CIM 2465 IP Subnetting 1

IP Subnetting

Textbook:

Networking Basics, CCNA 1 Companion Guide, Cisco Press

Cisco Networking Academy Program, CCNA 1 and 2, Companion Guide, Cisco Press, Latest Edition

CIM 2465 IP Subnetting 2

Brief Review

• Rules?

• Terms– Subnet– Subnet Number– Subnet Mask

CIM 2465 IP Subnetting 3

Determining the Number of Required Subnets and the Resulting Subnet Mask

• To come up with a subnetting plan (subnetting scheme), the engineer must answer the following questions:– How many subnets are required for a particular netwo

rk?– What is the largest number of hosts needed in the lar

gest subnet?– How many subnet bits does the subnet mask need to

support (at least) that many subnets?– How many host bits does the subnet mask need to su

pport (at least) that many hosts per subnet?– What subnet mask(s) support(s) the required number

of subnet and host bits?

CIM 2465 IP Subnetting 4

Number of Subnets and Hosts

CIM 2465 IP Subnetting 5

Number of Subnet and Host Bits

CIM 2465 IP Subnetting 6

How to find?

• Number of required subnet bits (in mask)– Find the number s such that 2S – 2 >= the

number of required subnets

• Number of required host bits (in mask)– Find the number h such that 2h – 2 >= the

number of required hosts

CIM 2465 IP Subnetting 7

Examples

Required Number of Subnets

Required Number of Hosts

Min Subnet bits needed

Min Host bits needed

200 200

5 15

12 3000

100 100

CIM 2465 IP Subnetting 8

Determine the Mask

Network Number of Subnet Bits

Number of Host Bits

Mask

128.1.0.0 8 8

192.168.9.0 3 5

172.31.0.0 4 12

130.10.0.0 7 7

CIM 2465 IP Subnetting 9

Determine Subnet Numbers and Assignable IP Addresses in Each Subnet

• Subnet number– The numerically lowest number in a subnet, represents a particul

ar IP subnet

• Subnet broadcast address– The highest numerical value in a subnet, for broadcast

• Assignable IP address– IP addresses that may be assigned to an interface

• Subnet zero (zero subnet)– The numerically smallest subnet number. With classful IP addres

sing, not be used

• Broadcast subnet– The numerically largest subnet number, With classful IP address

ing, not be used

CIM 2465 IP Subnetting 10

CIM 2465 IP Subnetting 11

Case

• Class C network: 192.168.9.0

• Mask: 255.255.255.224

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Finding Subnets

11000000 10101000 00001001 000 00000 192.168.9.0*

11000000 10101000 00001001 001 00000 192.168.9.32

11000000 10101000 00001001 111 00000 192.168.9.224*

CIM 2465 IP Subnetting 13

Finding Subnet Broadcast Address

11000000 10101000 00001001 000 11111 192.168.9.31*

11000000 10101000 00001001 001 11111 192.168.9.63

11000000 10101000 00001001 111 11111 192.168.9.255*

CIM 2465 IP Subnetting 14

Range of Valid Addresses

Subnet Lowest IP Highest IP Broadcast

192.168.9.0* 192.168.9.1 192.168.9.30 192.168.9.31

192.168.9.32 192.168.9.33 192.168.9.62 192.168.9.63

192.168.9.224*