Ccna notes: subnet division and vlsm

IP subnetting and variable length subnet masks (vlsm)

Subnetting Basics

Advantages of subnetting:

1. reduce network traffic

2. Improve Network Performance

3. simplified management

4. Easy to expand geographic scope

How to creat subnets

How do I divide subnets? First, you must note that the power of 2: The power of 2 0 to the power of 9 are: 1, 2, 4, 8, 16, 32, 64,128,256, and 512, respectively. it is also worth noting that the subnet division uses the host bit to take the removed part as the subnet bit. this means that the more subnets you divide, the fewer hosts you will have.

Subnet masks

The subnet mask is used to identify which part of the IP address is the network address and which part is the host address. It consists of 1 and 0 and has a length of 32 bits. All bits represent the network number. not all networks require subnets. Therefore, we introduce the default Subnet Mask (default subnet mask ). the default subnet mask of Class a ip addresses is 255.0.0.0; Type B is 255.255.0.0; Type C is 255.255.255.0.

Classless Inter-Domain Routing (CIDR)

CIDR is called classless Inter-Domain Routing. ISPs usually use this method to allocate addresses to customers. ISP provides customers with 1 block size, similar to 192.168.10.32/28, this row of numbers tells you what the subnet mask is,/28 indicates how many bits are 1, max/32. but the first point you must know is: whether it is Class A, Class B or other class addresses, the maximum available value is only 30/, that is, retain 2 bits to the host bit

CIDR value:

1. Mask 255.0.0.0:/8 (default mask for Class A addresses)

2. Mask listen 128.0.0:/9

3. Mask 255.192.0.0:/10

4. Mask 255.224.0.0:/11

5. Mask 255.240.0.0:/12

6. Mask 255.248.0.0:/13

7. Mask 255.252.0.0:/14

8. Mask 255.254.0.0:/15

9. Mask 255.255.0.0:/16 (default mask for Class B addresses)

10. Mask 255.255.128.0:/17

11. Mask 255.255.192.0:/18

12. Mask 127224.0:/19

13. Mask 255.255.240.0:/20

14. Mask 255.255.248.0:/21

15. Mask 255.255.252.0:/22

16. Mask 255.255.254.0:/23

17. Mask 255.255.255.0:/24 (default mask for Class C addresses)

18. Mask 255.255.255.128:/25

19. Mask 255.255.255.192:/26

20. Mask mask:/27

21. Mask 255.255.255.255.240:/28

22. Mask 255.255.255.248:/29

23. Mask 255.255.255.252:/30

Subnetting class A, B & c Address

Several shortcuts to dividing subnets:

1. How many subnets will the subnet mask you selected generate? : X to the power of 2-2 (X represents the mask bit, that is, the portion of the hexadecimal value 1)

2. How many hosts can each subnet have? : The power of Y of 2-2 (Y represents the host bit, that is, the part of the hexadecimal 0)

3. What is a valid subnet? : Valid subnet mask = 256-10 (block size or base number)

4. What is the broadcast address of each subnet? : Broadcast address = subnetwork ID-1

5. What are the valid hosts of each subnet? : Ignore all the addresses in the subnet that are 0 and 1. The remaining addresses are valid host addresses. The last valid host address is the next sub-network number-2 (that is, the broadcast address-1)

Based on the preceding shortcuts, you can divide the specific instance of the subnet:

Example of class C address: network address 192.168.10.0; subnet mask 255.255.255.192 (/26)

1. Number of subnets = 2*2-2 = 2

2. Number of hosts = 6 to the power of 2-2 = 62

3. valid subnet? : Block size = 256-192 = 64; therefore, the first subnet is 192.168.10.64, and the second subnet is 192.168.10.128.

4. broadcast address: The next subnet-1. The broadcast addresses of the two subnets are 192.168.10.127 and 192.168.10.191, respectively.

5. Valid host range: the host address of the first subnet is 192.168.10.65 to 192.168.10.126, and the second is 192.168.10.129 to 192.168.10.190.

Example 1: Network Address: 172.16.0.0; subnet mask 255.192.0 (/18)

1. Number of subnets = 2*2-2 = 2

2. Number of hosts = 14th power of 2-2 = 16382

3. valid subnet? : Block size = 256-192 = 64; therefore, the first subnetwork is 172.16.64.0, and the last one is 172.16.128.0.

4. broadcast address: The next subnet-1. Therefore, the broadcast addresses of the two subnets are 172.16.127.255 and 172.16.191.255, respectively.

5. Valid host range: the host address of the first subnet is 172.16.64.1 to 172.16.127.254; the second is 172.16.128.1 to 172.16.191.254.

Example 2 of type B address: Network Address: 172.16.0.0; subnet mask 255.255.255.255.255.255.255.255.(/27)

1. The number of subnets = the 11 power of 2-2 = 2046 (because the default mask of the B-type address is 255.255.0.0, the network bit is 8 + 3 = 11)

2. Number of hosts = 5 to the power of 2-2 = 30

3. valid subnet? : Block size = 256-224 = 32; therefore, the first subnetwork is 172.16.0.32, and the last one is 172.16.20.192.

4. broadcast address: The next subnet-1. Therefore, the broadcast addresses of the first subnet and the last subnet are 172.16.0.63 and 172.16.20.223, respectively.

5. Valid host range: the host address of the first subnet is 172.16.0.33 to 172.16.0.62, and the last one is 172.16.0000193 to 172.16.0000223.

Variable Length subnet masks (vlsm)

The function of a variable-length Subnet Mask (vlsm) is to save IP address space and reduce the route table size. when vlsm is used, the routing protocol used must support it. These routing protocols include ripv2, OSPF, OSPF, and BGP. for more vlsm knowledge, you can search at Google.com.

Troubleshooting IP Address

Network problems:

1. Open a DOS window in windows and ping the local loopback address 127.0.0.1. If the feedback fails, the IP protocol stack is incorrect. You must reinstall the TCP/IP protocol.

2. If 1 succeeds, ping the local IP address. If the feedback fails, your Nic cannot communicate with the IP protocol stack.