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| 2.3 |  | VLSM |
| 2.3.1 |  | Variable-Length Subnet Masks |

Variable-Length Subnet Mask (VLSM) allows an organization to use more than one subnet mask within the same network address space. Implementing VLSM is often referred to as subnetting a subnet and it can be used to maximize addressing efficiency.

Consider the subnets created by borrowing three bits from the host portion of the Class C address, 207.21.24.0.

If the

ip subnet-zero command is used, this mask creates seven usable subnets of 30 hosts each. Four of these subnets can be used to address remote offices at sites A, B, C, and D.

Unfortunately, there are only three subnets left for future growth and three point-to-point WAN links between the four sites remain to be addressed. If the three remaining subnets were assigned to the WAN links, the supply of IP addresses would be completely exhausted. This addressing scheme would also waste more than a third of the available address space.

There are ways to avoid this kind of waste. Over the past 20 years, network engineers have developed three critical strategies for efficiently addressing point-to-point WAN links:

- Use VLSM
- Use private addressing (RFC 1918)
- Use IP unnumbered

Private addresses and IP unnumbered are discussed in detail later in this module. This section focuses on VLSM. When VLSM is applied to an addressing problem, it breaks the address up into groups or subnets of various sizes. Large subnets are created for addressing LANs and very small subnets are created for WAN links and other special cases.

A 30-bit mask is used to create subnets with two valid host addresses. This is the exact number needed for a point-to-point connection. Figure illustrates what happens if one of the three remaining subnets is subnetted again, using a 30-bit mask.

Subnetting the 207.21.24.192/27 subnet in this way supplies another eight ranges of addresses to be used for point-to-point networks. For example, the network 207.21.24.192/30 can be used to address the point-to-point serial link between the Site A router and the Site B router.

Figure shows the commands needed to configure the Site A router, labled RTA, with a 27-bit mask on its Ethernet port and a 30-bit mask on its serial port.

| Interactive Media Activity Drag and Drop: VLSM Calculation Upon completion of this activity, the student will have a better understanding of VLSM.