CIDR Block
The slash notation that defines IP address ranges — and the one networking decision that becomes permanent the moment two networks need to talk.
What is a CIDR block?
CIDR notation (10.0.0.0/16) describes an IP range as a base address plus a prefix length: the /16 means the first 16 bits are fixed and the rest vary, yielding 65,536 addresses. Smaller suffix, bigger network — /8 is ~16.7 million addresses, /24 is 256, /32 is exactly one host. Every VPC, subnet, firewall rule, and route you will ever write speaks this notation.
The math worth memorizing
Each step in prefix length halves the range: /16 → two /17s → four /18s. Cloud subnetting is just this recursion — carve a /16 VPC into /20 subnets per AZ and tier. AWS quietly reserves 5 addresses per subnet (network, broadcast, router, DNS, future), so a “/28 with 16 IPs” gives you 11 — the classic capacity surprise in small subnets hosting autoscaling groups or EKS pods, which consume IPs far faster than instance counts suggest.
The permanence problem
CIDR choices are effectively forever: once workloads hold addresses, renumbering is a migration project. And the failure mode is collision — two networks both chose 10.0.0.0/16, and now peering, VPNs, or a merger demand they interconnect. Routing cannot distinguish overlapping ranges; the fixes (NAT gymnastics, renumbering) are all painful. The professional habit: keep an IP allocation plan (or IPAM tooling) from day one, assign non-overlapping supernets per environment/region, and always leave unallocated space.
What people get wrong
- Sizing subnets by server count — containers, load balancer ENIs, and Lambda interfaces consume IPs; EKS clusters exhaust /24s routinely.
0.0.0.0/0reflexes: it means “everywhere” in both routes and security rules; every accidental use is an exposure.- Forgetting IPv6 mostly ends the scarcity game — cloud VPCs hand out /56s; dual-stack designs sidestep future overlap entirely.
Primary source: RFC 4632 — Classless Inter-Domain Routing
Historical figures and technical concepts for informational purposes only. Not technical, professional, legal, or financial advice. Sources: Official Documentation.