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Subnetting: A Netmask Reference Guide

March 7, 2013 by Dan B. Lee Leave a Comment

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!

Filed Under: Networks Tagged With: DHCP, IP, LAN, Netmask, Networking, Subnet

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Dan Lee

Dan B. Lee works at SyApps, LLC., a Managed Hosting Solutions Firm, as a Senior Network Engineer. Dan has a decade of IT experience and specializes in a number of different disciplines including Virtualization, Web Site Hosting and Design, Network Security, Data Center Architecture, Local and Remote Server Hosting, and Backup & DRS Solutions. Read More…

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