Static vs dynamic IP: the difference and when you need one

Originally published on proxymint.

Static vs dynamic IP explained: how DHCP leases and ISP renumbering change an address, how to check which you have, and when a fixed IP is worth it.

TL;DR

• A static IP stays put; a dynamic IP is leased and can change. DHCP hands out dynamic addresses for a set period, and the ISP decides how long that is and when to renumber.
• Dynamic addresses change on three clocks. The lease, fixed renumbering schedules (often every 24 hours at ISPs that do it) and outages long enough for the lease to lapse.
• Check two layers. "DHCP Enabled" describes the LAN address only; the public address needs a week of logging and a look at the router WAN address for CGNAT.
• Direction decides the fix. Inbound services need a static address on their own line; outbound traffic only needs a fixed exit, such as a VPN gateway, a cloud server or a static ISP proxy.

Key takeaways

• Keep the dynamic address if nothing has to find or recognise you. Browsing, streaming and gaming gain nothing from a fixed IP.
• Rule out CGNAT before paying for anything. A WAN address between 100.64.0.0 and 100.127.255.255 means the line shares a public address and cannot accept inbound connections.
• Log the public IP every 15 minutes for a week. The pattern shows whether the address holds, moves daily by policy or changes after outages.
• Use a fixed exit when only outgoing requests must match. A static ISP proxy keeps one ISP-registered address on the order for the whole billing term.

Read the full guide on proxymint, with the commands, tables and sources.