Dynamic IP

A dynamic IP address is a public address your ISP assigns to your connection temporarily and can change over time — when your router reconnects, after a DHCP lease expires, or whenever the ISP reallocates its address pool. This is the default for most home and mobile connections, since it lets ISPs reuse a limited pool of addresses across customers who aren’t all online at once. Microsoft’s DHCP Server overview describes the same mechanism on a local network: a DHCP server maintains a pool of addresses and leases them to clients, and because those addresses are leased rather than permanently assigned, ones no longer in use automatically return to the pool for reallocation. The alternative is a static IP, which stays fixed and is usually a paid add-on. A changing address matters for anything that depends on reaching the same address reliably over time — remote access, self-hosting, dynamic DNS, whether kept current through a third-party DDNS provider’s API or the DNS protocol’s own DNS UPDATE mechanism sent straight to an authoritative server — and for long-term monitoring, since a single point-in-time check can miss an address that has since changed. The lease mechanism behind a changing dynamic IP is defined in RFC 2131 (DHCP) and its companion RFC 2132 (DHCP options), whose current option codes are tracked in IANA’s BOOTP/DHCP Parameters registry. Checking My IP periodically is the simplest way to notice when a dynamic address has changed.

Many dynamic IPs today sit behind CGNAT as well, adding a second layer of address sharing on top of the ISP’s own reassignment.

Dynamic IP is part of AdminKit’s IP & network cluster; the full AdminKit glossary lists other related terms.