Just for reference, here’s a list of popular prefixes for LANs based on the bits in their subnet. This can basically server as a guide to which subnet you’d like to configure your network with when determining how many IP’s you’d like available internally. Beyond this chart, LANs can become unmanageable and even subnets with higher available IPs are less likely to be needed.
A list of prefix sizes, network masks, and available hosts\IPs:
| Prefix | Netmask | Available Hosts\IPs |
| Slash 16 | 255.255.0.0 | 65536 |
| Slash 17 | 255.255.128.0 | 32768 |
| Slash 18 | 255.255.192.0 | 16384 |
| Slash 19 | 255.255.224.0 | 8192 |
| Slash 20 | 255.255.240.0 | 4096 |
| Slash 21 | 255.255.248.0 | 2048 |
| Slash 22 | 255.255.252.0 | 1024 |
| Slash 23 | 255.255.254.0 | 512 |
| Slash 24 | 255.255.255.0 | 256 |
| Slash 25 | 255.255.255.128 | 128 |
| Slash 26 | 255.255.255.192 | 64 |
| Slash 27 | 255.255.255.224 | 32 |
| Slash 28 | 255.255.255.240 | 16 |
| Slash 29 | 255.255.255.248 | 8 |
| Slash 30 | 255.255.255.252 | 4 |
| Slash 31 | 255.255.255.254 | 2 |
| Slash 32 | 255.255.255.255 | 1 |
A Quick Example:
Let’s say you have an office space with 200 users and 10 servers and you’re trying to determine which subnet to use. The gut instinct would be to choose the /24 as it offers more IP’s than needed with room for growth. But realistically, there are many more devices and tools that need IP addresses than one may originally think. Your DHCP Pool alone will need to be more than 200 and wireless devices or visitors to the office must be considered. At that point, you’ll probably want to go with a netmask that gives you more IP Addresses.
Best Practices:
The golden rule when working with IP Addresses, be it load balancing a DHCP Server or determining capacity, is 80\20. Find a netmask that’s going to offer IPs that are 80% of what you’ll have available, leaving 20% for growth or unexpected needs. Resubnetting after everything is in place can be a nightmare, so plan carefully.
Cheers!
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