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Karlyn Hemmerling

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Aug 2, 2024, 9:25:40 PM8/2/24

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In a traditional subnetting scheme, a fixed subnet mask is applied to all subnets in the network, which can lead to inefficient use of IP addresses. For example, if a network has two subnets, one with 10 hosts and another with 50 hosts, a traditional subnet mask of 255.255.255.0 would be used for both subnets, which means that each subnet would have 254 available IP addresses. This would result in wasted IP addresses for the smaller subnet.

VLSM allows network administrators to create subnets with different subnet masks to more effectively utilize IP addresses. Using the example above, VLSM could be used to assign a subnet mask of 255.255.255.128 to the smaller subnet with 10 hosts, which would provide 126 available IP addresses, and a subnet mask of 255.255.255.192 to the larger subnet with 50 hosts, which would provide 62 available IP addresses.

The exponential growth of the Internet in the last 30 years exposed shortcomings in the original IP protocol design. As the internet began to rapidly expand from its initial military network research status into commercial prominence, the demand for IP addresses (particularly in the class B space) skyrocketed.

Experts started to worry about the long-term scaling properties of classes A, B, and C IP address scheme, and began considering ways to modify IP assignment policy and routing protocols to accommodate the growth.

This led to the establishment of the Routing and Addressing (ROAD) group by the Internet Engineering Task Force (IETF) in the early 1990s to work out ways of restructuring the IP address space to increase its lifespan.

Subnet masks are used by a computer to determine if any computer is on the same network or on a different network. An IPv4 subnet mask is a 32-bit sequence of ones (1) followed by a block of zeros (0). The ones designate the network prefix, while the trailing block of zeros designates the host identifier. In shorthand, we use /24, which simply means that a subnet mask has 24 ones, and the rest are zeros.

As the name implies, subnetting is the process of dividing a single large network into multiple small networks known as subnets. The primary purpose of subnetting is to help relieve network congestion and improve efficiency in the utilization of the relatively small network address space available especially in IPv4.

Supernetting is the direct opposite of subnetting, in which multiple networks are combined into a single large network known as supernets. Supernetting provides route updates in the most efficient way possible by advertising many routes in one advertisement instead of individually.

The main objective of supernetting is to simplify or summarize network routing decisions to minimize processing overhead when matching routes, and storage space of route information on routing tables. A routing table is a summary of all known networks. Routers share routing tables to find the new path and locate the best path for the destination. Without Supernetting, the router will share all routes from routing tables as they are. With Supernetting, it will summarize them before sharing, which significantly reduces the size of routing updates.

There are two approaches to subnetting an IP address for a network: Fixed length subnet mask (FLSM) and variable-length subnet mask (VLSM). In FLSM subnetting, all subnets are of equal size with an equal number of host identifiers. You use the same subnet mask for each subnet, and all the subnets have the same number of addresses in them. It tends to be the most wasteful because it uses more IP addresses than are necessary.

VLSM is a subnet design strategy that allows all subnet masks to have variable sizes. In VLSM subnetting, network administrators can divide an IP address space into subnets of different sizes, and allocate it according to the individual need on a network. This type of subnetting makes more efficient use of a given IP address range. VLSM is the defacto standard for how every network is designed today. Table 2.0 below is a summary of the differences between FLSM and VLSM Subnetting. VLSM is supported by the following protocols: Open Shortest Path First (OSPF), Enhanced Interior Gateway Router Protocol (EIGRP), Border Gateway Protocol (BGP), Routing Information Protocol (RIP) version 2 and 3, and Intermediate System-to-Intermediate System (IS-IS). You need to configure your router for VLSM with one of those protocols.

Besides, wasting of public network IP addressing space has both technical and economic implications. On the technical side, it accelerates its exhaustion; and on the economic side, it costs a lot of money because public network IP addresses are expensive. Therefore, the introduction of VLSM allowed the IP address allocation of a smaller block.

As you can see from the diagram, we have six networks LAN A, LAN B, LAN C, and link A, link B and a link C. Links A, B, and C are also three separate networks and each requires two host identifiers. Thus our task is to design an IP plan for each of the six networks according to their stipulated sizes using VLSM subnetting method. We need five steps to solve the problem:

The largest network LAN A requires 60 hosts. From the Host section (row) of our subnetting chart below, the closest to the required 60 hosts is 64, which corresponds to 4 subnets and a new CIDR value of /26 (the column is in bold). From this relevant information, we will build a new table containing Network ID, Subnet Mask in CIDR notation, Usable, and Name of Local Area Network affected. Keep in mind the first host identifier is reserved for the network ID and the last host ID is reserved for the broadcast ID, so the total number of usable host IDs for each subnet in this particular case is 62 (64-2).

The second-largest network, LAN B, requires 29 hosts. The minimum number of hosts which can satisfy LAN B with the 29 hosts on our subnetting chart is 32. This corresponds to eight subnets and a new CIDR value of /27 (the column is in bold).

Now select the first unassigned large subnet in Table 5.0 above and subdivide into two smaller subnets. This gives us 192.168.4.64 and 192.168.4.96 marked in green in Table 6.0 below. Again the pattern is simple: The first network ID is always the original one. The next network ID is obtained by adding 32 to the previous one. We can then assign 192.168.4.64 to LAN B, and mark the second one (192.168.4.96) as unassigned and reserved for future use. We have completed designing the IP plan for LAN A.

This step repeats the process above. The minimum number of hosts which can satisfy LAN C with the 14 hosts on our subnetting chart is 16. This corresponds to 16 subnets and a new CIDR value of /28 (the column is in bold).

Now select the first unassigned subnet in Table 6.0 above and subdivide into two smaller subnets. This gives us 192.168.4.96 and 192.168.4.112 in Table 7.0 below. Again the pattern is simple: The first network ID is always the original one. The next network ID is obtained by adding 16 to the previous one. We can then assign 192.168.4.96 to LAN C, and mark the second one (192.168.4.112) as unassigned and reserved for future use. We have completed designing the IP plan for LAN C.

The last step is to assign three smaller subnets for serial links A, B, and C. Each link requires two host IDs. Therefore, the minimum number of hosts which can each link with two hosts on our subnetting chart is four. This corresponds to 64 subnets and a new CIDR value of /30 in our subnetting chart (the column is in bold).

Now select the unassigned subnet in Table 7.0 above and subdivide into four smaller subnets to accommodate the subnets for the three serial links. This gives us four unique IPs as shown in Table 8.0 below.

VLSM is a crucial technique in modern network design. If you want to design and implement scalable and efficient networks, you should definitely master the art of VLSM subnetting. One of the key objectives of VLSM subnetting in IPv4 is to improve efficiency in the utilization of the space available. This has managed to keep it going in the last 30 years. But on the 25th of November 2019, RIPE Network Coordination Centre announced that it made the final /22 IPv4 address allocation, and has officially run out of IPv4 addresses. A longer-term solution to the eventual exhaustion of the 32-bit IPv4 network address space is the 64-bit IPv6 protocol.

VLSM (Variable Length Subnet Mask) is a technique that allows network administrators to divide an IP address space into subnets of different sizes, rather than dividing it into subnets of the same size. This allows for more efficient use of IP addresses, as smaller subnets can be used for smaller networks, and larger subnets can be used for larger networks.

For example, if a network administrator is given a Class C IP address space of 192.168.1.0/24, they can use VLSM to divide it into two subnets: one with a /25 mask for a smaller network, and one with a /26 mask for a larger network. The smaller network would have 128 host addresses, and the larger network would have 64 host addresses.

This allows the administrator to assign IP addresses more efficiently, as they can use the larger subnet for a network that needs more IP addresses and the smaller subnet for a network that needs fewer IP addresses.

The link between two routers is a small network by itself and so it can be included in the wider network as a subnet. Imagine that the network is a WAN, in which case, the link between two routers could be across the internet. Both routers would need an internet-facing IP address. Within the private network, this still holds true; it is a network with two hosts and becomes a subnet with two hosts.

Variable Length Subnet Mask (VLSM) is a subnet -- a segmented piece of a larger network -- design strategy where all subnet masks can have varying sizes. This process of "subnetting subnets" enables network engineers to use multiple masks for different subnets of a single class A, B or C network.