IP Address Fundamentals
The whole reef, mapped. Every core idea about IP addresses, gathered on one page.
What this page is
This page is the overview: every core idea in this section, in order, in one place. Read it top to bottom and you will have the whole picture, and stopping here is a perfectly good outcome. Each section also links to its own article, which goes considerably deeper with worked examples and the edge cases this page skips. If you are revising, read this page. Go to an article when a particular section did not land, or when you need the detail behind it.
1. What an IP address is
Full article →An IP address identifies a device on a network, the way a street address identifies a building. Every request you send carries a return address, or the answer has nowhere to go. IP stands for Internet Protocol, the rules that define how those numbers are written and how traffic carrying them is moved along.
There is no master map of the internet. No single machine knows the route from your couch to a server in Virginia. Each device along the way knows only which neighbor gets the packet next in order to move it closer to its destination.
Addresses are handed down through a hierarchy rather than claimed first come, first served. IANA allocates very large blocks to five regional registries, those registries allocate smaller blocks to internet providers, and your provider assigns one address to your connection. Your own router then hands out private addresses inside your home. Every one of those handoffs is recorded, and that public record is what makes geolocation possible at all.
2. Public and private addresses
Full article →You have two addresses, and confusing them is the most common mistake in this whole subject. Your device holds a private address on your own network. Your household shares a single public address out on the internet. Your router sits between them and rewrites addresses in both directions, a process called NAT.
The mechanism is worth seeing, because it sounds impossible until you do. If three devices share one address, how does a reply find the right one? The answer is ports: the router picks an unused source port for each conversation and records the pairing.
Three ranges are reserved for private use and are not routable on the public internet. If the number you are looking at falls in one of them, it is not the number a game server, camera app, or vendor allowlist is asking for.
10.0.0.0 to 10.255.255.255 a single large block, 16.7 million addresses 172.16.0.0 to 172.31.255.255 sixteen contiguous blocks 192.168.0.0 to 192.168.255.255 the range most home routers use
3. IPv4, and how it ran out
Full article →IPv4 was defined in 1981 and is still the dominant protocol on the internet. An address is a 32-bit number, split into four 8-bit octets written in decimal with dots between them.
Thirty-two bits gives about 4.3 billion addresses. That felt limitless in 1981 and stopped being enough roughly twenty years later. IANA handed out the last unallocated blocks to the regional registries in 2011. Two techniques bought the time in between: NAT, which lets a whole household share one address, and CIDR, which allocates blocks in efficient sizes rather than in rigid classes. Both are life rafts rather than a permanent home.
4. IPv6, and why it took so long
Full article →IPv6 uses 128-bit addresses, written as eight groups of four hexadecimal digits separated by colons. The jump from 32 bits to 128 does not make the pool four times bigger. It makes it about 79 octillion times bigger.
Adoption has been slow for an unglamorous reason: the two protocols do not interoperate. An IPv6-only host cannot talk to an IPv4-only host without a translator in between, so nobody could switch until almost everybody had switched, and the incentive to go first was weak. The answer was dual stack, running both at once, which is what most networks do today and why our front page tests each protocol separately.
5. Why your address changes
Full article →Most home connections receive a dynamic address on a lease from the provider, handed out by DHCP. The part people miss is that a lease is renewed long before it expires, and a renewal normally hands back the same address.
Mobile networks are the most volatile of all. Carriers rotate enormous shared pools, and an address can change while you are standing still. They also place customers behind carrier-grade NAT, where thousands of subscribers share one public address, and that is increasingly true on fixed-line connections too.
A dynamic residential address will break that allowlist, often within days, and the failure will look like a mysterious permissions error rather than an address change. Ask about a static address, use a dynamic DNS hostname, or route through something with a fixed address instead.
6. How geolocation guesses your city
Full pillar →No part of an IP address encodes a location. There is no field inside 203.0.113.42 that says Pennsylvania. Every location you have been shown from an address is an inference built from other records.
Accuracy is not one number. It collapses as you zoom in, and the drop between region and city is where most people's expectations break.
A wrong record can be corrected, and it is worth knowing that there is no central registry to correct. Each provider keeps its own database, so a report has to be filed with each one you care about and will not propagate. Expect weeks rather than days.
That is expected often enough to be worth saying plainly. The estimate describes where your provider hands your traffic to the internet, which is not necessarily where you are sitting.
7. Quick reference tables
The numbers worth having in one place rather than hunting through six articles.
Reserved and special-purpose ranges
| Range | Name | What it means if you see it |
|---|---|---|
10.0.0.0/8 | Private | A local network address, often a larger or business network |
172.16.0.0/12 | Private | A local network address, common in corporate and container networks |
192.168.0.0/16 | Private | A local network address, the usual choice for home routers |
100.64.0.0/10 | Carrier-grade NAT | Your provider is sharing one public address among many customers |
127.0.0.0/8 | Loopback | The device talking to itself, never leaves the machine |
169.254.0.0/16 | Link-local | Usually a failure signal: no DHCP server answered |
::1 | IPv6 loopback | The IPv6 equivalent of 127.0.0.1 |
fe80::/10 | IPv6 link-local | Valid only on the local link, not routable |
IPv4 and IPv6 side by side
| IPv4 | IPv6 | |
|---|---|---|
| Address size | 32 bits | 128 bits |
| Written as | Four decimal octets | Eight hexadecimal groups |
| Example | 203.0.113.42 | 2001:db8::8a2e:370:7334 |
| Total addresses | About 4.3 billion | About 340 undecillion |
| Defined in | 1981 | 1998 |
| NAT | Nearly universal | Usually unnecessary |
8. Common questions
Which number do I give someone who asks for my IP?
The IPv4 line on our front page, the one that looks like 203.0.113.42. If they specifically asked for IPv6, give them the long one with colons.
Why does my computer show a different address than this site?
Your computer is showing its private address on your own network. This site shows the public address your whole household shares. Both are correct, and they answer different questions.
Can someone find my house from my IP address?
Not from the address alone. It generally reveals your provider and a rough area, frequently the wrong city. The record linking an address to a specific subscriber sits with the provider and normally requires legal process to obtain.
Does changing my IP address make me anonymous?
No. Cookies, browser fingerprints, and any account you log into all survive an address change. It is a much smaller move than it is usually made out to be.
Why do I have no IPv6 address?
Either your provider has not enabled it, your router is not configured for it, or something on your device or network is blocking it. It is common and mostly harmless, since the IPv4 internet still works everywhere.