Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 75
Variable-Length Subnet Masks

We know about the restriction of RIPv1. RIPv2 and OSPF do not have this restriction and can more efficiently use the address. The preceding examples show how to split up a network for subnets assuming one mask per network ID (discussed extensively previously in the book). A concept known as Variable-Length Subnet Mask (VLSM, detailed under the RIP and OSPF sections of this book), allows us to assign variable masks per network ID. We can move the mask around the single network ID. These protocols transmit the subnet mask along with the network ID in the routing update message.

VLSM can be very, very confusing. One rule you should follow: Do not make it overly complicated. As a general rule, do not VLSM more than three times. Yes, efficiency is important, but you must sit down with your team or customer and determine the network topology. For example, if you use the address 150.1.0.0 with a /16 prefix (255.255.0.0), a very effective method of using VLSM is /24 (for subnets with lots of networks), /27 (for subnets with fewer hosts or maybe higher-powered network-hogging apps), and /30 (mask for the serial lines). This is shown next. You can go wild and try to develop a mask for every subnet, but having a few leftover bits is fine. Also, using this method is not efficient as you will be spreading different subnets through the network in a noncontiguous fashion, which can become burdensome on the route tables. However, it does explain the variable-length subnet feature.

First, your base address is 150.1.0.0/16. This goes at the top of the chart. From here we will create 256 subnets using the /24 subnet mask. No hosts have been assigned yet. We currently have 50 serial (point-to-point) lines to work with and predict a growth of 100 more remote sites over the next two years. Therefore, we need 150 subnets for the serial lines and there are only two host addresses needed per serial line. We have reserved the 150.1.56.0, 150.1.57.0, and the 150.1.58.0 subnets for serial lines. The 150.1.56.0 network is further subdivided (sub-subnetted) using the first 6 bits of the fourth octet (255.255.255.252 or /30), yielding 64 subnets for serial lines. With each subnet (56, 57, and 58) supporting 64 subnets we now have 192 subnets allotted for serial lines. We leave 2 bits, which allows for two host addresses to be assigned (all 0s and all 1s are not allowed as host addresses). Seventy-five of the subnets will be assigned a another mask (/27) to allow for sub-subnets (subnets of subnets) with a smaller number of hosts per subnet.

Variable-Length Subnet Masks