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Cisco Cisco Certified Internetwork Expert (Routing and Switching) Exams

350-001 - CCIE Routing and Switching Written Exam #350-001, v4.0


350-018 - CCIE Security Written Exam v4.0


350-029 - CCIE SP Written Exam, V 3.0


350-030 - Cisco CCIE Voice Written (v3.0)


350-040 - Cisco CCIE Storage Networking


350-050 - CCIE Wireless Exam (V2.0)


400-051 - CCIE Collaboration


CCIE-LAB - CCIE LAB (Routing and Switching) 4.0


Defining Variable-length Subnet Mask (VLSM)


Introduction to VLSM

The purpose of Variable length subnet masks (VLSMs) is to conserve IP addresses. With VLSM, you can have different subnet masks for different subnets. It allows subnets to be further subnetted, and it also enables routers to handle different subnet masks. VLSMs provide the flexibility required to optimize the network configuration by allowing you to configure network subnets that meet the requirements of your organization. No IP addresses are wasted when you use variable length subnet masks. The concept used to describe the process wherein subnets are continuously subdivided, is recursive subnetting.


For VLSM to work, you need to use a routing protocol can send subnet mask information in its routing updates.


Routing protocols that support VLSM include:

  • Routing Information Protocol version 2 (RIPv2)

  • Open Shortest Path First (OSPF)

  • Intermediate System to Intermediate System (IS-IS)

  • Enhanced Interior Gateway Routing Protocol (EIGRP)

  • Border Gateway Protocol 4 (BGP4)


The routing protocols that do not support VLSM are:

  • Routing Information Protocol version 1 (RIPv1)

  • Interior Gateway Routing Protocol (IGRP)

  • Exterior Gateway Protocol (EGP)


The primary reasons for implementing VLSM are listed here:

  • Make efficient use of available addressing, that is, to not waste addresses.

  • Enable a hierarchical design that facilitates:

  • Summarization

  • Documentation


For VLSM to allow subnets to be further subnetted, it uses the subnet mask to distinguish between the network address and the host address of the IP address. The process of dividing a subnet into further segments starts with the subnet mask used on the original subnet. The default subnet mask is not the starting point. When you use VLSM to divide a network into different sized subnets, a definite pattern of subnet IDs with trailing 0s have to be used. The trailing 0s prevent the subnet address spaces from overlapping with each other. With a Class C network, a maximum of seven subnets can be defined.



Creating VLSM Masks

To efficiently create VLSM masks, you need to grasp how block sizes and charts are used to create the masks.


Table 1 contains the block sizes used for creating VLSMs with Class C networks:


TABLE 1: The block sizes used for creating VLSMs with Class C networks

Prefix

Mask

Hosts

Block size

/26

192

62

64

/27

224

30

32

/28

240

14

16

/29

248

6

8

/30

252

2

4



You can create a VLSM table to assist you with creating VLSMs. The steps for creating the VLSM table are outlined below:


First, create a chart that starts with 0 and ends at 255.

  • 0



  • 255


On the chart, place lines at intervals of 64:

  • 0


  • 64


  • 128


  • 192


  • 255


Next, place lines at intervals of 16:

  • 0

    • 16

    • 32

    • 48

  • 64

    • 80

    • 96

    • 112

  • 128

    • 144

    • 160

    • 176

  • 192

    • 208

    • 224

    • 240

  • 255


For WAN links, include two additional steps for creating the table in groups of 8 and 4.



VLSM Design Considerations

A few design considerations for VLSM are summarized below

  • The RIPv1 and IGRP routing protocols do not have a field for sending subnet mask information.

  • Only classless routing protocols contain a field for sending subnet mask information, and can therefore advertise subnet mask information.

  • When assigning addresses, you should try to not have discontiguous networks because the risk exists that some hosts could end up being unreachable. This is especially relevant when there are two subnets separated by a different network.

  • The rules for subnetting through VLSMs are listed next:

  • The routing protocol you use must be capable of communicating subnet masks in its routing updates.

  • A subnet can be used for either of the following purposes:

    • To address hosts

    • To enable further subnetting

  • The rule of not utilizing all zeros or ones is no longer applicable.

  • All routing decisions are made on the entire IP address.

  • If you want to summarize multiple IP address, the addresses must have the same high-order bits.





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