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shango-wk 889f08a5c2 reecriture de subnetting part 3

2026-05-05 12:38:30 +02:00

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Subnetting (Part 3 - VLSM)

Class B Reference Table

Prefix	Subnets	Hosts
/17	2	32766
/18	4	16382
/19	8	8190
/20	16	4094
/21	32	2046
/22	64	1022
/23	128	510
/24	256	254

Class C Reference Table

Prefix	Subnets	Hosts
/25	2	126
/26	4	62
/27	8	30
/28	16	14
/29	32	6
/30	64	2
/31		(0)
/32		(0–2)

Quiz Question 1

You have been given the network 172.30.0.0/16. Your company requires
100 subnets with at least 500 hosts per subnet.

Answer:

Prefix: /23
Network: 172.30.0.0/23
Subnet mask: 255.255.254.0

Quiz Question 2

What subnet does host 172.21.111.201/20 belong to?


10101100.00010101.01101111.11001001  (IP)
10101100.00010101.01100000.00000000  (Network)

Answer: 172.21.96.0/20

Quiz Question 3

What is the broadcast address of the network 192.168.91.78/26?


11000000.10101000.10110011.01001110  (IP)
11000000.10101000.10110011.01111111  (Broadcast)

Answer: 192.168.91.127

Quiz Question 4

You divide the 172.16.0.0/16 network into 4 equal subnets.
Identify the network and broadcast addresses.

Subnet 1

Network: 172.16.0.0/18
Broadcast: 172.16.63.255

Subnet 2

Network: 172.16.64.0/18
Broadcast: 172.16.127.255

Subnet 3

Network: 172.16.128.0/18
Broadcast: 172.16.191.255

Subnet 4

Network: 172.16.192.0/18
Broadcast: 172.16.255.255

Quiz Question 5

You divide the 172.30.0.0/16 network into subnets of 1000 hosts each.

Answer: 64 subnets

Subnetting Class A Networks

The process of subnetting Class A, B, and C networks is exactly the same.

Example 1

You are given the network 10.0.0.0/8.
You must create 2000 subnets.

Solution

Answer:

Prefix: /19
Usable hosts: 8192 - 2 = 8190

Example 2

PC1 has IP address 10.217.182.223/11

Subnet Details

Network: 10.192.0.0/11
Broadcast: 10.223.255.255
First usable: 10.192.0.1
Last usable: 10.223.255.254
Usable hosts: 2,097,150

VLSM (Variable-Length Subnet Masks)

Previously, we used FLSM (Fixed-Length Subnet Masks)
All subnets had the same prefix length
VLSM allows different subnet sizes for better efficiency
More flexible, slightly more complex

Example (VLSM Design)

Tokyo Lan A  = 110 Hosts  ---  Tokyo Lan B  = 8 Hosts
                           |
                         Router
                           |
                         Router
                           |
Toronto Lan A  = 29 Hosts --- Toronto Lan B  = 45 Hosts

We must divide 192.168.1.0/24 into 5 subnets:

Tokyo LAN A → 110 hosts
Tokyo LAN B → 8 hosts
Toronto LAN A → 29 hosts
Toronto LAN B → 45 hosts
Point-to-point link

Steps

Assign largest subnet first
Continue in descending order
Repeat until done

Allocation Order

Tokyo A → Toronto B → Toronto A → Tokyo B → Point-to-point

Tokyo LAN A

Network: 192.168.1.0/25
Broadcast: 192.168.1.127
Usable: 192.168.1.1 – 192.168.1.126
Hosts: 126

Toronto LAN B

Network: 192.168.1.128/26
Broadcast: 192.168.1.191
Usable: 192.168.1.129 – 192.168.1.190
Hosts: 62

Toronto LAN A

Network: 192.168.1.192/27
Broadcast: 192.168.1.223
Usable: 192.168.1.193 – 192.168.1.222
Hosts: 30

Tokyo LAN B

Network: 192.168.1.224/28
Broadcast: 192.168.1.239
Usable: 192.168.1.225 – 192.168.1.238
Hosts: 14

Point-to-Point

Network: 192.168.1.240/30
Broadcast: 192.168.1.243
Usable: 192.168.1.241 – 192.168.1.242
Hosts: 2

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