Variable eldest son netmask VLSM

Source: Internet
Author: User
Tags nets

2014-05-05 17:26:08

Tags: IP address router Technical Department routing table eldest son

Introduction of VLSM:

VLSM (VLSM (Variable Length subnetwork mask, variable eldest subnet mask) is used to effectively control the size of the routing table using CIDR (Classless Inter-domain routing) and route summarization, and can be hierarchical addressing to subnets To make the most efficient use of the existing address space. when using VLSM, the routing protocol used must be able to support it, including RIPv2,OSPF,EIGRP,is-is, and BGPv4 If you mix a different length subnet mask, VLSM, in a network running RIPV1 or IGRP, the network will not work correctly.

Advantages of VLSM:

1.IP address usage is more effective

2. Better performance when routing rollups are applied

3. Isolation from topology changes of other routers

When designing an internal network, have enough private address space why use VLSM?

A : because adjacent address blocks make it easier to generalize the network, and use a routing protocol to keep routing updates minimized. It saves a lot of bandwidth and reduces the processing time of the router.

Division of VLSM:

Example 1, a company has two main departments: Marketing department and Technical department. The technical department is divided into two departments of hardware department and software department. The company applied for a complete class C IP address segment:, subnet mask To facilitate hierarchical management, the company employs VLSM technology, which divides the primary network into a two-level subnet (without considering all 0 and all 1 subnets).

The market part got the 1th subnet in the first level subnet, namely, subnet mask, which has 62 IP addresses to allocate.

The 2nd subnet in the first-level subnet that the technology department will, the subnet mask It was further divided into two sub-nets of level two. wherein the 1th two-level subnet, subnet mask Division to the Ministry of Technology Division-Hardware department, the two-level subnet has a total of 30 IP addresses to allocate. Division of the Technical department-the software Section has got the 2nd level two subnet, subnet mask, the two-level subnet a total of 30 IP addresses to allocate.


Use Class C network address to properly plan your network based on the following topology diagram:

Method One:

1. Write down the number of IP addresses required for each network segment:

Zone A, area B, Area C, Area G, area 8, Zone H, Region J area, Zone 2

2. Arrange them in order from large to small:

3, according to the Order of VLSM Sub-network division (large address block and then small address block)












Method Two:
1. Write down the number of IP addresses required for each network segment:

Zone A, area B, Area C, Area G, area 8, Zone H, Region J area, Zone 2

2, refer to the following table to write out the network prefix and the maximum number of hosts:

A/27 B/28 C/28 G/28 h/26 16

J/26 K/28 D/30 4 E/30 4 F/30 4

3, the VLSM Sub-network division

A 0-31

B 32-47

C 48-63

G 64-79

H 80-143

I 144-159

J 160-223

K 224-239

D 240-243

E 244-247

F 248-252




VLSM variable eldest son netmask

VLSM (Variable Length Subnet mask variable eldest son netmask), which is a network allocation mechanism of different big kid nets, refers to a network can be configured with different masks. The idea of developing a variable-length subnet mask is that there is more flexibility in dividing a network into multiple subnets while keeping enough hosts on each subnet. If there is no vlsm, a subnet mask can be provided to only one network. This limits the number of hosts on the required number of subnets.

VLSM technology plays an important role in efficiently allocating IP addresses (less waste) and reducing the size of routing tables. However, it is important to note that when using VLSM, the routing protocol used must be able to support it, including RIP2,OSPF,EIGRP and BGP.

CIDR no category Addressing

The introduction of CIDR in 1992 meant that the concept of a "class" of network addresses at the routing table level had been removed and replaced with the concept of "network prefixes". The basic idea of CIDR classless Inter-Domain Routing classless Inter-domain routing in the Internet is to remove the classification structure of the address, instead of allowing the number of networks to be allocated in a variable-length demarcation. It supports routing aggregation, which restricts the growth of the necessary routing information in the Internet backbone routers. The IP address of Class A has been allocated, Class B has been almost the rest of the C-class address has become the goal of the partition. Obviously the address assigned to a country, region, organization is the best to be continuous so how to ensure this? The concept of CIDR was then proposed. CIDR is the acronym for Classless Inter Domain routing, which means classless inter-field routing. The "classless" means that the current routing decision is based on the mask operation of the entire 32-bit IP address. Regardless of whether the IP address is Class A, Class B, or Class C, there is no difference. Its idea is: to combine many C-type addresses for Class B address assignment. This method of assigning multiple IP addresses enables it to merge many of the table entries in the routing table into fewer summarization.

The two districts and contact


CIDR encompasses several concepts. It is based on the variable-length subnet masking (VLSM) technique with effective qualities of specifying Arbitra Ry-length prefixes.

Always thought there was no difference, because they are designed to save IP addresses,

Actually, they have a big area.

CIDR is the synthesis of several standard networks into a large network

VLSM is to divide a standard network into several small networks (subnets)

CIDR is the subnet mask shifted to the left, VLSM is the subnet mask to the right.

CIDR (classless Inter.domain Routing classless Inter-domain routing)

VLSM (Variable Length subnetwork mask variable eldest child netmask)

Variable eldest son netmask VLSM

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