Implemented in 1987; which states that it is possible to use more than one subnet due to the different sizes of the extended network prefixes. Having a flat network is a great disadvantage, because once the subnet mask is defined; it locks the network into a fixed number of subnets.

Other important aspects of VLSM are the further division of address space, by dividing an address into subnets, and then on to more subnets, which is known as aggregation.

Our base network is a /8 network, but then it is sub-netted into a /16 extended network prefix. Observe that in the second stage, the /16 network is sub-netted into two other subnets /24 networks and /19 extended network prefix. It doesn’t stop there. Note that the /24 network was sub-netted into a /27 extended network prefix.

This is the way underground networks are created. Each subnet can have its own router and can be interconnected within other networks through gateways, proxies, etc.

Observe the picture and note how VLSM can reduce the size of the routing table. First of all note that the entire sub-netted network is not visible to the outside world, and everything is going out trough router A. Depending on the size of the network, the bandwidth may be exhausted with all active connections, you could easily load balance this by treating any part of the network as a totally separated network and inserting another gateway.

IP PBX

An IP PBX phone system streamlines the way a business communicates internally as well as externally in two primary ways. First, an IP PBX is telephone switching equipment residing within the organization rather than the telephone company. Second, calls are transmitted via a data network instead of the traditional PSTN (public switched telephone network) and configuration is done using a PC browser. This means that all connections and extensions are controlled and administered by the organization without having to depend on — or pay for — PBX vendor service.

Transitioning from a legacy PBX to an IP PBX business phone system is a straightforward process that can be handled in stages or all at once. An organization may decide to convert completely from a traditional platform to an IP platform in one episode to maximize the economy of toll-free calling, may expand the network to connect to a branch office or home office to accommodate growth or move an endpoint or node from one location to another. With an IP PBX system, adding, moving, and changing users and sites is smooth and hassle-free. Because the new telephone system is now based on a data network, it is extremely flexible and scalable and is very economical to operate and maintain both in the short and long terms. It also enables the integration of call management applications with telephony, providing a revolutionary improvement in the analysis of calling patterns and deployment of telephony resources.

Vertical believes in an evolutionary rather than a revolutionary approach to IP telephony. Many organizations may not require a complete overhaul of their PBX systems, and can operate both traditional and new IP based systems side by side. Vertical products allow organizations to move over to IP PBX in stages and during the process to exploit the new merged voice-data network by installing applications that make operational and economic sense. Step-by-step migration ensures that needless costs are avoided and only productive application modules are integrated at each stage. Examples include: call routing and queuing; directed marketing messages; store service response; interactive voice response (IVR); connections to backend databases; and call management and other enterprise applications.