Subnetting – Is It Worth The Headache?

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Are there any benefits to subnetting your private network?

I cannot begin to tell you how many times a week I am asked by clients or students and sometimes even working professionals – what is a subnet, should I use them and what are the benefits, if any, of subnetting a network?

For this discussion we will be speaking of IPv4 – IPv6 can also be subnetted but for simplicity we will just be discussing IPv4.

The short and sweet – a sub network or subnet is a specific logical subdivision of an IP network. This means that a network is divided into two or more separate networks while still sharing a common, identical significant bit-grouping in their IP address. This creates a logical division of an IP address into two areas – a network number or routing prefix and the rest field or host identifier. The rest field is the identifier for a specific host or network interface.

It is not uncommon to see these different routing prefix’s displayed as a classless Inter-Domain Routing notation ( CIDR ) which would be the first address of a network follow by a forward slash and ending with the bit-length of the prefix. An example of this would be – this IPv4 network, starting at the first provided address and containing 24 bits allocated to the network prefix and the remaining 8 bits for the host address.

Within an IPv4 network, it may also be described via it’s subnet mask or netmask, which is the specific bitmask that yield the routing prefix. Subnet masks are also expressed in a dot-decimal notation similar to an IP address. Example – is the subnet ask for the prefix.

What does this all mean? Study the principals of subnetting to dig real deep but basically, the slash notation of the network defines how many separate IP address are available. The more one subnetts, or chops up, the available range the fewer IP address that are available per range.

Enough of that – what, if any, benefits are there to subnetting a network?


Some Potential Benefits of Subnetting on Private Networks

1.  Creating Logical Separations

Subnetting is a practice that helps maintain clean and clear separations of a network. One can define different boundaries between departments within the organization – a specific subnet for sales, a specific subnet for marketing and even a third, separate subnet for engineering.

One could also break up the network by floor designations or develop subnets for different devices such as VoIP phones, workstations, database servers or IoT devices. These logically divided networks would behave as if they were physically separate networks without the need to re-cable the entire office to make a change.

The reason this is so beneficial is that the overhead to achieve this ‘distinction’ of working parts is no longer specific to the physical separation of the networks but rather the configuration of the networks and the access rights associated. This means that if today marketing department doesn’t need to see the sales’s data but tomorrow they do, a few configuration changes can be made to achieve the change rather than purchasing hardware, running cables and touching each workstation.  


2. Improve Network Security

Once the logical divisions have been created, these subnets provide greater control over the entire network including who has access to what resources. With subnetting one could easily restrict an entire department from accessing a file server that contained payroll information or preventing the sales team from viewing classified records in HR.

The overall security of the network is also improved – worst case scenario there is a virus propagating throughout the network – by having subnets there is a clear separation between the different networks that can help contain the outbreak. Subnets also assist in limiting the different servers, direct information or even restrict access to different pieces of hardware.


3. Network Performance Enhancement

Lets view your network as a major metropolitan motorway – there are times when a large number of vehicles going across the road at the same time creates a slowdown by saturating the roadway with other vehicles. Imagine each vehicle as a packet – and in this scenario – it is possible that some cars ( or packets ) may not even make it to their destination or be dramatically delayed due to the congestion ( or collisions ) that may occur. This is especially an issue when – lets say when rush hour happens – that the probability of collision or delay is even further increased.

To help thwart this issue, subnetting reduces the amount of traffic by containing network segments to their own subnet, limiting the the broadcast from the entire network to the specific subnets.


4.  Adjust the Number Of Available IP’s

Aside from the logical separation of devices on various ranges, if you have a situation where you have maxed out the number of available IP addresses, you can assign a subnet to increase the number of available IPS – this is commonly referred to as supernetting. For example, if you need to extend a Class c IP range to accommodate more than 254 devices, you can double this number by changing the subnet mask.

In the reverse, if you have more IP addresses than devices you will ever need, you can half the size of the subnet by adjusting the subnet mask accordingly.

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