Chapter 5. Variable Length Subnet Masking

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Chapter 5

Variable Length Subnet Masking

Introduction

Why Are Variable Length Masks Necessary?

Rightsizing Your Subnets

More Addresses or More Useful Addresses?

The Importance of Proper Planning

Creating and Managing Variable Length Subnets

Analyze Subnet Needs

Enumerate Each Subnet and Number of Required Nodes

Determine Which Mask to Use

Allocate Addresses Based on Need

Routing Protocols and VLSM

Class C VLSM Problem

Completing the Class C Problem

Template-based Address Assignment

Summary

FAQs

 

Solutions in this chapter:

          Why variable length subnet masks (VLSM) are necessary.

          When to deploy VLSM.

          How to plan for creating VLSM administered networks.

          Address management in a VLSM administered network.

          Which routing protocols support VLSM.

          Creating an address plan using VLSM.

Introduction

Variable length subnet masks (VLSM) allow network administrators to right size each subnet. With fixed length subnet masks, however, each subnet in the network is the same size because each device has the same subnet mask, regardless of the need for addresses in each subnet. If we select a class mask of 255.255.254.0, each subnet is allocated 510 addresses. Most LANs have an upper limit of less than 150 devices due to traffic patterns and capacity of the physical LAN media.

In actual fact, each network, WAN or LAN, has a different number of devices. With VLSM, the address administrator can more closely match the addressing needs of each subnet and use the address space more efficiently .

Why Are Variable Length Masks Necessary?

Here is a parallel example of why VLSM is important. When restaurants sell pie, each piece of the pie is the same size. Each restaurant patron gets the same size pie slice regardless of the need (see Figure 5.1).

Figure 5.1 Uniform pie slices.

Ah, yes, the pie chart! If these people were subnets, they would all get the same size subnet regardless of how many addresses they really needed. Lets look at another possibility in Figure 5.2. Each person has a different appetite; they all need a different size slice of the pie.

Figure 5.2 Variable size pie slices.

When subnetting, it may be necessary to give each subnet an appropriate slice of the pie based on the actual number of addresses needed, rather than assume that all subnets need the same number of addresses. VLSM is the answer to this problem.

For IT Professionals Only

Developing an Addressing Plan Based on VLSM...

...requires complete knowledge of the network being subnetted . During the design phase of a new network, applying VLSM is complex but a lot easier than applying VLSM to an existing network. In an existing network, moving from fixed length subnet masks to variable length subnet masks will require a complete readdressing of the entire network. If you are considering readdressing, it may be simpler to use a private address, like 10.0.0.0, with a fixed length mask instead of trying to use VLSM. The private address gives you a lot of flexibility without the complexity of developing a VLSM addressing plan.

Rightsizing Your Subnets

What do we mean by right sizing subnets? Simply put, it is providing the right number of addresses for each subnet so that we dont end up wasting addresses. Figure 5.3 shows a simple network diagram.

Figure 5.3 The 153.90.0.0 network.

In this diagram there are three ethernet networks, one token-ring network, and four point-to-point WAN connections. Each of these is a subnet within the total corporate network. The actual number of host addresses for each subnet is in Table 5.1.

Subnet

Hosts

A

150

B

24

C

90

D

53

E

2

F

2

G

2

H

2

Table 5.1 Subnet Needs

If we were to assign a subnet mask of 255.255.255.0 for this class B addressed network, all subnets would be allocated 254 host addresses. This results in a lot of unused addresses, as shown in Table 5.2.

Subnet

Hosts

Allocated

Unused

A

150

254

104

B

24

254

230

C

90

254

164

D

53

254

201

E

2

254

252

F

2

254

252

G

2

254

252

H

2

254

252

Total

 

2032

1707

Table 5.2 Addresses Wasted

What if we choose a subnet mask for each subnet that allocates a more appropriate number of addresses in each subnet? Using the subnet masking tables from Chapter 1, we determine that the masks in Table 5.3 should be used in each of the subnets.

Subnet

Hosts

Mask Assigned

Addresses Allocated

Unused

A

150

255.255.255.0

254

104

B

24

255.255.255.224

30

6

C

90

255.255.255.128

126

36

D

53

255.255.255.192

62

9

E

2

255.255.255.252

2

0

F

2

255.255.255.252

2

0

G

2

255.255.255.252

2

0

H

2

255.255.255.252

2

0

Total

 

 

480

155

Table 5.3 Using Needs Appropriate Masks

How did we determine which mask to use? In the subnet mask table for class B networks, we located the mask that allowed the number of hosts required for each subnet.

Subnet A needed 150 hosts. In the class B subnet mask table we found that 255.255.255.0 allocates 254 addresses. We looked at some alternatives: The mask 255.255.255.192 allocates 126 addresses and cannot be used because the number of hosts is too small; on the other hand, the mask 255.255.254.0 allocates 510 hosts. This is far too many for our needs and should not be used. The mask 255.255.255.0 was selected for the best fit.

The rest of the subnets were evaluated using the same process. When it came to subnets E through H, the process was very simple. These subnets are actually point-to-point WAN links, and will have no more than two host addresses. Selecting the mask was easy because 255.255.255.252 allocates two addresses to each subnet and is an exact match for our needs.

More Addresses or More Useful Addresses?

You have probably noticed that we have saved addresses in the VLSM process. That is, we have not placed addresses in subnets that are not being used. Thats not quite truewith VLSM we have a much closer match in addresses allocated to addresses needed. We do assign addresses to subnets that are not used,   but we do not have as many unused addresses as we do with fixed length subnetting. Table 5.4 gives the results of the VLSM process for our simple network.

 

Allocated

Unused

Fixed Length Mask

2032

1707

Variable Length Masks

480

155

Table 5.4 Address Savings

Did we get more addresses? No, we still have a class B address. Are we using the addresses we have better than before? Yes; as a matter of fact, because we use the addresses more efficiently, VLSM allows us to implement more subnets with the same class of address. We didnt get more addresses, but   we do have more useful addresses.

For IT Professionals Only

Use an IP Address Only Once in a Network

Regardless of the subnetting method, variable or fixed length, you must make sure that an IP address is used only once in any network. A common mistake is to think that addresses can exist in different subnets, but that is not the case. As you develop an addressing plan using a VLSM structure, make sure you keep track of all address ranges that are allocated and those that are available for allocation. Tools presented later in this chapter will help you.

The Importance of Proper Planning

The processing of creating a VLSM subnetting structure requires a lot of advanced planning. A survey of the network is required. The survey must include the number of required subnets, the number of planned but not deployed subnets, the number of devices currently in each subnet and the number of planned but not deployed devices in each subnet. That seems like a lot of work but is necessary. You want to develop a plan that covers what you currently have and what you will probably have in the future.

Deciding to convert from fixed length masks to VLSM requires a large commitment on the part of the address planner, network managers, and users. If an existing network is changed from fixed length masks to VLSM, the entire network addressing structure will be affected. Every subnet will be assigned new address ranges and the administrative burden necessary to complete the process will be immense. Each and every device in the network will probably have to be readdressed. Do not underestimate the amount of work required to convert an existing network to VLSM.

If you fail to understand your network and subnet requirements adequately, you may develop a plan that cannot be deployed successfully. For existing networks, you may have done a lot of work planning and implementing only to find that your work must be discarded. You or other administrators and users may have readdressed thousands of devices only to find that they must be readdressed again due to a bad address plan. The following steps will help you make sure your address plan is successful the first time.

Creating and Managing Variable Length Subnets

Creating a variable length subnet mask addressing design requires the completion of four separate phases. Each of the phases must be completed before moving to the next phase.

1.    Analyze subnet needs.

2.    Enumerate each subnet and number of required nodes.

3.    Determine which mask to use in each subnet.

4.    Allocate addresses based on need for each subnet.

The details of each phase follow.

Analyze Subnet Needs

As we mentioned earlier, you need to know exactly what you have today and what you will need tomorrow for each subnet. A simple spreadsheet or matrix detailing each subnet will help you determine your needs. Remember to locate and list all networks.

For Managers Only

How Many Subnets?

The examples used in this chapter contain a limited number of subnets. In the large networks, there may be more than 500 subnets using VLSM allocations . The procedures we show you here will work with small networks with few subnets or large networks with many subnets.

Network

Location

Type

Status

Hosts Today

Hosts Future

Accounting

Building 3, 4 th floor

LAN-Ethernet

Operational

131

140

Accounting Link to Building 4

Building 34

WANPPP

Operational

2

2

Personnel

Building 4, 1 st floor

LAN-Ethernet

Operational

72

83

Personnel Expansion

Building 4, 2 nd floor

LAN-100mb Ethernet

PlannedSpring 200

 

29

Logistics

Warehouse

LANToken-ring

Operational

81

89

Shipping

Warehouse

LANEthernet

Operational

18

25

Warehouse to Building 4 link

WarehouseBuilding 4, 1 st floor

WAN-PPP

Operational

2

2

Loading Dock to Warehouse link

Loading DockWarehouse

WAN-PPP

Operational

2

2

Receiving

Loading Dock

LAN-Ethernet

Operational

14

17

Table 5.5 Network Survey

Enumerate Each Subnet and Number of Required Nodes

When the detailed needs survey is completed, the matrix you develop will contain all of the LANs and WANs with the number of hosts in each subnet (see Table 5.5). The matrix in Table 5.6 contains the survey information but has been sorted in descending sequence by the total number of hosts in each subnet in the future. The purpose of sorting the matrix is to group subnets together based on the number of hosts in the subnets. Subnets with similar numbers of host addresses will have the same subnet mask.

Network

Location

Type

Status

Hosts Today

Hosts Future

Accounting

Building 3, 4th floor

LAN-Ethernet

Operational

131

140

Logistics

Warehouse

LANToken-ring

Operational

81

89

Personnel

Building 4, 1st floor

LAN-Ethernet

Operational

72

83

Personnel Expansion

Building 4, 2nd floor

LAN-100mb Ethernet

PlannedSpring

 

29

Shipping

Warehouse

LANEthernet

Operational

18

25

Receiving

Loading Dock

LAN-Ethernet

Operational

14

17

Accounting Link to Building 4

Building 34

WANPPP

Operational

2

2

Warehouse to Building 4 link

WarehouseBuilding 4, 1st floor

WAN-PPP

Operational

2

2

Loading Dock to Warehouse link

Loading DockWarehouse

WAN-PPP

Operational

2

2

Table 5.6 Subnet Address Requirements

Determine Which Mask to Use

Using class B subnetting table, we select the subnet mask that allocates the necessary addresses for each subnet in Table 5.7.

Network

Location

Type

Status

Hosts Today

Hosts Future

Subnet Mask

Max Hosts/

Subnet

Accounting

Building 3, 4th floor

LAN-Ethernet

Operational

131

140

255.255.255.0

254

Logistics

Warehouse

LANToken-ring

Operational

81

89

255.255.255.128

126

Personnel

Building 4, 1st floor

LAN-Ethernet

Operational

72

83

255.255.255.128

126

Personnel Expansion

Building 4, 2nd floor

LAN-100mb Ethernet

PlannedSpring 2000

 

29

255.255.255.192

62

Shipping

Warehouse

LANEthernet

Operational

18

25

255.255.255.224

30

Receiving

Loading Dock

LAN-Ethernet

Operational

14

17

255.255.255.224

30

Accounting Link to Building 4

Building 34

WANPPP

Operational

2

2

255.255.255.252

2

Warehouse to Building 4 link

WarehouseBuilding 4, 1st floor

WAN-PPP

Operational

2

2

255.255.255.252

2

Loading Dock to Warehouse link

Loading Dock-Warehouse

WAN-PPP

Operational

2

2

255.255.255.252

2

Table 5.7 Subnet Mask Selection

When working with a real networking problem, make sure you leave enough room for growth in each subnet you specify. If you need 150 devices in a subnet, leave room for 200. If you need 40 devices, leave room for 60. Select a mask that gives you the allocation you need today and may need tomorrow.

Allocate Addresses Based on Need

Now it is time to determine which range of addresses will be assigned in each subnet. With fixed length subnetting, the ranges of addresses are uniform and easily determined; with variable length mask subnetting, the address ranges are just as important but more difficult to assign. A tool can be used to help determine which addresses to use.

For the purposes of this example, we will be subnetting the 172.38.0.0 class B network. The actual range of addresses that can be assigned in this network is 172.38.0.1 through 172.38.255.254. The addresses 172.38.0.0 and 172.38.255.255 are excluded as the network address and the network broadcast address. To simplify allocation, divide the network addresses into 256 groups based on the third octet of the address. In this case, the 256 blocks are given in Table 5.8.

0

16

32

48

64

80

196

112

128

144

160

176

192

208

224

240

1

17

33

49

65

81

197

113

129

145

161

177

193

209

225

241

2

18

34

50

66

82

198

114

130

146

162

178

194

210

226

242

3

19

35

51

67

83

199

115

131

147

163

179

195

211

227

243

4

20

36

52

68

84

100

116

132

148

164

180

196

212

228

244

5

21

37

53

69

85

101

117

133

149

165

181

197

213

229

245

6

22

38

54

70

86

102

118

134

150

166

182

198

214

230

246

7

23

39

55

71

87

103

119

135

151

167

183

199

215

231

247

8

24

40

56

72

88

104

120

136

152

168

184

200

216

232

248

9

25

41

57

73

89

105

121

137

153

169

185

201

217

233

249

10

26

42

58

74

90

106

122

138

154

170

186

202

218

234

250

11

27

43

59

75

91

107

123

139

155

171

187

203

219

235

251

12

28

44

60

76

92

108

124

140

156

172

188

204

220

236

252

13

29

45

61

77

93

109

125

141

157

173

189

205

221

237

253

14

30

46

62

78

94

110

126

142

158

174

190

206

222

238

254

15

31

47

63

79

95

111

127

143

159

175

191

207

223

239

255

Table 5.8 Address Block Matrix

The third octet contains the number of each possible block of 254 addresses that can be assigned if a subnet mask of 255.255.255.0 is used. If a subnet requires 300 addresses, you could use 172.38.1.0 and 172.38.2.0 to allocate a total of 510 addresses for the subnet. You would then strike through the two numbers 1 and 2 in Table 5.8 to show that the two complete 254 address blocks have been used and subsequently cannot be subdivided. Prepare a chart similar to this to keep track of the large address blocks that you have used.

For variable length masks with allocations less than 254 addresses, each of these blocks may be subdivided. Here is how we will address the example problem.

The first subnet we are going to allocate uses a subnet mask of 255.255.255.0. One address block in Table 5.8 will allocate 254 addresses. This is the number of hosts available in an address block when the mask is 255.255.255.0. We will use 172.38.1.0 for the first subnet. The range of addresses used in that subnet is 172.38.1.0 through 172.38.1.255. In case we need additional subnets requiring a mask of 255.255.255.0, we are reserving the blocks 172.38.2.0 through 172.38.31.0 for that purpose.

For Managers

Keep Blocks of Addresses Assigned by the Same Mask Together

It is good practice to keep blocks of addresses assigned by the same mask together. In this example, we are saving 32 blocks of addresses for mask 255.255.255.0. An additional 32 blocks will be saved for 255.255.255.128. Since other masks result in more subnets, an appropriate number of blocks should be saved and allocated together. Why? A uniform assignment of addresses results in a good distribution of addresses without stranding small address ranges that cannot be assigned because they contain a number of addresses that cannot be allocated in any subnet.

The second and third subnets use a mask of 255.255.255.128. This mask allocated one-half of a block of 256 addresses. There are 128 addresses in one allocation and 128 addresses in the other. In Table 5.9 we have allocated one-half of the 172.38.32.0 block to the second subnet and the other half to the second subnet. The 172.38.32.0 block is fully allocated and cannot be used for other subnets. In case we need additional subnets requiring a mask of 255.255.255.128, we are reserving blocks 172.38.33.0 through 172.38.63.0, which will allow us to allocate an additional 62 subnets of 126 addresses.

Network

Location

Subnet Mask

Max Hosts/

Subnet

Addresses Allocated

Accounting

Building 3, 4th floor

255.255.255.0

254

172.38.1.0172.38.1.255

Logistics

Warehouse

255.255.255.128

126

172.38.32.0172.38.32.127

Personnel

Building 4, 1st floor

255.255.255.128

126

172.38.32.128172.38.32.255

Personnel Expansion

Building 4, 2nd floor

255.255.255.192

62

172.38.64.64172.38.64.127

Shipping

Warehouse

255.255.255.224

30

172.38.128.32172.38.128.63

Receiving

Loading Dock

255.255.255.224

30

172.38.128.64172.38.128.95

Accounting Link to Building 4

Building 34

255.255.255.252

2

172.38.254.4172.38.254.7

Warehouse to Building 4 link

WarehouseBuilding 4, 1st floor

255.255.255.252

2

172.38.254.8172.38.254.11

Loading Dock to Warehouse link

Loading DockWarehouse

255.255.255.252

2

172.38.254.12-172.38.254.15

Table 5.9 Address Assignments

In completing this table, we assigned addresses from different blocks. To determine which addresses to use with different masks, use the address assignment templates found at the end of this chapter.

Routing Protocols and VLSM

Before you start a major VLSM implementation, there are a few particulars that must be understood . First, VLSM is difficult to implement. In our previous example we looked at a very small number of subnets. Creating a plan for a large number of subnets is time-consuming and requires accurate information. Making mistakes in a VLSM plan can cause a lot of extra administrative problems. Managers just dont appreciate renumbering the network over and over again because the address administer made a small mistake or two.

The second major issue is that the network routers must be using a routing protocol that supports VLSMthe Routing Information Protocol Version 2 (RIP2), the Open Shortest Path First protocol (OSPF), and Ciscos EIGRP. If your network does not use these protocols, dont use VLSM.

The protocols support VLSM because they share subnet mask information among all of the routers so the routers can make proper routing decisions. Without the subnet mask information provided by these protocols, routers assume that all subnets have the same mask. With VLSM they dont and routing will fail.

Class C VLSM Problem

You may also need to use VLSM to conserve addresses in a class C network. Some experts say that using VLSM in a class C network is too difficult because of the problems implementing the required routing protocols in a small network. If you are running out of addresses, you may not have a choice and VLSM may be your only solution.

In this example, the organization has four operational locations in the greater Chicago area. They have received a public class C address and wish to subnet it using VLSM. Since the network is small, the administrators have decided to implement the RIP2 routing protocol.

The choice to use VLSM was necessary due to the mix of subnets and sizes. There are seven subnets, and the largest subnet will eventually contain 95 devices; using a normal fixed length mask will not work. A mask of 255.255.255.224 will accommodate 15 subnets but will allow only 14 devices in each subnet. A mask of 255.255.255.128 will allow 126 devices in each subnet but will allow only two subnets. VLSM is a must for this implementation.

Network

Location

Type

Status

Hosts Today

Hosts Future

Accounting

Chicago

LAN-Ethernet

Operational

92

95

Logistics

Schaumburg

LAN-Ethernet

Operational

37

45

Personnel

Oak Park

LAN-Ethernet

Operational

11

13

Executive

Oak Brook

LAN-Ethernet

Operational

7

9

To Chicago

Schaumburg to Chicago

WAN-PPP

Operational

2

2

To Oak Park

Schaumburg to Oak Park

WAN-PPP

Operational

2

2

To Oak Brook

Schaumburg to Oak Brook

WAN-PPP

Operational

2

2

Table 5.10 Class C VLSM Problem

After reviewing the networking requirements survey, the administrators developed Table 5.10. Each subnet is described and the number of hosts required is also included in the table. The next step is to determine which masks to use for each subnet.

Network

Location

Type

Status

Hosts Today

Hosts Future

Subnet Mask

Max Hosts/

Subnet

Accounting

Chicago

LAN-Ethernet

Operational

92

95

255.255.255.128

126

Logistics

Schaumburg

LAN-Ethernet

Operational

37

45

255.255.255.192

62

Personnel

Oak Park

LAN-Ethernet

Operational

11

13

255.255.255.240

14

Executive

Oak Brook

LAN-Ethernet

Operational

7

9

255.255.255.240

14

To Chicago

Schaumburg to Chicago

WAN-PPP

Operational

2

2

255.255.255.252

2

To Oak Park

Schaumburg to Oak Park

WAN-PPP

Operational

2

2

255.255.255.252

2

To Oak Brook

Schaumburg to Oak Brook

WAN-PPP

Operational

2

2

255.255.255.252

2

Table 5.11 Class C VLSM Subnet Masks

Table 5.11 shows the subnet masks that the administrators determined were required for each of the subnets. The next part of the task is to determine which address ranges to apply to each subnet. Unlike the class B problem, where we selected large blocks of addresses to subdivide based on mask sizes, the class C problem requires a special technique. Class C networks contain 254 usable addresses. In the class C subnetting problem, we use addresses all from the same 254 address block but select ranges based on the mask in use.

The subnetting tables in Figures 5.4, 5.5, and 5.6show an easy way to select addresses for each range. At the top of the tables you will find labels for each of the possible fourth octet of the subnet mask. Each table has been assigned a label from A through H for each grouping of 32 available addresses. To assign addresses based on a given mask, select a column for the mask and follow the column down until the column ends. The addresses on the right-hand side table are included in the range.

For example, look at Figure 5.4, table A. To assign an address range based on the subnet mask 255.255.255.248, find the column marked 248. The first range of addresses for the .248 mask is from 0 through 7. The second range of addresses for the .248 mask is from 8 through 15, etc. The third range of addresses for mask .252 is 8 through 11. For each mask number, the tables indicate the range of addresses. Referring again to table A of Figure 5.4, look at the bottom of the 128 and 192 columns . You will see a down arrow   indicating that you need to go to the same column in the next table to continue the process of locating addresses. As a matter of fact, to assign addresses using the .128 mask, you will need to view tables A, B, and C of Figure 5.4 and table D of Figure 5.5 to determine all of the possible addresses. The end of the 128 segment on table D of Figure 5.5 does not contain an arrow. The address assignment stops when the arrow is missing. To assign the .128 addresses, you will use 0 through 31 from table A, 32 through 63 from table B, 64 through 95 from table C, and 96 through 127 from table D.

Figure 5.4 VLSM table addresses 0 through 95.

Figure 5.5 VLSM table addresses 96 through 191.

Figure 5.6 VLSM table addresses 192 through 255.

Completing the Class C Problem

Table 5.12 shows the completed address assignment matrix for the class C VLSM problem.

Network

Location

Subnet Mask

Max Hosts /Subnet

Subnet Table Column

Assigned Addresses

Accounting

Chicago

255.255.255.128

126

A,B,C,D

0 - 127

Logistics

Schaumburg

255.255.255.192

62

E,F

128 - 191

Personnel

Oak Park

255.255.255.240

14

G

192-207

Executive

Oak Brook

255.255.255.240

14

G

208-223

To Chicago

Schaumburg to Chicago

255.255.255.252

2

H

252-255

To Oak Park

Schaumburg to Oak Park

255.255.255.252

2

H

248-251

To Oak Brook

Schaumburg to Oak Brook

255.255.255.252

2

H

244-247

Table 5.12 Class C Address Assignments

Here is how the administrator decided on which address to assign. The first location required a mask of 255.255.255.128. The administrator looked at the VLSM tables, located the 128 column in table A, and followed the column down until the arrow at the end of the column was missing. Remember, the arrow says look at the next table and at the location associated with address 127, and the arrow was missing (see table D). The range of addresses is 0 through 127.

The next location required a mask of 255.255.255.192. Since all of the addresses from the first four tables were already assigned, the administrator then turned to the remaining tables E, F, G, and H. Looking at table E, the administrator found the 192 column and followed it down to the bottom of the table, found an arrow, and looked to table F. The end of the 192 column on table F did not contain an arrow so the administrator knew that the addresses for the second subnet started with 128, the first address in table E, and ended with 191, the last address in table F. The range of addresses for the second subnet is 128 through 191.

The next two subnets required a mask of 255.255.255.240. Since all of the addresses had been assigned from tables A, B, C, D, E, and F, the administrator started with table G. In table G, the administrator located the 240 column. The first subnet was assigned the range from 192 to 207, the first grouping in the 240 column, and the second subnet was assigned the range from 208 to 223, the second grouping in the 240 column.

The last three subnets required a mask of 255.255.255.252. The only assignable addresses are found in table H, so looking to the 252 column in table H, the administrator assigned the three subnets ranges 252255, 248251, and 244247. The administrator started at the bottom of the allocation to leave room in case a larger allocation is needed from the top of column H at a later time.

Using the subnet tables found in Figures 5.4, 5.5, and 5.6 simplify the procedures for VLSM. You might want to make copies of these pages or create a similar spreadsheet or form to keep track of address you have assigned. It is simple to cross out addresses as you assign them to various subnets. You have a hardcopy record of what you have assigned as well as a graphic representation of the address you have used and the addresses you have available.

Template-based Address Assignment

The following templates can be used in variable length subnetting when the last octet of the mask contains the given value. For example, with a mask of 255.255.255.252, you would look in the template labeled .25264 subnets. In that template, select a range of addresses for use in the subnet you wish to allocate. Once the range of addresses has been allocated from a block of addresses, it cannot be assigned to another subnet. Each of these templates represents a way to subnet the fourth octet of any network address.

 

.128two subnets

.0

.127

 

.128

.255

 

 

.192four subnets

.0

.63

 

.64

.127

 

.128

.191

 

.192

.255

 

 

.2248 subnets

.0

.31

 

.32

.63

 

.64

.95

 

.96

.127

 

.128

.159

 

.160

.191

 

.192

.223

 

.224

.255

 

.24016 subnets

.0

.15

 

.16

.31

 

.32

.47

 

.48

.63

 

.64

.79

 

.80

.95

 

.96

.111

 

.112

.127

 

.128

.143

 

.144

.159

 

.160

.175

 

.176

.191

 

.192

.207

 

.208

.223

 

.224

.239

 

.240

.255

 

.24832 subnets

.0

.7

 

.8

.15

 

.16

.23

 

.24

.31

 

.32

.39

 

.40

.47

 

.48

.55

 

.56

.63

 

.64

.71

 

.72

.79

 

.80

.87

 

.88

.95

 

.96

.103

 

.104

.111

 

.112

.119

 

.120

.127

 

.128

.135

 

.136

.143

 

.144

.151

 

.152

.159

 

.160

.167

 

.168

.175

 

.176

.183

 

.184

.191

 

.192

.199

 

.200

.207

 

.208

.215

 

.216

.223

 

.224

.231

 

.232

.239

 

.240

.247

 

.248

.255

 



.25264 subnets

.0

.3

 

.4

.7

 

.8

.11

 

.12

.15

 

.16

.19

 

.20

.23

 

.24

.27

 

.28

.31

 

.32

.35

 

.36

.39

 

.40

.43

 

.44

.47

 

.48

.51

 

.52

.55

 

.56

.59

 

.60

.63

 

.64

.67

 

.68

.71

 

.72

.75

 

.76

.79

 

.80

.83

 

.84

.87

 

.88

.91

 

.92

.95

 

.96

.99

 

.100

.103

 

.104

.107

 

.108

.111

 

.112

.115

 

.116

.119

 

.120

.123

 

.124

.127

 

.128

.131

 

.132

.135

 

.136

.139

 

.140

.143

 

.144

.147

 

.148

.151

 

.152

.155

 

.156

.159

 

.160

.163

 

.164

.167

 

.168

.171

 

.172

.175

 

.176

.179

 

.180

.183

 

.184

.187

 

.188

.191

 

.192

.195

 

.196

.199

 

.200

.203

 

.204

.207

 

.208

.211

 

.212

.215

 

.216

.219

 

.220

.223

 

.224

.227

 

.228

.231

 

.232

.235

 

.236

.239

 

.240

.243

 

.244

.247

 

.248

.251

 

.252

.255

 

Summary

Variable length subnet masking (VLSM) is often necessary when addresses are scarce and you need to use the addresses you have effectively. With legacy IP networks, implementing VLSM often requires a complete renumbering of the entire network so the decision to use VLSM must be made with full knowledge of the required administrative processes.

Regardless of the reason why VLSM is implemented, it requires the use of certain routing protocols to be successful. RIP2, OSPF, and EIGRP must be used on your network routers to insure that VLSM works correctly.

The process of creating a VLSM address plan includes the following steps:

1.          Analyze subnet needs.

2.          Enumerate each subnet and number of required nodes.

3.          Determine which mask to use in each subnet.

4.          Allocate addresses based on need for each subnet.

Survey your network to determine the number of subnets present. Determine the type of network and the current and future number of devices in each subnet. Create a list of the subnets you have, and group subnets together by size. With like size subnets together, determine the appropriate mask for each subnet. Use subnetting tables A through H to help assign addresses where the fourth octet of the mask contains .128, .192, .224, .240, .248, or .252.

Once addresses are allocated to a subnet, they cannot be allocated to other subnets. You must keep track of addresses carefully when you are creating the VLSM address plan to make sure that the subnet allocations for one subnet do not overlap other subnets. This often happens when an address is applied twice because it occurs in one size subnet and in another size subnet. No address ranges are allowed to overlap.

VLSM does save address, but if you have the ability to use a private address space with network or port address translation process, it may be just as useful to implement a fixed length subnet mask with a private address space. The administration is easier and address planning is simpler.

FAQs

Q: My coworker told me that VLSM gives me a lot more addresses than I had in the past. Is this true?

A: Not really. If you have been assigned a class B address, you have 65,534 addresses. Using VLSM does not increase the number of addresses, but it can allow to you have more useful addresses by allocating addresses based on subnet needs.

Q: We are running RIP in our network because it is an easy routing protocol to administer and because our network is small. Can we use VLSM?

A: RIP version 1 does not allow for the implementation of VLSM, but RIP version 2 does. Check to determine which version of RIP you are using before you commit to VLSM.

Q: Why do I have to do understand all about my network to do VLSM? When we did our original subnetting we just started assigning our addresses using a mask and it was very simple.

A: Using fixed length masks to create your addressing plan is much easier than using VLSM. With fixed length masks, each subnet is the same size and the number of addresses in each subnet is the same. A very simple process can be used to assign addresses. With VLSM, everything is more complex because all subnets are not all the same size and there is no simple process for allocating addresses. Using the tables found in this chapter is about the easiest manual process available.

Q: Why should I group subnets of similar sizes together before I start allocating addresses?

A: So that you assign uniform blocks of addresses from similar ranges. If you choose arbitrary ranges of addresses, you might create small address blocks in your plan that cannot be used because they are too small for your current requirements. You might actually need some subnets that have 30 addresses, but because you did not plan appropriately, you might be left with lots of 16 address blocks that are not contiguous and cannot be used.



IP Addressing and Subnetting, Including IPv6
IP Addressing and Subnetting, Including IPv6
ISBN: 672328704
EAN: N/A
Year: 1999
Pages: 15

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