# Learning notes on IP address planning in vlsm

Source: Internet
Author: User

First, we know that due to the limited number of IP addresses in the V4 version, we need to continue to divide subnets, reduce the number of hosts and increase the number of networks on the premise of the original subnet, to avoid wasting too many host addresses in a subnet as much as possible, vlsm works for this purpose.

Benefits of vlsm:

Efficient use of IP addresses

Routing aggregation is more powerful.

This reduces the router load by reducing the number of route table entries.

We try to separate the IP addresses used by the router Management Platform (loopback interface) from the IP addresses used by normal business traffic, and do not continue to divide the subnet (although sometimes this will waste, but in a large network will reflect this advantage ).

Assume that a Chinese branch of an international multinational company is assigned a network of 172.16.12.0/22. For this network, the/22 subnet mask is actually 255.255.252.0.

Therefore, we know that the available address range is 172.16.12.0 --- 172.16.15.255, where 172.16.12.0 is the network address, and 172.16.15.255 is the broadcast address in the network.

The specific network range is:

172.16.12.0 --- 172.16.12.255

172.16.htm --- 172.16.13.255

172.16.14.0 --- 172.16.14.255

172.16.15.0 --- 172.16.15.255

All the IP addresses here belong to 172.16.12.0/22. At this time, if we need more networks, we must continue to borrow the host bit to the network bit.

Based on the actual needs of the network, if each subnet needs no more than 254 hosts, you can allocate any of the above ranges to it (the subnet mask will be changed to/24. The calculation method is as follows: 254 hosts are required. Because two addresses cannot be used, 254 + 2 = 256, and the power of 2 is exactly 256, the host space is 8 bits, then the network bit is 32-8 = 24), so we can allocate 172.16.12.0/24,172.16 .20./ 24 ...... any subnet like 172.16.15.0/24 to this network

If we need a subnet of 30 hosts, 30 + 2 = 32, and 2 is 32 to the power of 5, the host space is enough as long as five digits, and the network bit is 32-5 = 27, so the mask is/27, so we can allocate 172.16.htm/27,172.16 .13.32/27,172.16 .13.64/27 .... any subnet such as 172.16.13.20./ 27. we must note that, assuming that the previous 172.16.12.0/24 is not used up, for example, if the IP address 172.16.12.31/24 is used up, can we use the 172.12.16.32/27 subnet? The answer is no. Once a subnet has been used, it cannot be further divided in this subnet. It must be further divided using an unused subnet. therefore, we cannot use a network like 172.16.13.4/30 (the network looks correct on the surface, but because 172.16.htm/27 is already in use .)

Experience: allocate a large IP address to the router interface and the loopback interface for management.

For example, allocate 254. * to manage 253. * To vro interfaces. This principle should also be followed after subnet partitioning.

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