Static vs dynamic IP addresses

The difference#

A static IP address is assigned to a connection or device and stays the same until someone changes it deliberately. A dynamic IP address is leased for a period and can change when the lease ends, when the router restarts, or when the ISP rebalances its pool. Both are ordinary addresses. The only difference is how long you keep one.

How a dynamic address is assigned#

A dynamic address is handed out by DHCP, the Dynamic Host Configuration Protocol (RFC 2131). Your router asks the ISP's server for an address, receives one with a lease time, and renews it partway through. If the router goes away for longer than the lease, the address may be given to someone else. On IPv6 the same idea exists as DHCPv6 (RFC 8415), often together with delegated prefixes.

In practice, how often the public address changes is a policy of each ISP. Some renew the same address for months so that it behaves almost statically. Others change it at every reconnect. There is no way to know from outside, only to watch.

Two layers, two meanings

"Static" is used at two levels, and mixing them up causes trouble. Your public address is static or dynamic according to your ISP. The private address a device has inside your home is static or dynamic according to your router: a DHCP reservation, which is the sensible way to pin a printer or a server to one address, keeps it the same every time while still being handed out by the router. Read public vs private IP addresses if the distinction is new.

When a static address is worth having#

• Hosting from home or a small office. A web, game or mail server needs an address people can find. A dynamic DNS name, which a small program on your router or computer keeps pointed at your changing address, is the cheap alternative.
• Allow-listing. A bank, a database or a vendor's API that only accepts your office needs a fixed address to recognise.
• Sending email. Receiving servers judge a sender by its address and its reverse DNS. Mail from a residential dynamic range is often rejected outright.
• Remote access to your own network. A VPN endpoint or a remote desktop needs somewhere stable to connect to.

None of that works if you are behind carrier-grade NAT, where the ISP shares one public address across many customers and you have no inbound path at all. In that case a static address is not for sale, and the answer is a tunnel or a cloud relay: see what NAT and CGNAT are.

How to tell which one you have#

1. Note your public address from the home page today.
2. Restart the router, wait for it to reconnect and check again. A different address means dynamic. The same address means static or a very sticky lease, so it proves little either way.
3. Look at the reverse DNS hostname in a lookup of your address. ISPs often put words such as dynamic, dhcp, pool or dsl in residential names, and static or a customer name in fixed ones. It is a hint, not a guarantee.
4. Ask your ISP, or check your bill or account page. They are the only authority.

What it means for privacy#

A dynamic address is a weak identifier: it is shared with the previous and next customer of the same pool over time, and it changes. A static address is a strong one, because it stays put and can be linked across sites and across years. For most people neither is the main tracking channel. Cookies and browser fingerprints do more of the work. But if your only concern is being recognised by address, a dynamic one helps a little, and a VPN helps more.

Common questions#