4.3.1. Subnet and IP Address Management
💡 First Principle: A subnet's IP range and an instance's IP address are two different operational levers — one adjusts capacity for future growth within a region, the other secures a stable address for a specific resource — and confusing which lever solves which problem is a common exam distractor.
Resizing a subnet's IPv4 address range (expanding it to a larger CIDR block) accommodates growth in the number of resources a subnet needs to support, and can typically be done without disrupting existing resources already using addresses in that subnet. Reserving static external or internal IP addresses guarantees a specific resource keeps the same address permanently, rather than receiving a new ephemeral address on every restart — essential for anything referenced by a fixed DNS record, firewall rule, or external partner allowlist.
⚠️ Exam Trap: A scenario describing a service that must always be reachable at the same IP address, referenced in an external partner's allowlist, needs a static IP address — an ephemeral IP (the default for most resources) can change on restart, which would silently break that external allowlist without a static reservation in place.
Reflection Question: A subnet originally sized for 250 addresses is running out of room as a team scales up. What operation solves this, and does it require rebuilding existing resources in that subnet?
