The Apartment & The Street: Understanding Public vs. Private IP Addresses

The Difference Between Public and Private IP

When you search for “what is my IP” on Google or sites like whatismyipaddress.com and ip.me, you may find the IP address (e.g., 210.252.239.212) different from what you find from your terminal (e.g., 10.20.19.110). You are seeing the distinction between your Public IP Address and your Private (Local) IP Address.

Think of your network like an apartment building:

• The Public IP (210.252.239.212) is the street address of the entire building. This is what the mailman (the Internet) sees.

• The Private IP (10.20.19.110) is your specific apartment number. It only makes sense inside the building.

The Public IP Address

The address 210.252.239.212 belongs to your Router (or your ISP’s gateway).

• Who gives it to you? — Your Internet Service Provider (ISP).
• What is it for? — It identifies your entire home or office network to the rest of the global internet.
• Visibility: Every website you visit sees this address. If you have five people in your house all visiting Google at the same time, Google sees all of them coming from this same single address.

The Private IP Address

The address 10.20.19.110 belongs specifically to your Device (laptop, phone, etc.).

• Who gives it to you? — Your Router.
• What is it for? — It allows your router to distinguish between your laptop and your roommate’s phone. When the router receives data from the internet, it uses this address to know which “apartment” to send the data to.
• Visibility: This address is invisible to the outside world. It is part of a reserved range (the 10.x.x.x block) that is specifically designated for private use and cannot be “routed” on the public internet.

The “Translator” (NAT)

You might wonder: If the internet only sees the Public IP, how does the data get back to my specific laptop?

This is handled by a process called NAT (Network Address Translation).

1. Request: Your laptop (10.20.19.110) sends a request to a website.
2. Translation: The router “strips off” your private IP, replaces it with the public IP (210.252.239.212), and makes a note in its internal table: “Traffic on Port X belongs to the laptop.”
3. Response: The website sends the data back to the Public IP.
4. Delivery: The router looks at its table, sees Port X, and forwards the data to your laptop’s Private IP.

Why do we do this?

• Security: Because your private IP is hidden, hackers on the internet cannot “see” your computer directly; they only see your router, which acts as a shield.

• IP Scarcity: There are only about 4 billion IPv4 addresses in existence. If every single smartphone, smart bulb, and laptop in the world needed a unique public address, we would have run out of addresses decades ago. NAT allows millions of devices to share a single public address.

Does Public IP Address change over time? When and Why?

For most residential users, your Public IP address is dynamic, meaning it will eventually change. It isn’t tied permanently to your house like a physical street address; it’s more like a temporary “lease” on a parking spot.

When does it change?

There isn’t a fixed schedule for everyone, but a change is typically triggered by:

• Router Reboots/Power Outages: If your router is turned off or loses power for a while, the “lease” on your current IP might expire. When you turn it back on, the ISP might give that old IP to someone else and hand you a new one.

• Lease Expiration: Even if your router stays on, your ISP sets a “lease time” (often 24 hours to a week). When the time is up, your router asks to renew it. Usually, it keeps the same one, but the ISP can force a change at this point.

• ISP Maintenance: If your provider performs network upgrades or reconfigures their hardware in your area, they may “refresh” all connections, resulting in new IPs for everyone in the neighborhood.

Why do ISPs change it?

It seems like a lot of work to keep moving addresses around, but it’s actually more efficient for the ISP:

• Conserving Addresses: There is a global shortage of IPv4 addresses. Not every customer is online 24/7. By using a “pool” of addresses, the ISP only needs enough for the people currently connected, rather than one for every single customer they’ve ever signed.

• Monetization: ISPs know that businesses need an address that never changes (a Static IP) so they can host servers. Because this is a premium requirement, they usually charge extra—often $10–$20 more per month—for the privilege of a fixed address.

• Security: A changing IP makes you a “moving target.” It’s slightly harder for a hacker to target your specific home network over a long period if your address keeps shifting.

Comparison: Dynamic vs. Static IP

Feature	Dynamic IP (Standard)	Static IP (Business/Add-on)
Price	Included in your plan	Extra monthly fee
Consistency	Changes periodically	Never changes
Best For	Browsing, Streaming, Gaming	Hosting websites, Home Servers, CCTV
Setup	Automatic (DHCP)	Manual configuration

What if you need it to stay the same?

If you want to access your home computer from work, or run a small game server, a changing IP is a headache. Since Static IPs are expensive, most people use DDNS (Dynamic DNS).

DDNS is a service that gives you a name (like myhome.ddns.net) and a tiny app on your router. Every time your Public IP changes, the app tells the DDNS service: “Hey, I’m at a new address now!” The name then automatically points to your new IP, so you never have to memorize the numbers.

Can you host servers without a static IP?

Strictly speaking, you do not need a static IP to host a server, but it is the “easy mode” for hosting. Without one, you simply need a way to track the “moving target” that is your dynamic IP address.

For a home server, there are three main ways to handle a changing IP, ranging from “easy” to “professional.”

The Standard Workaround: Dynamic DNS (DDNS)

This is the most popular solution for home users. Instead of giving people your IP address (which changes), you give them a hostname (like my-cool-server.ddns.net).

• How it works: You run a tiny piece of software (a “Client”) on your server or router.

• The Process: Every 5–10 minutes, the client checks: “What is my Public IP?” If it has changed, it sends an update to the DDNS provider. The provider immediately updates the DNS records so your hostname points to the new IP.

• Popular Services: No-IP, DuckDNS (Free), and DynDNS.

The Modern “Zero-Config” Way: Tunnels (Cloudflare)

If you don’t want to deal with IP addresses or port forwarding at all, you can use a Tunnel.

• How it works: You install a small program (like cloudflared) on your server. This program creates an outbound connection to Cloudflare’s network.

• The Benefit: Since your server reaches out to the internet first, it doesn’t matter what your IP is. Users visit your domain (e.g., server.yourdomain.com), Cloudflare receives the request, and sends it down the “tunnel” directly to your server.

• Security Bonus: This hides your Public IP entirely, protecting you from direct attacks.

The Private Way: Mesh VPNs (Tailscale / ZeroTier)

If your server is only for you or a few friends, you don’t even need a public domain or a public IP.

• How it works: You install a Mesh VPN like Tailscale. It assigns your server a permanent, “virtual” IP address (usually starting with 100.x.x.x).

• The Result: No matter where your server moves or how the ISP changes your real IP, you can always find it at that same virtual address, as long as your laptop is also logged into the same Tailscale account.

When is a Static IP “Non-Negotiable”?

You only truly need a static IP if you are running Email Servers. Most big providers (Gmail, Outlook) will automatically block or “spam-bin” any emails coming from a Dynamic IP range because they assume it’s a compromised home computer sending spam.