Subnetting is a fundamental practice for accommodating network growth and improving network performance and security, but it can be time-consuming and frustrating. While subnet calculators are available, subnetting is far more than just numbers and remains a core skill for networking professionals.
While there are obvious organizational variables that impact your overall subnetting plan, there are universal best practices you should follow. In this article, I cover several best practices to ensure your network's IPv4 subnetting strategy is both efficient and scalable. I focus on Internet Protocol version 4 since it's the most widely used IP version in the enterprise today.
Before getting into the details of subnetting best practices, it's important to first consider the big picture. So our first step is understanding RFC 1918 addressing.
RFC 1918 and NAT
Very few organizations own enough internet-routable public IP addresses to use for all internal subnetting and routing purposes. Fortunately, the combination of RFC 1918 reserved IP subnets combined with Network Address Translation (NAT) alleviates most of these types of public address shortage problems. In fact, the combination works so well, it's the sole reason why most companies have put off moving toward IPv6.
For those who aren't familiar with RFC 1918 addressing, these are IPv4 blocks of addresses designated by the Internet Engineering Task Force (IETF) and intended for private use. The caveat, however, is that these IP addresses cannot be routed on the internet. That means devices configured with an RFC 1918 address must use NAT at the internet edge to translate their private IP address into one that's publicly routable. Private address spaces designated by RFC 1918 include:
- 10.0.0.0 - 10.255.255.255 -- or 10.0.0.0/8
- 172.16.0.0 - 172.31.255.255 -- or 172.16.0.0/12
- 192.168.0.0 - 192.168.255.255 -- or 192.168.0.0/16
Businesses commonly leverage a single public IP or a small public IP block for their internet connectivity needs. Companies always use fewer public addresses than private IP addresses because network administrators can allow hundreds or even thousands of private IP addresses to share a single public address using a NAT extension feature called Port Address Translation (PAT).
On the next pages, I discuss seven subnetting best practices for IPv4 networks.
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