IP and InternetWhat the checker tells you
For any address you type, it answers three questions.
- Is it a valid address? An IPv4 address needs exactly four numbers from 0 to 255. An IPv6 address needs hex groups and colons in the right pattern. A typing slip such as
192.168.1.256or the letter l in place of the digit 1 is caught here. - What kind is it? Public, private, or one of the special kinds below.
- Can it be reached from the internet? Only public addresses can.
The check uses the fixed list of ranges that the internet's standards set aside, so it runs entirely in your browser. It does not look up who owns a public address or where it is, and it does not contact the address.
The kinds of IPv4 address
| Kind | Range | In plain words |
|---|---|---|
| Private | 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 | Home and office networks. Free for anyone to use inside their own network |
| Shared (carrier-grade NAT) | 100.64.0.0/10 | Between an internet provider and its customers when a public address is shared |
| Loopback | 127.0.0.0/8 | The device talking to itself |
| Link-local | 169.254.0.0/16 | Self-given when no router answered. A sign of a connection problem |
| Documentation | 192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24 | Reserved for examples, like the ones on this site |
| Multicast | 224.0.0.0/4 | One sender, many receivers |
| Reserved | 240.0.0.0/4 | Kept for future use |
| Public | Everything else | Ordinary internet addresses |
The kinds of IPv6 address
| Kind | Starts with | In plain words |
|---|---|---|
| Global (public) | 2 or 3 | Ordinary internet addresses |
| Unique local | fc or fd | Private addresses for inside a home or office |
| Link-local | fe80 | Every device has one; it works only on the local link |
| Multicast | ff | One sender, many receivers |
| Loopback | ::1 | The device talking to itself |
| Documentation | 2001:db8 | Reserved for examples |
When this is useful
- Your router's WAN address: if it turns out to be private or shared, you are behind carrier-grade NAT and port forwarding will not work.
- An address in a log or an email header: a private address there tells you about the sender's inner network only, not where they are.
- A device showing 169.254.x.x: it failed to get an address. Restart the router and reconnect.
- Homework and exam questions on address classes and ranges.
- Setting an address by hand: check that what you are about to type is valid and in a private range.
Tap the sample buttons above the Check button to see one of each kind.
Why is 172.32.0.1 public when 172.16.0.1 is private?
Only 172.16.0.0 to 172.31.255.255 is private. The rest of the 172 block is ordinary public space.
Is 192.168.1.1 dangerous to share?
No. It is a private address used by millions of routers and means nothing outside your own network.
Can the checker tell me who owns a public address?
No. That needs a look-up in the registries' "whois" records, which this page does not do. It only classifies the address.