1s and 0s. The 1s in the subnet mask
represent the positions that refer to the network or subnet addresses.
Next Table shows the
default subnet masks for Classes A, B, and C. These default masks cannot
change.
Classless Inter-Domain Routing (CIDR)
Another term you need to
familiarize yourself with is Classless Inter-Domain Routing (CIDR).
It’s basically the method
that ISPs (Internet service providers) use to allocate a number of addresses to
a company, a home—a customer. They provide addresses in a certain block size, something
I’ll be going into in greater detail later in this chapter.
When you receive a block
of addresses from an ISP, what you get will look something like this:
192.168.10.32/28. This is telling you what your subnet mask is. The slash
notation (/) means how many bits are turned on (1s). Obviously, the maximum
could only be /32 because a byte is 8 bits and there are 4 bytes in an IP
address. But keep in mind that the largest subnet mask available can only be a
/30 because you’ve got to keep at least 2 bits for host bits.
next Table has a listing
of every available subnet mask and its equivalent CIDR slash notation.
Class A Only
|
Subnet mask
|
CIDR value
|
Class A and B Only
|
Subnet mask
|
CIDR value
|
Class A,B and C
|
Subnet mask
|
CIDR value
|
255.0.0.0
|
/8
|
255.255.0.0
|
/16
|
255.255.255.0
|
/24
|
|||
255.128.0.0
|
/9
|
255.255.128.0
|
/17
|
255.255.255.128
|
/25
|
|||
255.192.0.0
|
/10
|
255.255.192.0
|
/18
|
255.255.255.192
|
/26
|
|||
255.224.0.0
|
/11
|
255.255.224.0
|
/19
|
255.255.255.224
|
/27
|
|||
255.240.0.0
|
/12
|
255.255.240.0
|
/20
|
255.255.255.240
|
/28
|
|||
255.248.0.0
|
/13
|
255.255.248.0
|
/21
|
255.255.255.248
|
/29
|
|||
255.252.0.0
|
/14
|
255.255.252.0
|
/22
|
255.255.255.252
|
/30
|
|||
255.254.0.0
|
/15
|
255.255.254.0
|
/23
|
Subnetting Class C Addresses
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?
-How many subnets? 2^x = number of subnets.
x is the number of masked bits, or the
1s. For example, in 11000000, the number of 1s gives us 2^2 subnets. In this
example, there are 4 subnets.
-How many hosts per
subnet? 2^y - 2 = number of hosts per
subnet.
y is the number of unmasked bits, or
the 0s. For example, in 11000000, the number of 0s gives us 2^6 – 2 hosts. In this
example, there are 62 hosts per subnet. You need to subtract 2 for the subnet address
and the broadcast address, which are not valid hosts.
_ What are the
valid subnets? 256 – subnet mask = block size.
An example would be 256 –
192 = 64. Start counting at zero in blocks of 64 until you reach the subnet
mask value and these are your subnets. 0, 64, 128, 192.
_ What’s the
broadcast address for each subnet? Since we counted our subnets
in the last section as 0, 64, 128, and 192, the broadcast address is always the
number right before the next subnet. For example, the 0 subnet has a broadcast address
of 63 because the next subnet is 64. The 64 subnet has a broadcast address of
127 because the next subnet is 128. And so on. And remember, the broadcast
address of the last subnet is always 255.
_What are the valid
hosts? Valid hosts are the numbers between the subnets,omitting
the all 0s and all 1s.
For example, if 64 is the
subnet number and 127 is the broadcast address, then 65–126 is the valid host
range—it’s always the numbers between the subnet address and the
broadcast address.
Subnetting Practice Examples: Class C Addresses
Practice Example #1C:
255.255.255.128 (/25)
192.168.10.0 = Network
address
255.255.255.128 = Subnet
mask
Now, let’s answer the big
five:
_ How many subnets?
Since 128 is 1 bit on (10000000), the answer would be 2^1 = 2.
_ How many hosts
per subnet? We have 7 host bits off (10000000), 2^7 – 2 = 126 hosts.
_ What are the
valid subnets? 256 – 128 = 128.so our subnets are 0, 128.
_ What’s the
broadcast address for each subnet? For the zero 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 The following table shows the 0 and 128
subnets, the valid host ranges of each, and the broadcast address of both
subnets:
Subnet 0 128
First host 1 129
Last host 126 254
Broadcast 127 255
Practice Example #2C:
255.255.255.192 (/26)
192.168.10.0 = Network
address
255.255.255.192 = Subnet
mask
Now, let’s answer the big
five:
_ subnets? Since 192 is 2 bits on
(11000000), the answer would be 2^2 = 4 subnets.
_ hosts? We have 6 host bits off
(11000000), 2^6 – 2 = 62 hosts.
_ valid subnets? 256 – 192 =
64.subnets are 0, 64, 128, and 192.
_ broadcast address for each subnet? For
the zero subnet, the next subnet is 64, so the broadcast address for the zero
subnet is 63.
_ the valid hosts? These are the
numbers between the subnet and broadcast address.
_ 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)?
The subnets (do this
first) 0
64 128 192
Our first host
(perform host addressing last) 1
65 129 193
Our last host 62
126 190 254
The broadcast address
(do this second) 63 127 191
255
Subnetting in Your Head: Class C Addresses
It really is possible to
subnet in your head. Even if you don’t believe me, I’ll show you how.
take the following
example:
192.168.10.33 = Node
address
255.255.255.224 = Subnet
mask
First, determine the subnet
and broadcast address of the above IP address. You can do this by answering
question 3 of the big five questions: 256 – 224 = 32. 0, 32, 64. The address of
33 falls between the two subnets of 32 and 64 and must be part of the
192.168.10.32 subnet.
The next subnet is 64, so
the broadcast address of the 32 subnet is 63. The valid host range is 33–62.
Okay, let’s try
another one. We’ll subnet another Class C address:
192.168.10.33 = Node
address
255.255.255.240 = Subnet
mask
What subnet and broadcast
address is the above IP address a member of? 256 – 240 = 16. 0, 16, 32, 48.
the host address is
between the 32 and 48 subnets. The subnet is 192.168.10.32, and the broadcast
address is 47 . The valid host range is 33–46 .
Regardless of whether you
have a Class A, Class B, or Class C address, the /30 mask will provide you with
only two hosts, ever. This mask is suited almost exclusively—as well as
suggested by Cisco—for use on point-to-point links.
Subnetting Class B Addresses
We know the Class B
network address has 16 bits available for host addressing. This means we can
use up to 14 bits for subnetting (because we have to leave at least 2 bits for
host addressing).
The process of subnetting
a Class B network is the same as it is for a Class C, except that you just have
more host bits and you start in the third octet.
Use the same subnet
numbers for the third octet with Class B that you used for the fourth octet
with Class C, but add a zero to the network portion and a 255 to the broadcast
section in the fourth octet. The following table shows you an example host
range of two subnets used in a Class B 240 (/20) subnet mask:
First subnet 16.0 32.0
Second subnet 31.255 47.255
The preceding example is
true only until you get up to /24 . after that it's the same as Class C
Subnetting Practice Examples: Class B Addresses
Practice Example #1B:
255.255.128.0 (/17)
172.16.0.0 = Network
address
255.255.128.0 = Subnet
mask
-Subnets? 2^1 = 2 (same as Class C).
-Hosts? 2^15 – 2 = 32,766 (7 bits
in the third octet, and 8 in the fourth).
_ Valid subnets? 256
– 128 = 128. 0, 128. Remember that subnetting is performed in the third octet,
so the subnet numbers are really 0.0 and 128.0, as shown in the next table.
_ Broadcast address
for each subnet?
_ Valid hosts?
Subnet 0.0 128.0
First host 0.1 128.1
Last host 127.254 255.254
Broadcast 127.255 255.255
255.255.255.0 This is a Class
B subnet mask with 8 bits of subnetting—it’s considerably different from a
Class C mask. Subnetting this address is fairly simple:
172.16.0.0 = Network
address
255.255.255.0 = Subnet
mask
_ Subnets? 2^8 = 256.
_ Hosts? 2^8 – 2 = 254.
_ Valid subnets? 256
– 255 = 1. 0, 1, 2, 3, etc., all the way to 255.
_ Broadcast address
for each subnet?
_ Valid hosts?
Subnet 0.0 1.0 2.0 3.0 .................. 254.0 255.0
First host 0.1 1.1
2.1 3.1
.................. 254.1 255.1
Last host 0.254 1.254 2.254 3.254 .............. 254.254 255.254
Broadcast 0.255 1.255
2.255 3.255
.............. 254.255 255.255
Practice Example #7B:
255.255.255.192 (/26)
Since the third octet has a 255 in the mask section,
whatever number is listed in the third octet is a subnet number. However, now
that we have a subnet number in the fourth octet, we can subnet this octet just
as we did with Class C subnetting. Let’s try it out:
172.16.0.0 = Network
address
255.255.255.192 = Subnet
mask
-Subnets? 2^10 = 1024.
-Hosts? 2^6 – 2 = 62.
-Valid subnets? 256
– 192 = 64. The subnets are shown in the following table.
-Broadcast address for
each subnet?
-Valid hosts?
Subnet 0.0 0.64 0.128 0.192 1.0
1.64 1.128 1.192
First host 0.1 0.65
0.129 0.193
1.1 1.65
1.129 1.193
Last host 0.62 0.126 0.190 0.254 1.62
1.126 1.190
1.254
Broadcast 0.63 0.127
0.191 0.255
1.63 1.127
1.191 1.255
Subnetting in Your Head: Class B Addresses
Question: What
subnet and broadcast address is the IP address 172.16.10.33 255.255.255.224
(/27) a member of?
Answer: The
interesting octet is the fourth octet. 256 – 224 = 32. 32 + 32 = 64. Bingo:33 is
between 32 and 64. However, remember that the third octet is considered part of
the subnet, so the answer would be the 10.32 subnet. The broadcast is 10.63,
since 10.64 is the next subnet. That was a pretty easy one.
Q: What subnet and
broadcast address is the IP address 172.16.66.10 255.255.192.0 (/18) a member of?
A: The interesting
octet is the third octet instead of the fourth octet. 256 – 192 = 64. 0, 64, 128. The subnet is 172.16.64.0. The
broadcast must be 172.16.127.255 since 128.0 is the next subnet.
Q: What subnet and
broadcast address is the IP address 172.16.50.10 255.255.224.0 (/19) a member
of?
A: 256 – 224 = 0,
32, 64 . The subnet is 172.16.32.0, and the broadcast must be 172.16.63.255
since 64.0 is the next subnet.
Q: What subnet and
broadcast address is the IP address 172.16.46.255 255.255.240.0 (/20) a member
of?
Answer: 256 – 240
= 16. The third octet is interesting to us. 0, 16, 32, 48. This subnet address
must be in the 172.16.32.0 subnet, and the broadcast must be 172.16.47.255 since
48.0 is the next subnet. So, yes, 172.16.46.255 is a valid host.
Q: What subnet and
broadcast address is the IP address 172.16.45.14 255.255.255.252 (/30) a member
of?
A: Where is the
interesting octet? 256 – 252 = 0, 4, 8, 12, 16 (in the fourth octet). The
subnet is 172.16.45.12, with a broadcast of 172.16.45.15 because the next
subnet is 172.16.45.16.
Q: A router
receives a packet on an interface with a destination address of 172.16.46.191/26.
What will the router do with this packet?
A: Discard it. Do
you know why? 172.16.46.191/26 is a 255.255.255.192 mask, which gives us a
block size of 64. Our subnets are then 0, 64, 128, 192. 191 is the broadcast address
of the 128 subnet, so a router, by default, will discard any broadcast packets.
Subnetting Class A Addresses
Class A subnetting is not
performed any differently than Classes B and C, but there are 24 bits to play
with instead of the 16 in a Class B address and the 8 in a Class C address.
Subnetting Practice Examples: Class A Addresses
Practice Example #1A:
255.255.0.0 (/16)
Class A addresses use a
default mask of 255.0.0.0, which leaves 22 bits for subnetting since you must
leave 2 bits for host addressing. The 255.255.0.0 mask with a Class A address
is using 8 subnet bits.
_ Subnets? 2^8 = 256.
_ Hosts? 2^16 – 2 = 65,534.
_ Valid subnets? What
is the interesting octet? 256 – 255 = 1. 0, 1, 2, 3, etc. (all in the second
octet). The subnets would be 10.0.0.0, 10.1.0.0, 10.2.0.0, 10.3.0.0, etc., up
to 10.255.0.0.
_ Broadcast address for
each subnet?
_ Valid hosts?
Subnet 10.0.0.0 10.1.0.0 … 10.254.0.0 10.255.0.0
First host 10.0.0.1 10.1.0.1 … 10.254.0.1 10.255.0.1
Last host 10.0.255.254 10.1.255.254 … 10.254.255.254 10.255.255.254
Broadcast 10.0.255.255 10.1.255.255 … 10.254.255.255 10.255.255.255
Practice Example #2A:
255.255.240.0 (/20)
255.255.240.0 gives us 12
bits of subnetting and leaves us 12 bits for host addressing.
_ Subnets? 2^12 = 4096.
_ Hosts? 2^12 – 2 = 4094.
_ Valid subnets? What
is your interesting octet? 256 – 240 = 16. The subnets in the second octet are
a block size of 1 and the subnets in the third octet are 0, 16, 32, etc.
_ Broadcast address
for each subnet?
_ Valid hosts?
Subnet 10.0.0.0 10.0.16.0 10.0.32.0
… 10.255.240.0
First host 10.0.0.1 10.0.16.1
10.0.32.1 … 10.255.240.1
Last host 10.0.15.254 10.0.31.254 10.0.47.254
… 10.255.255.254
Broadcast 10.0.15.255 10.0.31.255 10.0.47.255
… 10.255.255.255
Practice Example #3A:
255.255.255.192 (/26)
Let’s do one more example
using the second, third, and fourth octets for subnetting.
_ Subnets? 2^18 = 262,144.
_ Hosts? 2^8 – 2 = 62.
_ Valid subnets? In
the second and third octet, the block size is 1, and in the fourth octet, the
block size is 64.
_ Broadcast address
for each subnet?
_ Valid hosts?
The following table shows
the first four subnets and their valid hosts and broadcast addresses
Subnet 10.0.0.0 10.0.0.64 10.0.0.128
10.0.0.192
First host 10.0.0.1 10.0.0.65
10.0.0.129 10.0.0.193
Last host 10.0.0.62 10.0.0.126 10.0.0.190
10.0.0.254
Broadcast 10.0.0.63 10.0.0.127
10.0.0.191 10.0.0.255
The following table shows
the last four subnets and their valid hosts and broadcast addresses:
Subnet 10.255.255.0 10.255.255.64 10.255.255.128
10.255.255.192
First host 10.255.255.1 10.255.255.65 10.255.255.129
10.255.255.193
Last host 10.255.255.62 10.255.255.126 10.255.255.190 10.255.255.254
Broadcast 10.255.255.63 10.255.255.127 10.255.255.191 10.255.255.255
Subnetting in Your Head: Class A Addresses
This sounds hard, but as
with Class C and Class B, the numbers are the same; we just start in the second
octet. What makes this easy? You only need to worry about the octet that has
the largest block size (typically called the interesting octet; one that is
something other than 0 or 255)—for example, 255.255.240.0 (/20) with a Class A
network. The second octet has a block size of 1, so any number listed in that
octet is a subnet. The third octet is a 240 mask, which means we have a block
size of 16 in the third octet. If your host ID is 10.20.80.30, what is your subnet,
broadcast address, and valid host range?
The subnet in the second
octet is 20 with a block size of 1, but the third octet is in block sizes of
16, so we’ll just count them out: 0, 16, 32, 48, 64, 80, and 96 (By the way,
you can count by 16s by now, right?) This makes our subnet 10.20.80.0, with a
broadcast of 10.20.95.255 because the next subnet is 10.20.96.0. The valid host
range is 10.20.80.1 through 10.20.95.254. And yes, no lie! You really can do
this in your head if you just get your block sizes nailed!
Okay, let’s practice on
one more, just for fun!
Host IP: 10.1.3.65/23
First, /23, is 255.255.254.0. The interesting octet
here is the third one: 256 – 254 = 2. Our subnets in the third octet are 0, 2,
4, 6, etc. The host in this question is in subnet 2.0, and the next subnet is
4.0, so that makes the broadcast address 3.255. And any address between 10.1.2.1
and 10.1.3.254 is considered a valid host.
