A comprehensive understanding of how networks function is one of the trickiest aspects faced by businesses trying to configure their IT infrastructure. Nevertheless, it is imperative to maintain server efficiency and develop watertight security protocols. 

Some of the most common queries we see relate to setting up a logical network environment. How many public IP addresses are required to meet the needs of our business’ IT operations? What is a /29 subnet mask? How many IP addresses are in a /24? All these questions need answering before you dive into arranging your computer network system. So, we put together this guide to shed some light on subnet mask values and what they mean in real terms for you and your business. 

What is an IP Address?

Before we can begin arranging your network, first we must understand what an IP address is and why it’s important. 

When you’re navigating the internet, your device is identified by what’s known as an ‘IP address.’ IP stands for Internet Protocol; the framework that allows information to be communicated between devices on a given network and is critical to the stability of the web’s data-sharing processes. In simple terms: in order for two computers to interact, first, they have to find one another. Think of it as an autonomous switchboard operator directing your call to the appropriate person you wish to speak with. 

For that reason, each IP address within a network must be distinct from one another. A standard IPv4 address appears as a unique code of four numbers (punctuated by full stops) with each ranging from 0 all the way up to 255. Generally, the first three numbers of the IP address will remain the same as a way to define the network, while the fourth number identifies the host. The process of splitting IP addresses into a group like this is referred to as ‘subnetting.’ 

Given that there are more people online than ever before, and with demand exceeding supply, the emerging IPv6 offers a different structure. Whereas the pool of IPv4 addresses are 32-bits in size (as each block is 8-bit, known as an ‘octet’), IPv6 addresses are a whopping 128-bits. However, IPv4 remains the most commonly used Internet Protocol and is more than likely to be the one you encounter. 

CIDR Subnets and IP Blocks

In 1993, Classless Inter-Domain Routing (CIDR) was launched as a more sophisticated form of subnetting. It works by simply adding a qualifier to the IP address itself. For example, in the notation 123.4.10.1/28, the ‘28’ indicates the CIDR value. That doesn’t mean that there are exactly 28 usable IPs within the block, but it is a code that correlates to the number of IP addresses available. 

All CIDR IPs can range anywhere between 0 and 32. The higher the number, the fewer IP addresses that are available. The CIDR notation doubles in value with each jump. So, /32 gives you one IP address. /31 gives you two. /30 gives you four. And so on, until you reach /0 and a massive 4,294,967,296 total IP addresses. 

Remember, total IP addresses are different from usable IP addresses, as ‘0’ and ‘255’ are reserved for the network address and broadcast address respectively. That means you’ll always have two fewer usable IP addresses than total IPs, which is something to keep in mind when you make a subnet request to your Internet Service Provider (ISP). 

As mentioned earlier, due to a shortage of IPv4 IP addresses, you will face an uphill battle trying to get anything more than a /29 CIDR network subnet mask. Higher than that will almost definitely require the submission of a RIPE justification, in which you’ll have to provide adequate evidence of the necessity to Europe’s IP address administrator. 

Here’s an IP address table for subnets to make things simple:

CIDR  Subnet Mask  Total IPs  Usable IPs 
/32  255.255.255.255 
/31  255.255.255.254 
/30  255.255.255.252 
/29  255.255.255.248 
/28  255.255.255.240  16  14 
/27  255.255.255.224  32  30 
/26  255.255.255.192  64  62 
/25  255.255.255.128  128  126 
/24  255.255.255.0  256  254 
/23  255.255.254.0  512  510 
/22  255.255.252.0  1024  1022 
/21  255.255.248.0  2048  2046 
/20  255.255.240.0  4096  4094 
/19  255.255.224.0  8192  8190 
/18  255.255.192.0  16,384  16,382 
/17  255.255.128.0  32,768  32,766 
/16  255.255.0.0  65,536  65,534 
/15  255.254.0.0  131,072  131,070 
/14  255.252.0.0  262,144  262,142 
/13  255.248.0.0  524,288  524,286 
/12  255.240.0.0  1,048,576  1,048,574 
/11  255.224.0.0  2,097,152  2,097,150 
/10  255.192.0.0  4,194,304  4,194,302 
/9  255.128.0.0  8,388,608  8,388,606 
/8  255.0.0.0  16,777,216  16,777,214 
/7  254.0.0.0  33,554,432  33,554,430 
/6  252.0.0.0  67,108,864  67,108,862 
/5  248.0.0.0  134,217,728  134,217,726 
/4  240.0.0.0  268,435,456  268,435,454 
/3  224.0.0.0  536,870,912  536,870,910 
/2  192.0.0.0  1,073,741,824  1,073,741,822 
/1  128.0.0.0  2,147,483,648  2,147,483,646 
/0  0.0.0.0  4,294,967,296  4,294,967,294 

At Custard, we can help you identify which IP address type you need for your business, assist in allocating and configuring subnet masks, and provide guidance and clarity on IP address requests (including RIPE justification) and any other IT consultancy challenges your business may face in a language you can understand. 

Get in touch with us today.