Your IP Address Is a Borrowed Number
If you have ever checked your IP address on this site, you may have noticed that the number changes from day to day. IP addresses are not permanently assigned - in most cases, they are temporarily leased numbers.
The IP address assigned to your home internet connection is temporarily lent from a pool of numbers managed by your ISP (Internet Service Provider). Understanding this mechanism reveals why your IP address changes and how it affects your privacy.
DHCP - The Automatic IP Distribution System
The protocol responsible for IP address assignment is DHCP (Dynamic Host Configuration Protocol). Every time you restart your router or switch Wi-Fi networks on your smartphone, DHCP is working behind the scenes. For a foundational explanation of what IP addresses are and how they work, see our guide on what an IP address is.
DHCP operates in four steps:
- Discover: Your device broadcasts "I need an IP address" to the entire network
- Offer: The DHCP server (usually your router) responds with "You can use this IP address"
- Request: Your device formally requests "I would like that IP address, please"
- Acknowledge: The server confirms "Granted - this IP address is yours for X hours"
The "X hours" is the lease period. Home routers typically set this to 24 hours, while ISPs may configure it from a few hours to several days. When the lease expires, you might receive the same IP address again, or you might get a different one.
Dynamic IP vs. Static IP
| Aspect | Dynamic IP | Static IP |
|---|---|---|
| Change frequency | Changes on router restart or lease expiry | Never changes unless contract is modified |
| Cost | Included in standard plan | Additional monthly fee |
| Primary use | Home users, mobile connections | Server hosting, VPN, remote access |
| Privacy | Harder to track long-term | Easier to track (always the same) |
| Security | Harder to target (IP keeps changing) | Easier to target for attacks |
Over 95% of home internet connections use dynamic IPs. Static IPs are only needed when hosting a public-facing server or when corporate VPN connections require a fixed source address.
Five Moments When Your IP Address Changes
- Router restart: The most reliable way to get a new IP. The connection to your ISP is reset, and a new address is often assigned upon reconnection
- DHCP lease expiration: When the lease period set by your ISP (hours to days) expires, automatic reassignment occurs
- Network switching: Switching from Wi-Fi to mobile data gives you a completely different IP. Connecting to a cafe's Wi-Fi does the same
- ISP maintenance: When your provider updates network equipment, IP address redistribution may occur
- VPN connection/disconnection: Connecting to a VPN shows the VPN server's IP; disconnecting restores your original IP
The Privacy Trade-off of Changing IPs
Regular IP changes offer a privacy advantage: websites and ad networks find it harder to track you by IP address alone over extended periods.
However, modern browser fingerprinting techniques do not rely on IP addresses. Even when your IP changes, the combination of your browser settings and hardware information can still identify you. Relying solely on IP changes for privacy is overly optimistic.
How to Check Your IP Address Changes
Visit IP確認さん's homepage to see your current IP address, location information, and security score at any time. By checking periodically, you can observe how frequently your IP address changes.
To learn more about how IP addresses and networking work, networking fundamentals books are a great resource.