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Subnetting Application

Learn how to assess a corporate network's subnetting requirements by calculating the number of needed hosts and networks, including planning for future growth.

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About this video

Before subnetting a corporate network, two requirements must be established: the total number of hosts needed and the total number of networks to be supported. The host calculation begins with the largest single segment on the network and then accounts for anticipated growth, typically projected over a three-year window. Adding expected device additions to the current peak figure produces a working host requirement that prevents the design from becoming obsolete shortly after deployment. Calculating network count requires more attention than it might initially seem. Beyond the access-layer segments that connect end-user devices, every point-to-point link between routers and distribution switches also constitutes a network and must be included in the total. In a moderately sized corporate topology, these inter-router links can nearly double the raw segment count, significantly affecting which subnet mask is appropriate for the design. Once the host and network requirements are established, standard subnetting methods can be applied to select an appropriate address block and mask combination. The only additional input needed at that stage is the address space assigned to the organization. This requirements-gathering process is a critical first step that ensures the resulting subnet design is both accurate and scalable.

What you'll learn

What's covered

Corporate Network Subnetting Requirements

Aligned to

CompTIA Network+
1.7 Given a scenario, use appropriate IPv4 network addressing.
Cisco CCNA
1.6 Configure and verify IPv4 addressing and subnetting.
1.7 Describe private IPv4 addressing.
Cisco CCST Networking
2.2 Identify IPv4 addresses and subnet formats.

Key terms

Subnet
A logical subdivision of an IP network, used to organize and segment network traffic.
Subnet Mask
A 32-bit number that divides an IP address into network and host portions.
IP Address
A numerical label assigned to each device connected to a network that uses the Internet Protocol.
Network Segmentation
The practice of dividing a network into smaller segments to improve performance and limit the spread of security threats.
Host
An end device or node on a network that is assigned an IP address and communicates with other devices.
Usable Host Range
The set of IP addresses within a subnet that can actually be assigned to network devices, spanning from one address above the network address to one address below the broadcast address. For example, in the 192.168.1.0/24 network, the usable host range is 192.168.1.1 through 192.168.1.254.
Scalability
The ability of a system to handle increased load by adding resources without degrading performance.

Transcript

Determining the Requirements

Here's our corporate network. If we were going to be doing subnetting with this corporate network, we'd first of all have to figure out how many hosts we need and how many networks. Hosts is the easy part, so let's start out with hosts.

How Many Hosts

To figure out the hosts, all I need to do — since we're not doing VLSM at this point — is just look at the largest network that we need, which is right here, 1351. That's how many usable hosts we need, that's at least how many we need right now.

We may want to factor in some growth, so let's say we're going to factor in maybe around 600 more end users or end devices going to be added to this network over the next three years. Generally you plan things at least three years out. So if I factor that in, that's going to be about 1900. We'll round it up, we'll say about 2,000. So let's say that network is going to need to be at 2,000, and that's our needs. Since we're just doing basic subnetting, that's going to be the needs across the board. So that's how many hosts.

How Many Networks

Let's take a look at how many networks we need. We count them up. We've got the end user device, or the access network: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. So there are ten of those right there.

But we also have the connections between all of these different networks. The connections between these are networks as well that we need to count. There are just probably going to be two usable hosts on each one, because you have an IP address on one side and an IP address on the other side, but we need to count those. So if I were to count those out: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, and then we have 16, 17, 18, 19, 20, 21. So 21.

In total we have a need for 31 networks right here. So we need to plan for 31 networks and 2,000 hosts.

At this point you know where to take it from there, because you've been practicing the IP subnetting, so now you'd be able to take this and use the other method that we've already talked about to figure out how to subnet this network. Of course, the one other piece of information you would need to know is what address you're given, in order to figure out what the network looks like. But essentially this is how you determine those requirements for the network.

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