Variable Length Subnet Mask (VLSM)

Previously, the Solaris operating environment supported the use of a single netmask for each network number in use on a network. The netmask determines which bits of an IP network number represent the subnet, and which represent the host number. Using variable-length subnet masks (VLSM) enables more efficient use of IP addresses in large networks. With VLSM, the TCP/IP administrator can now use multiple netmasks for a given network number, tailoring each mask according to the size of each individual subnetwork. When deciding how to route an IP packet, Solaris automatically uses the longest matching netmask.

For example, in prior releases an administrator could subdivide the network 192.168.1.0 into eight subnets by using a netmask of 255.255.255.224. By using VLSM, this network number can instead be broken into

• Two subnets with a netmask of 255.255.255.192, and

• Four subnets with a netmask of 255.255.255.224

In addition, the use of VLSM allows supernetting or aggregation of network numbers, which can result in substantial savings in the sizes of routing tables carried by network routers. These two techniques make it possible to implement Classless Inter-Domain Routing (CIDR) as described in RFC1519.

To fully utilize VLSM in a network, CIDR-aware protocols such as OSPF, BGP-4, or RIPv2 must be used.

For more information, see TCP/IP and Data Communications Administration Guide.