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Chapter 9 - Subnetting IP Networks

9.0 Subnetting IP Networks

9.0.1 Introduction >9.0.1.1 Introduction

Upon completion of this chapter you will be able to:

  • Explain why routing is necessary for hosts on different subnets to communicate.
  • Describe IP as a communication protocol used to identify a single device on a network.
  • Given a network and subnet mask, calculate the number of host addresses available.
  • Calculate the necessary subnet mask in order to accommodate a given number of hosts.
  • Describe the benefits of variable length subnet masking (VSLM)
  • Design and implement a hierarchical addressing scheme.
  • Explain how IPv6 address assignments are implemented in a business network.

9.0.1 Introduction >9.0.1.2 Activity - Call Me!

The figure on this page shows that an IPv6 address is split into 3 parts.

IPv6 /48 Address Block
Global Routing PrefixSubnet IDInterface ID
48 bits16 bits64 bits
Subnet ID
0000, 0001, 0002, 0003, 0004, 0005, 0006, 0007, 0008, 0009, 000A
-
FFFE, FFFF
65,536 subnets

The description given for this figure is "IP network addresses can be structured to represent smaller groups of numbers..."

9.1.1 Network Segmentation >9.1.1.1 Reasons for Subnetting

The figure on this page shows a router with a LAN connected. A host makes a broadcast and the packet is forwarded to every other device on the LAN as well as the router's LAN interface:

DeviceIP Address
Source: PC1172:16:4:1
Destination: Network255.255.255.255
PC2172:16:4:2
PC3172:16:4:3
PC4172:16:4:253

9.1.1 Network Segmentation >9.1.1.2 Communication Between Subnets

The figure on this page shows 2 routers connected. A router with 2 subnets on network 1 is communicating with a router on network 2. This is demonstrating that a router is required to move traffic from one LAN to another.

9.1.2 IP Subnetting is FUNdamental >9.1.2.1 The Plan

The figure on this page shows 3 LANs, the student LAN, Faculty LAN, and Administrative LAN. The figure has a question mark above it.

The description given for this figure is "Planning requires decisions on each subnet in terms of size, the number of hosts per subnet, and how host addresses will be assigned.".

9.1.2 IP Subnetting is FUNdamental >9.1.2.2 The Plan - Address Assignment

The image on this page shows people collaborating on a plan for subnetting.

9.1.3 Subnetting an IPv4 Network >9.1.3.1 Basic Subnetting

Figure 1 on this page shows how an IPv4 address is separated into network portion and host portion.

192.168.1.0/24 Network

 Network PortionHost Portion
Address    192     168       10000     0000
Mask255     255     2550000     0000

The description given for this figure is, "With no host bits borrowed, the host portion of both the network address and mask are all 0 bits.".

Figure 2 on this page shows how we can borrow bits from the host portion to make subnets. This figure is borrowing 1 bit and as a result can make 2 subnets. With one bit there are 2 values. The bit can be a zero or a one.

 Network PortionHost Portion
Original    192     168       1     0000     0000
Mask255     255     255     0000     0000

The borrowed bit value is 0 for the Net 0 address:

 Network PortionHost Portion
Net 0192.    168.    1.    0000    0000

The borrowed bit value is 1 for the Net 1 address:

 Network PortionHost Portion
Net 1192.    168.    1.    1000    0000

Net 0 and Net 1 are 2 subnet

The new subnets have the SAME subnet mask:

 Network PortionHost Portion
Mask255.    255.    255.    1000    0000

Figure 3 on this page shows the network ID and broadcast address for each of the two subnets.192.168.1.0/24 network.

 Network PortionHost PortionDecimal
Original192     168       1       0000     0000192.168.1.0/24
Mask255     255     255     0000     0000255.255.255.0

Borrowing 1 bit creates 2 subnets with the same mask:

 Network PortionHost PortionDecimal
Net 0192.    168.      1.      0000    0000192.168.1.0/25
Mask255     255     255       0000     0000255.255.255.128
 Network PortionHost PortionDecimal
Net 1192.    168.      1.      1000    0000192.168.1.128/25
Mask255     255     255     0000     0000255.255.255.128

9.1.3 Subnetting an IPv4 Network >9.1.3.2 Subnets in Use

Figure 1 on this page shows a router with 2 GigabitEthernet interfaces, G0/0 and G0/1. A LAN is connected to each GigabitEthernet interface:

GigabitEthernet
Interface
Subnet IP Address
G0/0192.168.1.0/25
G0/1192.168.1.128/25

Figure 2 on this page shows the address range for the 192.168.1.0/25 subnet:

AddressNetwork PortionHost portionDecimal
Network192.    168.    1.    0   000    0000  =  192.168.1.0
First Host192.    168.    1.    0   000    0001  =  192.168.1.1
Last Host192.    168.    1.    0   111    1110  =  192.168.1.126
Broadcast192.    168.    1.    0   111    1111  =  192.168.1.127

Figure 3 on this page shows the address range for the 192.168.1.128/25 subnet:

AddressNetwork PortionHost portionDecimal
Network192.    168.    1.    1   000    0000=  192.168.1.128
First Host192.    168.    1.    1   000    0001=  192.168.1.129
Last Host192.    168.    1.    1   111    1110=  192.168.1.254
Broadcast192.    168.    1.    1   111    1111=  192.168.1.255

Figure 4 on this page shows the network and the CLI of the router. The CLI shows how to configure the IP address for each LAN on the appropriate gigabit interface using the ip address command:

R1(config)#interface gigabitethernet 0/0
R1(config-if)#ip address 192.168.1.1 255.255.255.128
R1(config-if)#exit
R1(config)#interface gigabitethernet 0/1
R1(config-if)#ip address 192.168.1.129 255.255.255.128

Figure 5 on this page shows the IP configuration screen of a PC with the correct addresses:

IP address192.168.1.130
subnet mask255.255.255.128
Default gateway192.168.1.129

9.1.3 Subnetting an IPv4 Network >9.1.3.3 Subnetting Formulas

Figure 1 on this page shows an IP address with the first three octets in decimal. The fourth octet is in binary with the first bit highlighted indicating that it was borrowed for subnetting. This allows us to make 2 (2 raised to the first power) subnets.

Subnets = 2^n (2 to the the power of n)
(where n = bits borrowed)

Network PortionHost portion
192.    168.    1.   0000    0000

1 bit was borrowed
2^1 = 2 subnets

Figure 2 on this page shows an IP address with the first three octets in decimal. The fourth octet is in binary with the last 7 bits for host addressing. This allows us to make 126 (2 raised to the seventh power minus 2) host addresses.

Hosts = 2^n (2 to the the power of n)
(where n = host bits remaining)

Network PortionHost portion
192.    168.    1.   0000    0000

7 bits remain in host field
2^7 = 128 hosts per subnet
2^7-2 = 126 valid hosts per subnet

9.1.3 Subnetting an IPv4 Network >9.1.3.4 Creating 4 Subnets

Figure 1 on this page shows an IP address with the first three octets in decimal. The fourth octet is in binary with the first two bits highlighted indicating that they were borrowed for subnetting:

AddressNetwork PortionHost portion
Original192.    168.      1.      0000    0000
Mask255.    255.    255.   0000    0000

Borrowing 2 bits creates 4 subnets:

AddressNetwork PortionHost portionDecimal
Net 0192.    168.    1.   0000    0000192.168.1.0/26
Net 1192.    168.    1.   0100    0000192.168.1.64/26
Net 2192.    168.    1.   1000    0000192.168.1.128/26
Net 3192.    168.    1.   1100    0000192.168.1.192/26

All 4 subnets use the same mask:

AddressNetwork PortionHost portionDecimal
Mask255.    255.    255.   1100    0000255.255.255.192

Figure 2 on this page shows how to calculate the number of hosts on each subnet:

Network PortionHost portion
192.    168.    1.   0000    0000

6 bits remain in host field 2^6 = 64 hosts per subnet
2^6-2 = 62 valid hosts per subnet

Figure 3 on this page shows the address range for 192.168.1.0/26 subnet:

AddressNetwork PortionHost portionDecimal
Network192.    168.    1.    0000    0000=  192.168.1.0
First Host192.    168.    1.    0000    0001=  192.168.1.1
Last Host192.    168.    1.    0011    1110=  192.168.1.62
Broadcast192.    168.    1.    0011    1111=  192.168.1.63

Figure 4 on this page shows the address ranges for nets 0-2:

NetAddressNetwork PortionHost portionDecimal
0Network192.    168.    1.    0000    0000  =  192.168.1.0
0First Host192.    168.    1.    0000    0001  =  192.168.1.1
0Last Host192.    168.    1.    0011    1110  =  192.168.1.62
0Broadcast192.    168.    1.    0011    1111  =  192.168.1.63
1Network192.    168.    1.    0100    0000  =  192.168.1.64
1First Host192.    168.    1.    0100    0001  =  192.168.1.65
1Last Host192.    168.    1.    0111    1110  =  192.168.1.126
1Broadcast192.    168.    1.    0111    1111  =  192.168.1127
2Network192.    168.    1.    1000    0000  =  192.168.1.128
2First Host192.    168.    1.    1000    0001  =  192.168.1.129
2Last Host192.    168.    1.    1011    1110  =  192.168.1.190
2Broadcast192.    168.    1.    1011    1111  =  192.168.1.191-

Figure 5 on this page shows a router with 2 LANs and one WAN connected. The CLI is also shown with each interface configured with the appropriate interface address from one of the subnets.:

R1(config)#interface gigabitethernet 0/0
R1(config-if)#ip address 192.168.1.1 255.255.255.192
R1(config-if)#exit
R1(config)#interface gigabitethernet 0/1
R1(config-if)#ip address 192.168.1.65 255.255.255.192
R1(config-if)#exit
R1(config)#interface serial 0/0/0
R1(config-if)#ip address 192.168.1.129 255.255.255.192

9.1.3 Subnetting an IPv4 Network >9.1.3.5 Creating 8 Subnets

Figure 1 on this page shows two routers connected with a serial link, each with two LANs.

Figures 2 and 3 on this page show the address ranges for nets 0-7:

NetAddressNetwork PortionHost portionDecimal
0Network192.    168.    1.    0000    0000  =  192.168.1.0
0First Host192.    168.    1.    0000    0001  =  192.168.1.1
0Last Host192.    168.    1.    0001    1110  =  192.168.1.30
0Broadcast192.    168.    1.    0001    1111  =  192.168.1.31
1Network192.    168.    1.    0010    0000  =  192.168.1.32
1First Host192.    168.    1.    0010    0001  =  192.168.1.33
1Last Host192.    168.    1.    0011    1110  =  192.168.1.62
1Broadcast192.    168.    1.    0011    1111  =  192.168.163
2Network192.    168.    1.    0100    0000  =  192.168.1.64
2First Host192.    168.    1.    0100    0001  =  192.168.1.65
2Last Host192.    168.    1.    0101    1110  =  192.168.1.94
2Broadcast192.    168.    1.    0101    1111  =  192.168.1.95
3Network192.    168.    1.    0110    0000  =  192.168.1.96
3First Host192.    168.    1.    0110    0001  =  192.168.1.97
3Last Host192.    168.    1.    0111    1110  =  192.168.1.126
3Broadcast192.    168.    1.    0111    1111  =  192.168.1.127
4Network192.    168.    1.    1000    0000  =  192.168.1.128
4First Host192.    168.    1.    1000    0001  =  192.168.1.129
4Last Host192.    168.    1.    1001    1110  =  192.168.1.158
4Broadcast192.    168.    1.    1001    1111  =  192.168.1.159
5Network192.    168.    1.    1010    0000  =  192.168.1.160
5First Host192.    168.    1.    1010    0001  =  192.168.1.161
5Last Host192.    168.    1.    1011    1110  =  192.168.1.190
5Broadcast192.    168.    1.    1011    1111  =  192.168.1.191
6Network192.    168.    1.    1100    0000  =  192.168.1.192
6First Host192.    168.    1.    1100    0001  =  192.168.1.193
6Last Host192.    168.    1.    1101    1110  =  192.168.1.222
6Broadcast192.    168.    1.    1101    1111  =  192.168.1.223
7Network192.    168.    1.    1110    0000  =  192.168.1.224
7First Host192.    168.    1.    1110    0001  =  192.168.1.225
7Last Host192.    168.    1.    1111    1110  =  192.168.1.254
7Broadcast192.    168.    1.    1111    1111  =  192.168.1.255

Figure 4 on this page shows the subnet allocation for two routers, R1 and R2 connected with a serial link, subnet 192.168.1.64/27 each with two LAN's with the network ID's assigned to corresponding networks:

R1 subnet allocation:

GigabitEthernet interface G0/0192.168.1.1/27
S1192.168.1.0/27
PC1192.168.1.2/27
GigabitEthernet interface G0/1192.168.1.33/27
S2192.168.1.32/27
PC2192.168.1.34/2 7
Serial interface S0/0/0192.168.1.65/27

R2 subnet allocation:

GigabitEthernet interface G0/0192.168.1.97/27
S1192.168.1.96/27
PC1192.168.1.98/27
GigabitEthernet interface G0/1192.168.1.129/27
S2192.168.1.128/27
PC2192.168.1.130/27
Serial interface S0/0/0192.168.1.66/27

Figure 5 on this page shows the CLI output from two routers with the configuration from all six interfaces including the IP address and subnet mask:

R1:

R1(config)#interface gigabitethernet 0/0
R1(config-if)#ip address 192.168.1.1 255.255.255.224
R1(config-if)#exit
R1(config)#interface gigabitethernet 0/1
R1(config-if)#ip address 192.168.1.33 255.255.255.224
R1(config-if)#exit
R1(config)#interface serial 0/0/0
R1(config-if)#ip address 192.168.1.65 255.255.255.224

R2:

R1(config)#interface gigabitethernet 0/0
R1(config-if)#ip address 192.168.1.97 255.255.255.224
R1(config-if)#exit
R1(config)#interface gigabitethernet 0/1
R1(config-if)#ip address 192.168.1.129 255.255.255.224
R1(config-if)#exit
R1(config)#interface serial 0/0/0
R1(config-if)#ip address 192.168.1.66 255.255.255.224

9.1.3 Subnetting an IPv4 Network >9.1.3.6 Activity - Determining the Network Address - Basic

The figure on this page is an interactive activity that allows the learner to practice converting IP addresses into Network Addresses in binary and then dotted decimal.

The learner is given random IP addresses and subnet masks. For each conversion, the learner is asked to enter the correct network address in binary and in decimal format. For example:

Host Address192168162116
Subnet Mask255255255252
Host Address in binary11000000101010001010001001100100
Subnet Mask in binary11111111111111111111111111111100
Network Address in binary    
Network Address in decimal    

9.1.3 Subnetting an IPv4 Network >9.1.3.7 Activity - Calculate the Number of Hosts - Basic

The figure on this page is an interactive activity that allows the learner to practice calculating the number of valid hosts.

The learner is given an address and a subnet mask. The learner must determine the number of valid hosts per network. For example:

Network Address1921682143
Subnet Mask255255255224
Network Address in binary11000000101010001010001010001111
Subnet Mask in binary11111111111111111111111111100000
Nunber of Valid Hosts 

9.1.3 Subnetting an IPv4 Network >9.1.3.8 Activity - Determining the Valid Addresses for Hosts - Basic

The figure on this page is an interactive activity that allows the learner to practice calculating the network address, first valid host address, last valid host address, and broadcast address.

The learner is given the network address and subnet mask and asked to define the range of hosts, the broadcast address, and the next network address. For example:

Network Address in decimal1921682143
Subnet Mask in decimal255255255224
Network Address in binary11000000101010001010001010001111
Subnet Mask in binary11111111111111111111111111100000
First Usable Host IP Address in decimal    
Last Usable Host IP Address in decimal    
Broadcast Address in decimal    
Next Network address in decimal    

9.1.3 Subnetting an IPv4 Network >9.1.3.9 Activity - Calculate the Subnet Mask

The figure on this page is an interactive activity that allows the learner to practice converting a subnet mask in dotted decimal into a subnet mask in binary and prefix notation (slash notation).

The learner is given a subnet mask in decimal format and asked to determine the binary representation of the subnet mask. The learner is asked to also convert the mask to prefix notation (/x) format. For example:

Subnet Mask2552552400
Subnet Mask in binary    
Prefix notation 

9.1.3 Subnetting an IPv4 Network >9.1.3.10 Creating 100 Subnets with a /16 prefix

The figure on this page shows that by borrowing 7 host bits from a /16, you can make 128 subnets. These subnets would be a /23:

Network PortionHost portion
172.    16.    0000.   0000    0000
255.    255.    1111.   1110    0000

Borrowing 7 bits creates 128 subnets

Network PortionHost portionDecimal
172.    16.    0000.   0000    0000172.16.0.0/23
172.    16.    0000.   0010    0000172.16.2.0/23
172.    16.    0000.   0100    0000172.16.4.0/23
...to...
172.    16.    1111.   1110    0000172.16.254.0/23

9.1.3 Subnetting an IPv4 Network >9.1.3.11 Calculating the Hosts

Figure 1 on this page shows that with a /23, there are 9 host bits, and two raised to the ninth power minus 2 (or 510) valid hosts:

Hosts = 2^n
(where n = host bits remaining)

Network PortionHost portion
172.    16.    00  00. 00 00    0000    0000

9 bits remain in host field

2^9 = 512 hosts per subnet
2^9-2 = 510 valid hosts per subnet

Figure 2 on this page shows the address range for the 172.16.0.0/23 subnet:

AddressNetwork PortionHost portionDecimal
Network172.    16.    00  00. 00 00    0000    0000=  172.16.0.0/23
First Host172.    16.    00  00. 00 00    0000    0001=  172.16.0.1/23
Last Host172.    16.    00  00. 00 01    1111    1110=  172.16.1.254/23
Broadcast172.    16.    00  00. 00 01    1111    1111=  172.16.1.255/23

9.1.3 Subnetting an IPv4 Network >9.1.3.12 Calculating the Hosts

Figure 1 on this page shows that given a 10.0.0.0/8 network, if you borrow ten host bits for subnetting, you can create two raised to the tenth power (2^10) or 1024 subnets:

Network PortionHost portionDecimal
10.     0000   0000.     0000   0000.     0000   0000 
255.     1111   1111.     1100   0000.     0000   0000 
Borrowing 10 bits creates 1024 subnets
10.     0000   0000.     0000   0000.     0000   000010.0.0.0/18
10.     0000   0000.     0100   0000.     0000   000010.0.64.0/18
10.     0000   0000.     1000   0000.     0000   000010.0.128.0/18
10.     0000   0000.     1100   0000.     0000   000010.0.192.0/18
10.     0000   0001.     0000   0000.     0000   000010.1.0.0/18
..to...
10.     1111   1111.     1000   0000.     0000   000010.255.255.0/18

Figure 2 on this page shows how to calculate the number of hosts. If you borrow ten bits for subnetting from a /8, you have 18 network bits and therefore 14 host bits. With 14 host bits, you have two raised to the fourteenth power minus two ((2^14)-2) or 16,382 valid hosts on each subnet:

Network PortionHost portion
10.     00   00   00   00.     0000   0000.     0000   0000

14 bits remain in host field.

2^14 = 16384 hosts per subnet. (2^14)-2 = 16382 valid hosts per subnet.

Figure 3 on this page shows the address range for 10.0.0.0/18 subnet:

AddressNetwork PortionHost portionDecimal
Network10.     00   00   00   00.     0000   0000.     0000   000010.0.0.0/18
First Host10.     00   00   00   00.     0000   0000.     0000   000110.0.0.1/18
Last Host10.     00   00   00   00.     0011   1111.     1111   111010.0.63.254/18
Broadcast10.     00   00   00   00.     0011   1111.     1111   111110.0.63.255/18

9.1.3 Subnetting an IPv4 Network >9.1.3.13 Activity - Determining the Network Address - Advanced

The figure is an interactive activity that allows the learner to practice calculating the network address for more advanced subnetting examples.

In this activity you are given the hiost address, subnet mask, host address (in binary format), and subnet mask (in binary format).

The learner is asked to enter the network address in binary and decimal formats in the octet fields. For example:

Host Address10104191119
Subnet Mask255255255252
Host Address in binary00001010011010001011111101110111
Subnet Mask in binary11111111111111111111111111111100
Network Address in binary    
Network Address in decimal    

The figure also has the following 3 buttons:

  • Check
  • Reset
  • New Number

9.1.3 Subnetting an IPv4 Network >9.1.3.14 Activity - Calculating the Number of Hosts - Advanced

The figure is an interactive activity that allows the learner to practice calculating the number of valid host addresses for more advanced subnetting examples.

In this activity, you are given the network address, subnet mask, binary network address, and binary subnet mask.

The learner is asked to enter the number of hosts that are available for addressing in the Number of Valid Hosts field.

Network Address10000
Subnet Mask255255255240
Network Addres in Binary00001010000000000000000000000000
Subnet Mask in Binary11111111111111111111111111110000
Number of Valid Hosts 

The figure also has the following 3 buttons:

  • Check
  • Reset
  • New Number

9.1.3 Subnetting an IPv4 Network >9.1.3.15 Activity – Determining the Valid Addresses for Hosts

The figure is an interactive activity that allows the learner to practice calculating the network address, first valid host address, last valid host address, and broadcast address for a given host address and subnet mask for more advanced subnetting examples.

In the activity the learner is given the network address and subnet mask. The learner is asked to define the range of hosts, the broadcast address and the next network address and enter the answers in decimal format in the octet fields provided. For example

Network Address102061120
Subnet Mask2552552400
Network Addres in Binary00001010110011100111000000000000
Subnet Mask in Binary11111111111111111111000000000000
First Usable Host IP address in decimal    
Last Usable Host IP address in decimal    
Broadcast address in decimal    
Next network address in decimal    

The figure also has the following 3 buttons:

  • Check
  • Reset
  • New Number

9.1.4 Determining the Subnet Mask >9.1.4.1 Subnetting Based on Host Requirements

The animation on this page shows a table that displays the subnet possibilities for the 192.168.1.0 network.

Network Address in decimal192.168.1.0
Network Address in Binary11000000 10100000 00000001 hhhhhhhh
Subnet
ID Bits
Host
ID Bits
Number
of Subnets
Number of
Hosts per
Subnet
Bit pattern
081254hhhhhhhh
172126shhhhhhh
26462sshhhhhh
35830ssshhhhh
441614sssshhhh
53326ssssshhh
62642sssssshh
  • When the Subnet ID Bits = 0 the network has one subnet
  • As soon as one of the host bits is designated as a subnet bit, the network will have two subnets. Remember, in binary, a bit can have two states, 1 or 0, so the number of subnets is 2^s (2 to the power of s), where s is the number of subnet ID bits.
  • Notice the inverse relationship between the number of subnets and the number of hosts per subnet.
  • Our example network has fewer than 6 hosts in it. If we had to really subnet this network, would we choose to break it into two subnets, or would ewe choose top break it into the number of subnets that support 6 hosts?

9.1.4 Determining the Subnet Mask >9.1.4.2 Subnetting Network-Based Requirements

The figure on this page shows a cloud with smaller clouds within. This figure demonstrates subnetting based on organizational structure. The large cloud represents the Corporate Network. The smaller clouds represent:

  • Research and Development
  • Executive management
  • Sales
  • Technical Support
  • Human Resources
  • Engineers

9.1.4 Determining the Subnet Mask >9.1.4.3 Subnetting to Meet Network Requirements

Figure 1 on this page shows a corporate network divided by department. The host requirements for each department are:

LANDepartmentHosts Required
1Engineering30
2Human Resources23
3Sales40
4Technical Support35
5Executive Management10

Figure 2 on this page shows how a /22 has ten host bits which allows for 1022 valid hosts.

Network portionHost PortionDecimal
10101100.00010100.00000000.   00000000172.16.0.0/22
 10 host bits
2^10-2=1,022 hosts
 

9.1.4 Determining the Subnet Mask >9.1.4.4 Subnetting to Meet Network Requirements (Cont.)

Figure 1 on this page shows the 16 subnets that are formed when creating /26s out of a /22. You can see that by alternating bits 23, 24, 25, and 26 in a logical order, you can easily determine the network IDs for all sixteen subnets.

 Network PortionHost PortionDecimal
Original
Network Address
10101100.00010000.00000000.00000000172.16.0.0/22
SubnetNetwork PortionSubnet
Portion
Host
Portion
Decimal
010101100.00010000.00000000.00000000172.16.0.0/26
110101100.00010000.00000000.01000000172.16.0.64/26
210101100.00010000.00000000.10000000172.16.0.128/26
310101100.00010000.00000000.11000000172.16.0.192/26
410101100.00010000.00000001.00000000172.16.1.0/26
510101100.00010000.00000001.01000000172.16.1.64/26
610101100.00010000.00000010.00000000172.16.1.128/26
Subnets 7-13 not shown
1410101100.00010000.00000011.10000000172.16.3.128/26
1510101100.00010000.00000011.11000000172.16.3.192/26
  4 bits borrowed from host portion to create subnets  

Figure 2 on this page shows the same table as figure 1 a callout pointing to the 4 subnet bits, which demonstrates how 16 subnets are formed (2^4 = 16 subnets). There is also a callout pointing to the 6 remaining host bits showing that there are 62 valid hosts on each of these subnets (2^6-2 = 62 hosts per subnet).

Figure 3 on this page shows how these subnets might be assigned to the departments in a corporate network. Each subnet is the same size.

LANDepartmentAssigned Subnets
1Engineering172.16.0.0/26
2Human Resources172.16.0.64/26
3Sales172.16.1.0/26
4Technical Support172.16.0.128/26
5Executive Management172.16.0.192/26

9.1.4 Determining the Subnet Mask >9.1.4.5- Activity: Determine the Number of Bits to Borrow.

The figure on this page is an interactive activity that allows the learner to practice calculating the network address for more advanced subnetting examples.

The learner is given the number of hosts that are needed. The learner is then asked to determine the subnet mask that would support the number of hosts as specified and enter the answers in binary, decimal, and prefix notation format in the fields provided.

Hosts
Needed
Subnet Mask (Binary)Subnet Mask
(decimal)
Prefix
Notation (/x)
25011111111.11111111.11111111.00000000255.255.255.0/24
25   
1000   
75   
10   
500   

The figure also has the following 2 buttons:

  • Check
  • Reset

9.1.4 Determining the Subnet Mask >9.1.4.6: Subnetting Scenario 1

Objectives:

Part 1: Design an IP Addressing Scheme
Part 2: Assign IP Addresses to Network Devices and Verify Connectivity

9.1.4 Determining the Subnet Mask >9.1.4.6: Subnetting Scenario 2

Objectives:

Part 1: Design an IP Addressing Scheme
Part 2: Assign IP Addresses to Network Devices and Verify Connectivity

9.1.4 Determining the Subnet Mask >9.1.4.8: Calculating IPv4 Subnets

See Lab Descriptions.

9.1.4 Determining the Subnet Mask >9.1.4.9: Subnetting Network Topologies

See Lab Descriptions.

9.1.4 Determining the Subnet Mask >9.1.4.10: Researching Subnet Calculators

See Lab Descriptions.

9.1.5 Benefits of Variable Length Subnet Masking >9.1.5.1 Traditional Subnetting Wastes Addresses

Figure 1 on this page shows four routers with different host requirements. Each router is connected to the adjacent router with a WAN link. This shows a need for seven subnets as described in the page notes.

RouterLocationHosts
R1Building A25
R2Building B20
R3Building C15
R4Building D28

Figure 2 on this page shows that by borrowing three host bits from a /24, we can create 8 subnets. These subnets will be a /27.

 Network PortionHost PortionDecimal
Original
Network Address
11000000.10101000.00010100.000000000192.168.20.0/24
SubnetNetwork PortionSubnet
Portion
Host
Portion
Decimal
011000000.10101000.00010100.00000000192.168.20.0/27
111000000.10101000.00010100.00100000192.168.20.32/27
211000000.10101000.00010100.01000000192.168.20.64/27
311000000.10101000.00010100.01100000192.168.20.96/27
411000000.10101000.00010100.10000000192.168.20.128/27
511000000.10101000.00010100.10100000192.168.20.160/27
611000000.10101000.00010100.11000000192.168.20.192/27
711000000.10101000.00010100.11100000192.168.20.224/27
  Subnet portion
2^3 = 8
Host portion
2^5-2 = 30 hosts per subnet
 
SubnetAddressAssigned to
0192.168.20.0/27Buliding LAN A
1192.168.20.32/27Buliding LAN B
2192.168.20.64/27Buliding LAN C
3192.168.20.96/27Buliding LAN D
4192.168.20.128/27Site to Site WAN
5192.168.20.160/27Site to Site WAN
6192.168.20.192/27Site to Site WAN
7192.168.20.224/27Unused/Available

Figure 3 on this page shows that with the remaining five bits we will have 30 valid hosts on each of the subnets.

SubnetNetwork PortionSubnet
Portion
Host
Portion
Decimal
411000000.10101000.00010100.10000000192.168.20.128/27
511000000.10101000.00010100.10100000192.168.20.160/27
611000000.10101000.00010100.11000000192.168.20.192/27
   Host Portion
2^5-2 = 30 hosts per subnet

30-2 = 28
Each WAN subnet wastes 28 addresses

28*3 = 84
84 addresses are unused
 

9.1.5 Benefits of Variable Length Subnet Masking >9.1.5.2 Variable Length Subnet Masks (VLSM)

Figure 1 on this page shows a pie chart representing a network that has been split into eight equal subnets of 30 hosts each.

Figure 2 on this page shows one of the subnets further subnetted to /30s, and each hold 2 valid host addresses.

9.1.5 Benefits of Variable Length Subnet Masking >9.1.5.3 Basic VLSM

Figure 1 on this page shows the binary representation of the eight subnets formed by borrowing 3 bits from a /24 making /27s.

 Network PortionHost PortionDecimal
Original
Network Address
11000000.10101000.00010100.000000000192.168.20.0/24
SubnetNetwork PortionSubnet
Portion
Host
Portion
Decimal
011000000.10101000.00010100.00000000192.168.20.0/27
111000000.10101000.00010100.00100000192.168.20.32/27
211000000.10101000.00010100.01000000192.168.20.64/27
311000000.10101000.00010100.01100000192.168.20.96/27
411000000.10101000.00010100.10000000192.168.20.128/27
511000000.10101000.00010100.10100000192.168.20.160/27
611000000.10101000.00010100.11000000192.168.20.192/27
711000000.10101000.00010100.11100000192.168.20.224/27
SubnetAddressAssigned to
0192.168.20.0/27Buliding LAN A
1192.168.20.32/27Buliding LAN B
2192.168.20.64/27Buliding LAN C
3192.168.20.96/27Buliding LAN D
4192.168.20.128/27Unused/Available
5192.168.20.160/27Unused/Available
6192.168.20.192/27Unused/Available
7192.168.20.224/27 

Figure 2 on this page shows the binary representation of the eight subnets formed by borrowing 3 bits from a /27 making /30s.

SubnetNetwork PortionHost PortionDecimal
711000000.10101000.00010100.11100000192.168.224.0/27

3 more bit borrowed from subnet 7:

SubnetNetwork PortionSubnet
Portion
Host
Portion
Decimal
7:011000000.10101000.00010100.11100000192.168.20.224/30
7:111000000.10101000.00010100.11100100192.168.20.228/30
7:211000000.10101000.00010100.11101000192.168.20.232/30
7:311000000.10101000.00010100.11101100192.168.20.236/30
7:411000000.10101000.00010100.11110000192.168.20.240/30
7:511000000.10101000.00010100.11110100192.168.20.244/30
7:611000000.10101000.00010100.11111000192.168.20.248/30
7:711000000.10101000.00010100.11111100192.168.20.252/30
SubnetAddressAssigned to
7:0192.168.20.224/30WANs
7:1192.168.20.228/30WANs
7:2192.168.20.232/30WANs
7:3192.168.20.236/30Unused/Available
7:4192.168.20.240/30Unused/Available
7:5192.168.20.244/30Unused/Available
7:6192.168.20.248/30Unused/Available
7:7192.168.20.252/30Unused/Available

9.1.5 Benefits of Variable Length Subnet Masking >9.1.5.4 VLSM in Practice

Figure 1 on this page shows four routers connected with a WAN link.

RouterLocationSubnet Address
R1Building A192.168.20.0/27
R2Building B192.168.20.32/27
R3Building C192.168.20.64/27
R4Building D192.168.20.96/27
R1 - R2WAN link192.168.20.224/30
R2 - R3WAN link192.168.20.228/30
R3 - R4WAN link192.168.20.232/30

Figure 1 on this page also shows the CLI configuration commands to configure the interfaces on Router 1 for building A:

R1 (config)#interface gigabitethernet 0/0
R1 (config-if)#ip address 192.168.20.1 255.255.255.224
R1 (config-if)#exit
R1 (config)#interface serial 0/0/0
R1 (config-if)#ip address 192.168.20.225 255.255.255.252
R1 (config-if)#end
R1#

Figure 2 on this page shows the CLI configuration commands to configure the interfaces on Router 2 for building B:

R2 (config)#interface gigabitethernet 0/0
R2 (config-if)#ip address 192.168.20.33 255.255.255.224
R2 (config-if)#exit
R2 (config)#interface serial 0/0/0
R2 (config-if)#ip address 192.168.20.226 255.255.255.252
R2 (config-if)#exit
R2 (config)#interface serial 0/0/1
R2 (config-if)#ip address 192.168.20.229 255.255.255.252
R2 (config-if)#end
R2#

Figure 3 on this page shows the CLI configuration commands to configure the interfaces on Router 3 for building C:

R3 (config)#interface gigabitethernet 0/0
R3 (config-if)#ip address 192.168.20.65 255.255.255.224
R3 (config-if)#exit
R3 (config)#interface serial 0/0/0
R3 (config-if)#ip address 192.168.20.230 255.255.255.252
R3 (config-if)#exit
R3 (config)#interface serial 0/0/1
R3 (config-if)#ip address 192.168.20.233 255.255.255.252
R3 (config-if)#end
R3#

Figure 4 on this page shows the CLI configuration commands to configure the interfaces on Router 4 for building D:

R4 (config)#interface gigabitethernet 0/0
R4 (config-if)#ip address 192.168.20.97 255.255.255.224
R4 (config-if)#exit
R4 (config)#interface serial 0/0/0
R4 (config-if)#ip address 192.168.20.234 255.255.255.252
R4 (config-if)#end
R4#

9.1.5 Benefits of Variable Length Subnet Masking >91.5.5 VLSM Chart

Figure 1 on this page shows a chart using basic subnetting to assign equal sized subnets to the LANs and WANs.

 /27 NetworkHosts
Building A.0.1 - .30
Building B.32.33 - .62
Building C.64.65 - .94
Building D.96.97 - .126
WAN R1-R2.128.129 - .158
WAN R2-R3.160.161 - .190
WAN R3-R4.192.193 - .222
Unused.224.225 - .254

Figure 2 on this page shows a chart using VLSM subnetting to assign equal sized (slash twenty sevens) subnets to each of the LAN links and different but equal sized (slash thirties) subnets to each of the WAN links.

 /30 NetworkHosts
WAN R1-R2.224.225 - .226
WAN R2-R3.228.227 - .230
WAN R3-R4.232.233 - .234
Unused.236.237 - .238
Unused.240.241 - .242
Unused.244.245 - .246
Unused.248.249 - .250
Unused.252.253 - .254

9.1.5 Benefits of Variable Length Subnet Masking >9.1.5.6 Activity - Practicing VLSM

Figure 1 on this page is an interactive activity that allows the learner to practice regular subnetting by matching the addresses with their subnet description.

The following table uses regular subnetting to accommodate the network shown.

Match the addresses to the appropriate subnet description.

192.168.5.0/24//First Subnets Calculation
Calcualte 50 users per subnet.
New subnet Msk (decimal) 
First Prefix Notation 
First full Subnet Range 
Second full Subnet Range 
Last full Subnet Range 

The Subnet Addresses are:

  • 192.168.5.0 - 192.168.5.63
  • /26
  • 255.255.255.192
  • 192.168.5.192 - 192.168.5.255
  • 192.168.5.64 - 192.168.5.127

The figure also has the following 2 buttons:

  • Check
  • Reset

Figure 2 on this page is an interactive activity that allows the learner to practice subnetting by matching the parts from VLSM subnetting with their description.

192.168.5.0/24//VLSM Calculation
Use the table in figure 1's second full subnet range and VLSM to calculate for 20 users per subnet.
Second full subnet range (/26) from table in figure 1 
New VSLM Subnet Mask (decimal) 
VLSM prefix notation 
First full VLSM subnet range 
Last full VLSM subnet range 

The Subnet Addresses are:

  • 192.168.5.96 - 192.168.5.127
  • /27
  • 192.168.5.64 - 192.168.5.127
  • 255.255.255.224
  • 192.168.5.64 - 192.168.5.95

The figure also has the following 2 buttons:

  • Check
  • Reset

9.2 Addressing Schemes

9.2.1 Structured Design >9.2.1.1 Planning to Address the Network

The figure on this page shows two groupings of computers, printers, and servers. This illustrates that planning before subnetting is important.

The description given for this figure is "Network addressing should be based on network segmentation."

9.2.1 Structured Design >9.2.1.2 Assigning Addresses to Devices

The figure on this page shows a table with a breakdown of IP address assignment based on network design.

Network: 192.168.1.0/24
UseFirstLast
Host Devices.1.229
Servers.230.239
Printers.240.249
Intermediary Devices.250.253
Gateway (router LAN interface).254 

9.2.1 Structured Design >9.2.1.3: Lab - Designing and Implementing a Subnetted IPv4 Addressing Scheme

See Lab Descriptions.

9.2.1 Structured Design >9.2.1.4: Lab - Designing and Implementing a VLSM Addressing Scheme

See Lab Descriptions.

9.2.1 Structured Design >9.2.1.5: Packet Tracer - Designing and Implementing a VLSM Addressing Scheme

Objectives:

Part 1: Examine the Network Requirements
Part 2: Design the VLSM Addressing Scheme
Part 3: Assign IP Addresses to Devices and Verify Connectivity

9.3 Design Considerations for IPv6

9.3.1 Subnetting an IPv6 Network >9.3.1.1 Subnetting Using the Subnet ID

Figure 1 on this page shows the structure of an IPv6 /48 address block.

Global routing prefixSubnet IDInterface ID
48 Bit16 bits64 bits

The subnet ID in the address block has 65,536 subnets in the range:

  • 0000
  • 0001
  • 0002
  • 0003
  • 0004
  • 0005
  • 0006
  • 0007
  • 0008
  • 0009
  • 000A

to

  • FFFE
  • FFFF

Figure 2 on this page shows that the 16 bits, represented in hexadecimal, can make 65,536 subnets. These subnets are 0000 through FFFF.

Increment Subnet ID to create 65,536 subnets.

Address Block: 2001:0DB8:ACAD::/48
Subnet Address List
2001:0DB8:ACAD:0000::/64
2001:0DB8:ACAD:0001::/64
2001:0DB8:ACAD:0002::/64
2001:0DB8:ACAD:0003::/64
2001:0DB8:ACAD:0004::/64
2001:0DB8:ACAD:0005::/64
2001:0DB8:ACAD:0006::/64
2001:0DB8:ACAD:0007::/64
2001:0DB8:ACAD:0008::/64
2001:0DB8:ACAD:0009::/64
2001:0DB8:ACAD:000A::/64
2001:0DB8:ACAD:000B::/64
2001:0DB8:ACAD:000C::/64
Subnets 13 - 65,534
not shown
2001:0DB8:ACAD:FFFF::/64

9.3.1 Subnetting an IPv6 Network >9.3.1.2 IPv6 Subnet Allocation

Figure 1 on this page shows two routers connected by a serial link. Each router has 2 switches attached and each switch has one computer attached.

RouterSwitchComputer
R1S1PC1
R1S2PC2
R2S3PC3
R2S4PC4

Figure 2 on this page shows several IPv6 subnet IDs and highlighting 5 which will be used for the networks previously displayed.

Address Block: 2001:0DB8:ACAD::/48
Subnet Addresses to be used
2001:0DB8:ACAD:0001::/64
2001:0DB8:ACAD:0002::/64
2001:0DB8:ACAD:0003::/64
2001:0DB8:ACAD:0004::/64
2001:0DB8:ACAD:0005::/64

Figure 3 on this page shows the previous network topology with the IPv6 subnets assigned to the networks.

DeviceSubnet
S12001:0DB8:ACAD:0001::/64
S22001:0DB8:ACAD:0002::/64
R1 - R2
Serial Link
2001:0DB8:ACAD:0003::/64
S32001:0DB8:ACAD:0004::/64
S42001:0DB8:ACAD:0005::/64

Figure 4 on this page shows a router with the CLI showing how to configure each interface with an I.P. version 6 address.

@@R1 (config)#interface gigabitethernet 0/0

R1 (config-if)#ipv6 address 2001:db8:acad:1::1/64 R1 (config-if)#exit R1 (config)#interface gigabitethernet 0/1 R1 (config-if)#ipv6 address 2001:db8:acad:2::1/64 R1 (config-if)#exit R1 (config)#interface serial 0/0/0 R1 (config-if)#ipv6 address 2001:db8:acad:3::1/64 R1 (config-if)#end R1#@@

9.3.1 Subnetting an IPv6 Network >9.3.1.3 Subnetting into the Interface ID

The figure on this page shows the structure of an IPv6 address as a /68. This address borrows 4 more bits (or 1 nibble) from the host addresses.

Nibble = 4 bits (1 hexadecimal digit)

Global Routing PrefixSubnet IDInterface ID
48 bits16 + 4 = 20 bits64 – 4 = 60 bits
Subnet prefix /68
2001:0DB8:ACAD:0000:0000::/68
2001:0DB8:ACAD:0000:1000::/68
2001:0DB8:ACAD:0000:2000::/68
2001:0DB8:ACAD:0000:3000::/68
2001:0DB8:ACAD:0000:4000::/68
to
2001:0DB8:ACAD:FFFF:E000::/68
2001:0DB8:ACAD:FFFF:F000::/68

Subnet ID extended 1 nibble (4 bits).

9.3.1 Subnetting an IPv6 Network >9.3.1.4: Packet Tracer - Implementing a Subnetted IPv6 Addressing Scheme

Objectives:

Part 1: Determint the IPv6 Subnets and Addressing Scheme
Part 2: Configure the IPv6 Addressing on Routers and PCs and Verify Connectivity

9.4 Summary

9.4.1 Summary >9.4.1.1 Activity - Can you call me now?

The figure on this page shows a pie chart with 8 equal sections. One of the sections is further broken down into 8 more equal sections. This figure demonstrates how to incorporate VLSM into a network design.

Careful planning is required to make best use of an IP addressing scheme.

Plan for:

  • Size
  • Location
  • Use
  • Access requirements

9.4.1 Summary >9.4.1.2: Packet Tracer - Skills Integration Challenge

9.4.1 Summary >9.4.1.3 Summary

The figure on this page shows how to separate a slash twenty into two slash twenty five by changing the twenty fifth bit from a zero to one.

Original192.   168.   1.   0   000   0000Network: 192.168.1.0/24
Mask255.   255   .255   0   000   0000Mask: 255.255.255.0

Borrowing 1 bit creates 2 subnets with the same mask.

Net 0192.   168.   1.   0   000   0000Network: 192.168.1.0/25
Mask255.   255.   255.   1   000 0000Mask: 255.255.255.128
Net 1192.   168.   1.   1   000   0000Network: 192.168.1.128/25
Mask255.   255.   255.   1   000 0000Mask: 255.255.255.128

End of Chapter 9: Subnetting IP Networks.

Next - Chapter 10: Application Layer.||

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