CCNA 200-301 Portable Command Guide 5th Edition 36
Learn how to subnet IPv4 addresses using binary math, including Class B and C subnetting, binary ANDing, and shortcuts—essential skills for mastering the CCNA 200-301 exam.
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CHAPTER 2
How to Subnet IPv4 Addresses
This chapter provides information concerning the following topics:
■ Subnetting a Class C network using binary
■ Subnetting a Class B network using binary
■ Binary ANDing
■ So Why AND?
■ Shortcuts in binary ANDing
In the previous chapter, we looked at how IPv4 addressing works, and the idea that it
is possible to break a single large networks into multiple smaller networks for more
flexibility in your network design. This chapter shows you how to perform this task. This
is known as subnefting.
NOTE: Some students (and working IT professionals) are intimidated by subnetting
because it deals with math; more specifically, binary math. While some people pick
this up quickly, some take more time than others. And this is OK. Just keep practicing.
The ability to subnet IPv4 addresses is a key skill that is required to pass the CCNA
200-301 exam. This makes some people nervous during an exam. Just remember that
this is math, and therefore there has to be an absolute correct answer. If you follow the
steps, you will come up with the correct answer. I always tell my students that
subnetting and working with binary should be the easiest questions you have on an
exam, because you know that if you follow the steps you will arrive at the correct
answer. Keep calm, remember the rules, and you will be fine. After all, it's just math,
and math is easy.
NOTE: Remember from the previous chapter that there are network bits (N bits) and
host bits (H bits) in an IPv4 address and they follow a specific pattern:
Octet # I 2 3 4
Class A Address N H H H
Class B Address N N H H
Class C Address N N N H
All Os in host portion = network or subnetwork address
All 1s in host portion = broadcast address
Combination of 1s and Qs in host portion = valid host address
CHAPTER 2
How to Subnet IPv4 Addresses
This chapter provides information concerning the following topics:
■ Subnetting a Class C network using binary
■ Subnetting a Class B network using binary
■ Binary ANDing
■ So Why AND?
■ Shortcuts in binary ANDing
In the previous chapter, we looked at how IPv4 addressing works, and the idea that it
is possible to break a single large networks into multiple smaller networks for more
flexibility in your network design. This chapter shows you how to perform this task. This
is known as subnefting.
NOTE: Some students (and working IT professionals) are intimidated by subnetting
because it deals with math; more specifically, binary math. While some people pick
this up quickly, some take more time than others. And this is OK. Just keep practicing.
The ability to subnet IPv4 addresses is a key skill that is required to pass the CCNA
200-301 exam. This makes some people nervous during an exam. Just remember that
this is math, and therefore there has to be an absolute correct answer. If you follow the
steps, you will come up with the correct answer. I always tell my students that
subnetting and working with binary should be the easiest questions you have on an
exam, because you know that if you follow the steps you will arrive at the correct
answer. Keep calm, remember the rules, and you will be fine. After all, it's just math,
and math is easy.
NOTE: Remember from the previous chapter that there are network bits (N bits) and
host bits (H bits) in an IPv4 address and they follow a specific pattern:
Octet # I 2 3 4
Class A Address N H H H
Class B Address N N H H
Class C Address N N N H
All Os in host portion = network or subnetwork address
All 1s in host portion = broadcast address
Combination of 1s and Qs in host portion = valid host address
How to Subnet IPv4 Addresses
This chapter provides information concerning the following topics:
■ Subnetting a Class C network using binary
■ Subnetting a Class B network using binary
■ Binary ANDing
■ So Why AND?
■ Shortcuts in binary ANDing
In the previous chapter, we looked at how IPv4 addressing works, and the idea that it
is possible to break a single large networks into multiple smaller networks for more
flexibility in your network design. This chapter shows you how to perform this task. This
is known as subnefting.
NOTE: Some students (and working IT professionals) are intimidated by subnetting
because it deals with math; more specifically, binary math. While some people pick
this up quickly, some take more time than others. And this is OK. Just keep practicing.
The ability to subnet IPv4 addresses is a key skill that is required to pass the CCNA
200-301 exam. This makes some people nervous during an exam. Just remember that
this is math, and therefore there has to be an absolute correct answer. If you follow the
steps, you will come up with the correct answer. I always tell my students that
subnetting and working with binary should be the easiest questions you have on an
exam, because you know that if you follow the steps you will arrive at the correct
answer. Keep calm, remember the rules, and you will be fine. After all, it's just math,
and math is easy.
NOTE: Remember from the previous chapter that there are network bits (N bits) and
host bits (H bits) in an IPv4 address and they follow a specific pattern:
Octet # I 2 3 4
Class A Address N H H H
Class B Address N N H H
Class C Address N N N H
All Os in host portion = network or subnetwork address
All 1s in host portion = broadcast address
Combination of 1s and Qs in host portion = valid host address
CHAPTER 2
How to Subnet IPv4 Addresses
This chapter provides information concerning the following topics:
■ Subnetting a Class C network using binary
■ Subnetting a Class B network using binary
■ Binary ANDing
■ So Why AND?
■ Shortcuts in binary ANDing
In the previous chapter, we looked at how IPv4 addressing works, and the idea that it
is possible to break a single large networks into multiple smaller networks for more
flexibility in your network design. This chapter shows you how to perform this task. This
is known as subnefting.
NOTE: Some students (and working IT professionals) are intimidated by subnetting
because it deals with math; more specifically, binary math. While some people pick
this up quickly, some take more time than others. And this is OK. Just keep practicing.
The ability to subnet IPv4 addresses is a key skill that is required to pass the CCNA
200-301 exam. This makes some people nervous during an exam. Just remember that
this is math, and therefore there has to be an absolute correct answer. If you follow the
steps, you will come up with the correct answer. I always tell my students that
subnetting and working with binary should be the easiest questions you have on an
exam, because you know that if you follow the steps you will arrive at the correct
answer. Keep calm, remember the rules, and you will be fine. After all, it's just math,
and math is easy.
NOTE: Remember from the previous chapter that there are network bits (N bits) and
host bits (H bits) in an IPv4 address and they follow a specific pattern:
Octet # I 2 3 4
Class A Address N H H H
Class B Address N N H H
Class C Address N N N H
All Os in host portion = network or subnetwork address
All 1s in host portion = broadcast address
Combination of 1s and Qs in host portion = valid host address
To subnet a network address space, we will use the following formulae:
2N (where N is equal to the number of network
bits borrowed)
Number of XoXa/ subnets created
2n (where El is equal to the number of host bits) Number of tota/ hosts per subnet
2" - 2 Number of valid hosts per subnet
Subnetting a Class C Network Using Binary
You have an address of 192.168.100.0 724. You need nine subnets. What is the IP plan of
network numbers, broadcast numbers, and valid host numbers? What is the subnet mask
needed for this plan?
You cannot use N bits, only H bits. Therefore, ignore 192.168. 100. These numbers
cannot change. You only work with host bits. You need to borrow some host bits and lum
them into network bits (or in this ease, subnetwork bits; I use the variable N to refer to
both network and subnetwork bits).
Step 1. Determine how many II bits you need to borrow to create nine valid subnets.
2* > 9
N = 4, so you need to borrow 4 El bits and lum them into N bits.
Start with 8 II bits HHHHHHHH
Borrow 4 bits NNNNHHHH
Step 2. Determine the first subnet in binary.
OOOOHHHH
(XMX)0000 All 0s in host portion = subnetwork number
00000001 First valid host number
oooootio Second valid host number
00000011 rITiird valid host number
...
00001110 Ijist valid host number
00001111 .All Is in host portion = broadcast number
Step 3. Convert binary to decimal.
(XXXMJOOO = 0 Subnetwork number
00000001 = 1 First valid host number
00000010 = 2 Second valid host number
00000011 = 3 I’hird valid host number
. ...
00001110= 14 Ijst valid host number
00001111 = 15 ?MI Is in host portion = broadcast number
To subnet a network address space, we will use the following formulae:
2N (where N is equal to the number of network
bits borrowed)
Number of XoXa/ subnets created
2n (where El is equal to the number of host bits) Number of tota/ hosts per subnet
2" - 2 Number of valid hosts per subnet
Subnetting a Class C Network Using Binary
You have an address of 192.168.100.0 724. You need nine subnets. What is the IP plan of
network numbers, broadcast numbers, and valid host numbers? What is the subnet mask
needed for this plan?
You cannot use N bits, only H bits. Therefore, ignore 192.168. 100. These numbers
cannot change. You only work with host bits. You need to borrow some host bits and lum
them into network bits (or in this ease, subnetwork bits; I use the variable N to refer to
both network and subnetwork bits).
Step 1. Determine how many II bits you need to borrow to create nine valid subnets.
2* > 9
N = 4, so you need to borrow 4 El bits and lum them into N bits.
Start with 8 II bits HHHHHHHH
Borrow 4 bits NNNNHHHH
Step 2. Determine the first subnet in binary.
OOOOHHHH
(XMX)0000 All 0s in host portion = subnetwork number
00000001 First valid host number
oooootio Second valid host number
00000011 rITiird valid host number
...
00001110 Ijist valid host number
00001111 .All Is in host portion = broadcast number
Step 3. Convert binary to decimal.
(XXXMJOOO = 0 Subnetwork number
00000001 = 1 First valid host number
00000010 = 2 Second valid host number
00000011 = 3 I’hird valid host number
. ...
00001110= 14 Ijst valid host number
00001111 = 15 ?MI Is in host portion = broadcast number
2N (where N is equal to the number of network
bits borrowed)
Number of XoXa/ subnets created
2n (where El is equal to the number of host bits) Number of tota/ hosts per subnet
2" - 2 Number of valid hosts per subnet
Subnetting a Class C Network Using Binary
You have an address of 192.168.100.0 724. You need nine subnets. What is the IP plan of
network numbers, broadcast numbers, and valid host numbers? What is the subnet mask
needed for this plan?
You cannot use N bits, only H bits. Therefore, ignore 192.168. 100. These numbers
cannot change. You only work with host bits. You need to borrow some host bits and lum
them into network bits (or in this ease, subnetwork bits; I use the variable N to refer to
both network and subnetwork bits).
Step 1. Determine how many II bits you need to borrow to create nine valid subnets.
2* > 9
N = 4, so you need to borrow 4 El bits and lum them into N bits.
Start with 8 II bits HHHHHHHH
Borrow 4 bits NNNNHHHH
Step 2. Determine the first subnet in binary.
OOOOHHHH
(XMX)0000 All 0s in host portion = subnetwork number
00000001 First valid host number
oooootio Second valid host number
00000011 rITiird valid host number
...
00001110 Ijist valid host number
00001111 .All Is in host portion = broadcast number
Step 3. Convert binary to decimal.
(XXXMJOOO = 0 Subnetwork number
00000001 = 1 First valid host number
00000010 = 2 Second valid host number
00000011 = 3 I’hird valid host number
. ...
00001110= 14 Ijst valid host number
00001111 = 15 ?MI Is in host portion = broadcast number
To subnet a network address space, we will use the following formulae:
2N (where N is equal to the number of network
bits borrowed)
Number of XoXa/ subnets created
2n (where El is equal to the number of host bits) Number of tota/ hosts per subnet
2" - 2 Number of valid hosts per subnet
Subnetting a Class C Network Using Binary
You have an address of 192.168.100.0 724. You need nine subnets. What is the IP plan of
network numbers, broadcast numbers, and valid host numbers? What is the subnet mask
needed for this plan?
You cannot use N bits, only H bits. Therefore, ignore 192.168. 100. These numbers
cannot change. You only work with host bits. You need to borrow some host bits and lum
them into network bits (or in this ease, subnetwork bits; I use the variable N to refer to
both network and subnetwork bits).
Step 1. Determine how many II bits you need to borrow to create nine valid subnets.
2* > 9
N = 4, so you need to borrow 4 El bits and lum them into N bits.
Start with 8 II bits HHHHHHHH
Borrow 4 bits NNNNHHHH
Step 2. Determine the first subnet in binary.
OOOOHHHH
(XMX)0000 All 0s in host portion = subnetwork number
00000001 First valid host number
oooootio Second valid host number
00000011 rITiird valid host number
...
00001110 Ijist valid host number
00001111 .All Is in host portion = broadcast number
Step 3. Convert binary to decimal.
(XXXMJOOO = 0 Subnetwork number
00000001 = 1 First valid host number
00000010 = 2 Second valid host number
00000011 = 3 I’hird valid host number
. ...
00001110= 14 Ijst valid host number
00001111 = 15 ?MI Is in host portion = broadcast number
Step 4. Determine the second subnet in binary.
0001 HHHH
00010000 All Os in host portion = subnetwork number
00010001 First valid host number
00010010 Second valid host number
00011110 Last valid host number
00011111 All Is in host portion = broadcast number
Step 5. Convert binary to decimal.
00010000= 16 Subnetwork number
00010001 = 17 First valid host number
00011 110 = 30 Last valid host number
00011111 = 31 All Is in host portion = broadcast number
Step 6. Create an IP plan table.
Subnet Network
Number
Range of
Valid Hosts
Broadcast
Number
1 0 1-14 15
2 16 17-30 31
3 32 33-46 47
Notice a pattern? Counting by 16.
Step 7. Verify the pattern in binary. (The third subnet in binary is used here.)
OOIOHHHH 'ITiird subnet
00100000 = 32 Subnetwork number
00100001 = 33 First valid host number
00100010 = 34 Second valid host number
00101110=46 Last valid host number
00101 II 1 = 47 Broadcast number
Step 8. Finish the IP plan tabic.
Subnet Network
Address (0000)
Range of Valid
Hosts (0001-1110)
Broadcast
Address (1111)
1 (0000) I92J 68. 100.0 I92.168.100.1-
192. 168. J 00.14
I92J68.I00.15
2(0001) 192. 168. J 00.16 192. 168. 100.17-
192.168.100.30
I92J68.I00.31
Step 4. Determine the second subnet in binary.
0001 HHHH
00010000 All Os in host portion = subnetwork number
00010001 First valid host number
00010010 Second valid host number
00011110 Last valid host number
00011111 All Is in host portion = broadcast number
Step 5. Convert binary to decimal.
00010000= 16 Subnetwork number
00010001 = 17 First valid host number
00011 110 = 30 Last valid host number
00011111 = 31 All Is in host portion = broadcast number
Step 6. Create an IP plan table.
Subnet Network
Number
Range of
Valid Hosts
Broadcast
Number
1 0 1-14 15
2 16 17-30 31
3 32 33-46 47
Notice a pattern? Counting by 16.
Step 7. Verify the pattern in binary. (The third subnet in binary is used here.)
OOIOHHHH 'ITiird subnet
00100000 = 32 Subnetwork number
00100001 = 33 First valid host number
00100010 = 34 Second valid host number
00101110=46 Last valid host number
00101 II 1 = 47 Broadcast number
Step 8. Finish the IP plan tabic.
Subnet Network
Address (0000)
Range of Valid
Hosts (0001-1110)
Broadcast
Address (1111)
1 (0000) I92J 68. 100.0 I92.168.100.1-
192. 168. J 00.14
I92J68.I00.15
2(0001) 192. 168. J 00.16 192. 168. 100.17-
192.168.100.30
I92J68.I00.31
0001 HHHH
00010000 All Os in host portion = subnetwork number
00010001 First valid host number
00010010 Second valid host number
00011110 Last valid host number
00011111 All Is in host portion = broadcast number
Step 5. Convert binary to decimal.
00010000= 16 Subnetwork number
00010001 = 17 First valid host number
00011 110 = 30 Last valid host number
00011111 = 31 All Is in host portion = broadcast number
Step 6. Create an IP plan table.
Subnet Network
Number
Range of
Valid Hosts
Broadcast
Number
1 0 1-14 15
2 16 17-30 31
3 32 33-46 47
Notice a pattern? Counting by 16.
Step 7. Verify the pattern in binary. (The third subnet in binary is used here.)
OOIOHHHH 'ITiird subnet
00100000 = 32 Subnetwork number
00100001 = 33 First valid host number
00100010 = 34 Second valid host number
00101110=46 Last valid host number
00101 II 1 = 47 Broadcast number
Step 8. Finish the IP plan tabic.
Subnet Network
Address (0000)
Range of Valid
Hosts (0001-1110)
Broadcast
Address (1111)
1 (0000) I92J 68. 100.0 I92.168.100.1-
192. 168. J 00.14
I92J68.I00.15
2(0001) 192. 168. J 00.16 192. 168. 100.17-
192.168.100.30
I92J68.I00.31
Step 4. Determine the second subnet in binary.
0001 HHHH
00010000 All Os in host portion = subnetwork number
00010001 First valid host number
00010010 Second valid host number
00011110 Last valid host number
00011111 All Is in host portion = broadcast number
Step 5. Convert binary to decimal.
00010000= 16 Subnetwork number
00010001 = 17 First valid host number
00011 110 = 30 Last valid host number
00011111 = 31 All Is in host portion = broadcast number
Step 6. Create an IP plan table.
Subnet Network
Number
Range of
Valid Hosts
Broadcast
Number
1 0 1-14 15
2 16 17-30 31
3 32 33-46 47
Notice a pattern? Counting by 16.
Step 7. Verify the pattern in binary. (The third subnet in binary is used here.)
OOIOHHHH 'ITiird subnet
00100000 = 32 Subnetwork number
00100001 = 33 First valid host number
00100010 = 34 Second valid host number
00101110=46 Last valid host number
00101 II 1 = 47 Broadcast number
Step 8. Finish the IP plan tabic.
Subnet Network
Address (0000)
Range of Valid
Hosts (0001-1110)
Broadcast
Address (1111)
1 (0000) I92J 68. 100.0 I92.168.100.1-
192. 168. J 00.14
I92J68.I00.15
2(0001) 192. 168. J 00.16 192. 168. 100.17-
192.168.100.30
I92J68.I00.31
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