Variable Length Subnetting
Introduction
Variable length subnetting and the subnetting of class C networks can cause misunderstanding and difficulties for many people, it is necessary that we use VSLM to conserve IP address space. This tutorial will assist you in understanding how best to subnet IP address space and how to fill out IP request templates to request more IP addresses. Included are three tables of a subnetted class C address as an example of what can be done with variable length subnetting. By using these techniques, much can be done to reduce the size of routing tables and to provide enough IP addresses to allow all of our networks to grow. You need to keep in mind that making the best use of address space using these techniques assumes that your routing equipment is capable of addressing subnet zero and that your are using a routing protocol that understands variable length subnets. Protocols like OSPF and EIGRP are capable of dealing with this implementation. Protocols like RIP are not capable of dealing with subnets. The following example is formulated to assume that a new ISP is forming and requires their initial IP address space. According to the ARIN allocation guidelines, the ISP would have to create an engineering plan and a IP address plan as part of the required documentation. We will assume that the ISP will provide the following services.
ISP Services Plan
- Local dialup modems
- Web hosting
- Some dedicated access to remote customers
- Their own corp backbone
Typical Engineering Plan
- Class C for dialup modems
- Class C for internal addresses for web service and corp. network
- Class C for dedicated customers
- Class C for connected networks
At first glance, it would make sense for our ISP to request a /22 or 4 Class C addresses. In the past, this, is what most ISPs have done and the Internic used to grant requests like that. With IP address exhaustion being a real threat, ARIN requires that IP addresses only be allocated for current use. The problem is that a /22 or 4 class C addresses, provides support for about 1000 hosts and most ISP never pass that mark in the first two years of operation. The result is that much of the IP address space is unused. To facilitate our fictional ISPs needs and still meet the ARINs guide lines, the engineering plan could be built on the following assumptions to start out. If we use variable length subnetting we can reduce the number of IP addresses that we need by 75%. Here is how we would create our IP subnet plan.
Startup Configuration or Phase I
- 30 IP addresses for dialup modems
- 12 IP addresses for internal addresses for web service and corp. network
- 2 small networks for dedicated customers 14 IPs each
- 2 small networks for connected networks 2 IPs each
Phase II Configuration
- 60 IP addresses for dialup modems
- 22 IP addresses for internal addresses for web service and corp. network
- 4 small networks for dedicated customers 14 IPs each
- 8 small networks for connected networks 2 IPs each
Phase III Configuration
- 60 IP addresses for dialup modems
- 40 IP addresses for internal addresses for web service and corp. network
- 10 small networks for dedicated customers 14 IPs each
- 15 small networks for connected networks 2 IPs each
By the time our fictional ISP reaches the end of phase two, they would ask for additional IP address space to meet the growth in dedicated customers. Up to this point in time, we have been able to operate our ISP with only one class C address, instead of the initial four. Many ISPs start deploying remote access devices at about this time and it would be easy to subnet a second Class C address for that expansion. The down side of this type of planning is that the ISP may have to renumber portions of its network from time to time, and reassign or reuse IP address space. Unfortunately the requirement to renumber networks is part of the Internet as it is today, and network planners need to take this into consideration when they address their network equipment.
Table Descriptions
Table 1 illustrates the networks that can be derived from one class C network using VLSM (Variable Length Subnet Masks). Note that the subnet masks are different for the different types of networks. In this example you can see three masks used, 255.255.255.192, 255.255.255.240 and 255.255.255.252. The 255.255.255.192 mask effectively cuts the class C into four(4) pieces. The first two pieces are used as 64 address networks. The second two are cut in different sizes using the 255.255.255.240 subnet mask to cut four customer networks out of the third quarter and the 255.255.255.252 mask to cut the last quarter into 16 pieces. See RFC1878 for more information.
Table 2 illustrates one way of describing the subnetting of the class C in a template that you would fill out for IP address space. Note that this IP address plan uses 28% of the available address space at first use and that reaches 64% within one year. This would meet with the ARINS requirement for IP address space penetration. One thing you should note; ARIN only allows delegation of 90 to 120 days on IP address space to an ISP. They use a slow start process and watch the subsequent allocations to determine the proper use of IP addresses used by ISPs.
Table 3 is a complete list of the IP address plan in our example.
Table 1 Networks for Our Class C
Network no. Description Subnet mask 206.26.97.0 Dial up modem network 255.255.255.192 206.26.97.64 Corporate Network Backbone 255.255.255.192 206.26.97.128 Dedicated Customer Net 255.255.255.240 206.26.97.144 Dedicated Customer Net 255.255.255.240 206.26.97.160 Dedicated Customer Net 255.255.255.240 206.26.97.175 Broadcast Address 255.255.255.240 206.26.97.192 Connected Network 1 255.255.255.252 206.26.97.196 Connected Network 2 255.255.255.252 206.26.97.200 Connected Network 3 255.255.255.252 206.26.97.204 Connected Network 4 255.255.255.252 206.26.97.208 Connected Network 5 255.255.255.252 206.26.97.212 Connected Network 6 255.255.255.252 206.26.97.216 Connected Network 7 255.255.255.252 206.26.97.220 Connected Network 8 255.255.255.252 206.26.97.224 Connected Network 9 255.255.255.252 206.26.97.228 Connected Network 10 255.255.255.252 206.26.97.232 Connected Network 11 255.255.255.252 206.26.97.236 Connected Network 12 255.255.255.252 206.26.97.240 Connected Network 13 255.255.255.252 206.26.97.244 Connected Network 14 255.255.255.252 206.26.97.248 Connected Network 15 255.255.255.252 206.26.97.252 Connected Network 16 255.255.255.252
Table 2 Networks as You Would List Them in Frontier's Template
Expansion Network Details:Number of hosts per subnet for Net/Subnet Subnet mask Max 0-3Mo Year+1 Year+2 Remarks ------------------------------------------------------------------------ 1.0 255.255.255.192 62 30 60 60 dialup modems 1.1 255.255.255.192 62 12 22 40 corp backbone 1.2 255.255.255.240 14 10 10 10 xyz company 1.3 255.255.255.240 14 5 10 14 abc company 1.4 255.255.255.240 14 0 8 12 growth cust 1.5 255.255.255.240 14 0 8 12 growth cust 1.6-21 255.255.255.252 32 4 16 30 connt'd nets Totals 212 61 134 178 IP utilization 100% 28% 64% 84%
Table 3 Complete Breakdown of Our Class C
Address Description of Host Subnet Mask 206.26.97.0 Dial up modem network 255.255.255.192 206.26.97.1 Dial up modems DHCP 255.255.255.192 206.26.97.2 Dial up modems DHCP 255.255.255.192 206.26.97.3 Dial up modems DHCP 255.255.255.192 206.26.97.4 Dial up modems DHCP 255.255.255.192 206.26.97.5 Dial up modems DHCP 255.255.255.192 206.26.97.6 Dial up modems DHCP 255.255.255.192 206.26.97.7 Dial up modems DHCP 255.255.255.192 206.26.97.8 Dial up modems DHCP 255.255.255.192 206.26.97.9 Dial up modems DHCP 255.255.255.192 206.26.97.10 Dial up modems DHCP 255.255.255.192 206.26.97.11 Dial up modems DHCP 255.255.255.192 206.26.97.12 Dial up modems DHCP 255.255.255.192 206.26.97.13 Dial up modems DHCP 255.255.255.192 206.26.97.14 Dial up modems DHCP 255.255.255.192 206.26.97.15 Dial up modems DHCP 255.255.255.192 206.26.97.16 Dial up modems DHCP 255.255.255.192 206.26.97.17 Dial up modems DHCP 255.255.255.192 206.26.97.18 Dial up modems DHCP 255.255.255.192 206.26.97.19 Dial up modems DHCP 255.255.255.192 206.26.97.20 Dial up modems DHCP 255.255.255.192 206.26.97.21 Dial up modems DHCP 255.255.255.192 206.26.97.22 Dial up modems DHCP 255.255.255.192 206.26.97.23 Dial up modems DHCP 255.255.255.192 206.26.97.24 Dial up modems DHCP 255.255.255.192 206.26.97.25 Dial up modems DHCP 255.255.255.192 206.26.97.26 Dial up modems DHCP 255.255.255.192 206.26.97.27 Dial up modems DHCP 255.255.255.192 206.26.97.28 Dial up modems DHCP 255.255.255.192 206.26.97.29 Dial up modems DHCP 255.255.255.192 206.26.97.30 Dial up modems DHCP 255.255.255.192 206.26.97.31 Dial up modems DHCP 255.255.255.192 206.26.97.32 Dial up modems DHCP 255.255.255.192 206.26.97.33 Dial up modems DHCP 255.255.255.192 206.26.97.34 Dial up modems DHCP 255.255.255.192 206.26.97.35 Dial up modems DHCP 255.255.255.192 206.26.97.36 Dial up modems DHCP 255.255.255.192 206.26.97.37 Dial up modems DHCP 255.255.255.192 206.26.97.38 Dial up modems DHCP 255.255.255.192 206.26.97.39 Dial up modems DHCP 255.255.255.192 206.26.97.40 Dial up modems DHCP 255.255.255.192 206.26.97.41 Dial up modems DHCP 255.255.255.192 206.26.97.42 Dial up modems DHCP 255.255.255.192 206.26.97.43 Dial up modems DHCP 255.255.255.192 206.26.97.44 Dial up modems DHCP 255.255.255.192 206.26.97.45 Dial up modems DHCP 255.255.255.192 206.26.97.46 Dial up modems DHCP 255.255.255.192 206.26.97.47 Dial up modems DHCP 255.255.255.192 206.26.97.48 Dial up modems DHCP 255.255.255.192 206.26.97.49 Dial up modems DHCP 255.255.255.192 206.26.97.50 Dial up modems DHCP 255.255.255.192 206.26.97.51 Dial up modems DHCP 255.255.255.192 206.26.97.52 Dial up modems DHCP 255.255.255.192 206.26.97.53 Dial up modems DHCP 255.255.255.192 206.26.97.54 Dial up modems DHCP 255.255.255.192 206.26.97.55 Dial up modems DHCP 255.255.255.192 206.26.97.56 Dial up modems DHCP 255.255.255.192 206.26.97.57 Dial up modems DHCP 255.255.255.192 206.26.97.58 Dial up modems DHCP 255.255.255.192 206.26.97.59 Dial up modems DHCP 255.255.255.192 206.26.97.60 Dial up modems DHCP 255.255.255.192 206.26.97.61 Dial up modems DHCP 255.255.255.192 206.26.97.62 Dial up modems DHCP 255.255.255.192 206.26.97.63 Broadcast Address 255.255.255.192 206.26.97.64 Corporate Network Backbone 255.255.255.192 206.26.97.65 Host address 255.255.255.192 206.26.97.66 Host address 255.255.255.192 206.26.97.67 Host address 255.255.255.192 206.26.97.68 Host address 255.255.255.192 206.26.97.69 Host address 255.255.255.192 206.26.97.70 Host address 255.255.255.192 206.26.97.71 Host address 255.255.255.192 206.26.97.72 Host address 255.255.255.192 206.26.97.73 Host address 255.255.255.192 206.26.97.74 Host address 255.255.255.192 206.26.97.75 Host address 255.255.255.192 206.26.97.76 Host address 255.255.255.192 206.26.97.77 Host address 255.255.255.192 206.26.97.78 Host address 255.255.255.192 206.26.97.79 Host address 255.255.255.192 206.26.97.80 Host address 255.255.255.192 206.26.97.81 Host address 255.255.255.192 206.26.97.82 Host address 255.255.255.192 206.26.97.83 Host address 255.255.255.192 206.26.97.84 Host address 255.255.255.192 206.26.97.85 Host address 255.255.255.192 206.26.97.86 Host address 255.255.255.192 206.26.97.87 Host address 255.255.255.192 206.26.97.88 Host address 255.255.255.192 206.26.97.89 Host address 255.255.255.192 206.26.97.90 Host address 255.255.255.192 206.26.97.91 Host address 255.255.255.192 206.26.97.92 Host address 255.255.255.192 206.26.97.93 Host address 255.255.255.192 206.26.97.94 Host address 255.255.255.192 206.26.97.95 Host address 255.255.255.192 206.26.97.96 Host address 255.255.255.192 206.26.97.97 Host address 255.255.255.192 206.26.97.98 Host address 255.255.255.192 206.26.97.99 Host address 255.255.255.192 206.26.97.100 Host address 255.255.255.192 206.26.97.101 Host address 255.255.255.192 206.26.97.102 Host address 255.255.255.192 206.26.97.103 Host address 255.255.255.192 206.26.97.104 Host address 255.255.255.192 206.26.97.105 Host address 255.255.255.192 206.26.97.106 Host address 255.255.255.192 206.26.97.107 Host address 255.255.255.192 206.26.97.108 Host address 255.255.255.192 206.26.97.109 Host address 255.255.255.192 206.26.97.110 Host address 255.255.255.192 206.26.97.111 Host address 255.255.255.192 206.26.97.112 Host address 255.255.255.192 206.26.97.113 Host address 255.255.255.192 206.26.97.114 Host address 255.255.255.192 206.26.97.115 Host address 255.255.255.192 206.26.97.116 Host address 255.255.255.192 206.26.97.117 Host address 255.255.255.192 206.26.97.118 Host address 255.255.255.192 206.26.97.119 Host address 255.255.255.192 206.26.97.120 Host address 255.255.255.192 206.26.97.121 Host address 255.255.255.192 206.26.97.122 Host address 255.255.255.192 206.26.97.123 Host address 255.255.255.192 206.26.97.124 Host address 255.255.255.192 206.26.97.125 Host address 255.255.255.192 206.26.97.126 Host address 255.255.255.192 206.26.97.127 Broadcast Address 255.255.255.192 206.26.97.128 Dedicated Customer Net 255.255.255.240 206.26.97.129 Customer Host 255.255.255.240 206.26.97.130 Customer Host 255.255.255.240 206.26.97.131 Customer Host 255.255.255.240 206.26.97.132 Customer Host 255.255.255.240 206.26.97.133 Customer Host 255.255.255.240 206.26.97.134 Customer Host 255.255.255.240 206.26.97.135 Customer Host 255.255.255.240 206.26.97.136 Customer Host 255.255.255.240 206.26.97.137 Customer Host 255.255.255.240 206.26.97.138 Customer Host 255.255.255.240 206.26.97.139 Customer Host 255.255.255.240 206.26.97.140 Customer Host 255.255.255.240 206.26.97.141 Customer Host 255.255.255.240 206.26.97.142 Customer Host 255.255.255.240 206.26.97.143 Broadcast Address 255.255.255.240 206.26.97.144 Dedicated Customer Net 255.255.255.240 206.26.97.145 Customer Host 255.255.255.240 206.26.97.146 Customer Host 255.255.255.240 206.26.97.147 Customer Host 255.255.255.240 206.26.97.148 Customer Host 255.255.255.240 206.26.97.149 Customer Host 255.255.255.240 206.26.97.150 Customer Host 255.255.255.240 206.26.97.151 Customer Host 255.255.255.240 206.26.97.152 Customer Host 255.255.255.240 206.26.97.153 Customer Host 255.255.255.240 206.26.97.154 Customer Host 255.255.255.240 206.26.97.155 Customer Host 255.255.255.240 206.26.97.156 Customer Host 255.255.255.240 206.26.97.157 Customer Host 255.255.255.240 206.26.97.158 Customer Host 255.255.255.240 206.26.97.159 Broadcast Address 255.255.255.240 206.26.97.160 Dedicated Customer Net 255.255.255.240 206.26.97.161 Customer Host 255.255.255.240 206.26.97.162 Customer Host 255.255.255.240 206.26.97.163 Customer Host 255.255.255.240 206.26.97.164 Customer Host 255.255.255.240 206.26.97.165 Customer Host 255.255.255.240 206.26.97.166 Customer Host 255.255.255.240 206.26.97.167 Customer Host 255.255.255.240 206.26.97.168 Customer Host 255.255.255.240 206.26.97.169 Customer Host 255.255.255.240 206.26.97.170 Customer Host 255.255.255.240 206.26.97.171 Customer Host 255.255.255.240 206.26.97.172 Customer Host 255.255.255.240 206.26.97.173 Customer Host 255.255.255.240 206.26.97.174 Customer Host 255.255.255.240 206.26.97.175 Broadcast Address 255.255.255.240 206.26.97.176 Dedicated Customer Net 255.255.255.240 206.26.97.177 Customer Host 255.255.255.240 206.26.97.178 Customer Host 255.255.255.240 206.26.97.179 Customer Host 255.255.255.240 206.26.97.180 Customer Host 255.255.255.240 206.26.97.181 Customer Host 255.255.255.240 206.26.97.182 Customer Host 255.255.255.240 206.26.97.183 Customer Host 255.255.255.240 206.26.97.184 Customer Host 255.255.255.240 206.26.97.185 Customer Host 255.255.255.240 206.26.97.186 Customer Host 255.255.255.240 206.26.97.187 Customer Host 255.255.255.240 206.26.97.188 Customer Host 255.255.255.240 206.26.97.189 Customer Host 255.255.255.240 206.26.97.190 Customer Host 255.255.255.240 206.26.97.191 Customer Host 255.255.255.240 206.26.97.192 Connected Network 1 255.255.255.252 206.26.97.193 Near end 255.255.255.252 206.26.97.194 Far end 255.255.255.252 206.26.97.195 Broadcast Address 255.255.255.252 206.26.97.196 Connected Network 2 255.255.255.252 206.26.97.197 Near end 255.255.255.252 206.26.97.198 Far end 255.255.255.252 206.26.97.199 Broadcast Address 255.255.255.252 206.26.97.200 Connected Network 3 255.255.255.252 206.26.97.201 Near end 255.255.255.252 206.26.97.202 Far end 255.255.255.252 206.26.97.203 Broadcast Address 255.255.255.252 206.26.97.204 Connected Network 4 255.255.255.252 206.26.97.205 Near end 255.255.255.252 206.26.97.206 Far end 255.255.255.252 206.26.97.207 Broadcast Address 255.255.255.252 206.26.97.208 Connected Network 5 255.255.255.252 206.26.97.209 Near end 255.255.255.252 206.26.97.210 Far end 255.255.255.252 206.26.97.211 Broadcast Address 255.255.255.252 206.26.97.212 Connected Network 6 255.255.255.252 206.26.97.213 Near end 255.255.255.252 206.26.97.214 Far end 255.255.255.252 206.26.97.215 Broadcast Address 255.255.255.252 206.26.97.216 Connected Network 7 255.255.255.252 206.26.97.217 Near end 255.255.255.252 206.26.97.218 Far end 255.255.255.252 206.26.97.219 Broadcast Address 255.255.255.252 206.26.97.220 Connected Network 8 255.255.255.252 206.26.97.221 Near end 255.255.255.252 206.26.97.222 Far end 255.255.255.252 206.26.97.223 Broadcast Address 255.255.255.252 206.26.97.224 Connected Network 9 255.255.255.252 206.26.97.225 Near end 255.255.255.252 206.26.97.226 Far end 255.255.255.252 206.26.97.227 Broadcast Address 255.255.255.252 206.26.97.228 Connected Network 10 255.255.255.252 206.26.97.229 Near end 255.255.255.252 206.26.97.230 Far end 255.255.255.252 206.26.97.231 Broadcast Address 255.255.255.252 206.26.97.232 Connected Network 11 255.255.255.252 206.26.97.233 Near end 255.255.255.252 206.26.97.234 Far end 255.255.255.252 206.26.97.235 Broadcast Address 255.255.255.252 206.26.97.236 Connected Network 12 255.255.255.252 206.26.97.237 Near end 255.255.255.252 206.26.97.238 Far end 255.255.255.252 206.26.97.239 Broadcast Address 255.255.255.252 206.26.97.240 Connected Network 13 255.255.255.252 206.26.97.241 Near end 255.255.255.252 206.26.97.242 Far end 255.255.255.252 206.26.97.243 Broadcast Address 255.255.255.252 206.26.97.244 Connected Network 14 255.255.255.252 206.26.97.245 Near end 255.255.255.252 206.26.97.246 Far end 255.255.255.252 206.26.97.247 Broadcast Address 255.255.255.252 206.26.97.248 Connected Network 15 255.255.255.252 206.26.97.249 Near end 255.255.255.252 206.26.97.250 Far end 255.255.255.252 206.26.97.251 Broadcast Address 255.255.255.252 206.26.97.252 Connected Network 16 255.255.255.252 206.26.97.253 Near end 255.255.255.252 206.26.97.254 Far end 255.255.255.252 206.26.97.255 Broadcast Address 255.255.255.252Written by Geoffrey Williams, Data Engineering Manager