VLSM Subnet Allocation

So far in this chapter, most of the discussion has been about examining existing addresses and subnets. Before deploying new networks, or new parts of a network, you must give some thought to the ranges of IP addresses to be allocated. Also, when assigning subnets for different locations, you should assign the subnets with thought for how routes could then be summarized. This section covers some of the key concepts related to subnet allocation and summarization. (This section focuses on the concepts behind summarization; the configuration of route summarization is routing protocol-specific and thus is covered in the individual chapters covering routing protocols.)

Many organizations purposefully use SLSM to simplify operations. Additionally, many internetworks also use private IP network, with an SLSM prefix length of /24, and use NAT for connecting to the Internet. By using SLSM, particularly with a nice, easy prefix like /24, operations and troubleshooting can be a lot easier.

In some cases, VLSM is required or preferred when allocating addresses. VLSM is typically chosen when the address space is constrained to some degree. The VLSM subnet assignment strategy covered here complies with the strategy you may remember from the Cisco BSCI course or from reading the Cisco Press CCNP Routing certification books.

Similar to when assigning subnets with SLSM, you should use an easily summarized block of addresses for a new part of the network. Because VLSM network addresses are likely constrained to some degree, you should choose the specific subnets wisely. The general rules for choosing wisely are as follows:

KEY Step 1 Determine the shortest prefix length (in other words, the largest block) P°'NT required.

Step 2 Divide the available address block into equal-sized prefixes based on the shortest prefix from Step 1.

Step 3 Allocate the largest required subnets/prefixes from the beginning of the IP address block, leaving some equal-sized unallocated address blocks at the end of the original large address block.

Step 4 Choose an unallocated block that you will further subdivide by repeating the first three steps, using the shortest required prefix length (largest address block) for the remaining subnets.

Step 5 When allocating very small address blocks for use on links between routers, consider using subnets at the end of the address range. This leaves the largest consecutive blocks available in case future requirements change.

For instance, imagine that a network engineer plans a new site installation. He allocates the address block for the new site, expecting to use the block as a single summarized route. When planning, the engineer then subdivides per the subnet requirements for the new installation, as shown in Figure 4-3. The figure shows three iterations through the VLSM subnet assignment process, because the requirements call for three different subnet sizes. Each iteration divides a remaining block into equal sizes, based on the prefix requirements of the subnets allocated at that step. Note that the small /30 prefixes were allocated from the end of the address range, leaving the largest possible consecutive address range for future growth.

Figure 4-3 Example of VLSM Subnet Allocation Process

KEY POINT ( Through Requirements: 3 /25's

Pass 1: /25 prefixes



Allocated -


Unallocated -

Pass 2: /27 prefixes


Step 3: /30 prefixes Allocate High End:,,

Allocated Allocated Unallocated Unallocated



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  • Berylla
    How to do vlsm shortest prefix?
    3 years ago

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