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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               255.255.255.252
Written by Geoffrey Williams, Data Engineering Manager
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