Most tech-savvy people know that
127.0.0.1 is the Internet Protocol (IP) address of
localhost, and many tech-savvy people know
::1 is the Internet Protocol version 6 (IPv6) address of
::1 the only ways of representing the IP address of
No, there are an infinite different valid ways to represent it 💥
One IP address, many ways of representing it#
Here are the nine broad ways an IP address can be represented. We will use
127.0.0.1 as the subject of our example. You might want to run some server listening on
127.0.0.1 to confirm my claims.
1. Dotted-decimal notation#
We are most familiar with the dotted-decimal notation, which has the format of
N can range from
In this notation,
localhost is the familiar
2. 0-optimized dotted-decimal notation#
The digitally correct format of
127.0.0.1 is actually
127.0.0.1 is the zero suppressed form.
Zero compression is the exlusion of segments whose value sums up to zero.
Using zero compression, the
0 value segments of an IP address can be ommitted. So
Try pinging it for proof:
$ ping 127.1 PING 127.1 (127.0.0.1): 56 data bytes 64 bytes from 127.0.0.1: icmp_seq=0 ttl=64 time=0.051 ms
Because of zero suppression and zero compression, the following also is interpreted as
Go ahead, ping it.
000127.0.1 will also resolve to
000127 will be read as octal
0127, which equals to
87 in decimal. So
000127.0.1 will resolve to
3. Octal notation#
Each number of the dotted-decimal IP address can be represented in the octal format too. So in octal notation
It's important to note that the leading
0 before the numbers are required for marking them as octal. And you can put as many
0s before them. So, the following also refer to
00000000177.000.0.00000001 0177.0.0.0000001 000177.0000.00000.01 0000177.000000000000000000.00000000000.00000000001 00000000000000000000000000000000000000000000000000177.0.0.01
Don't believe me?
Trying pinging them:
$ ping 00000000000000000000000000000000000000000000000000177.0.0.01 PING 00000000000000000000000000000000000000000000000000177.0.0.01 (127.0.0.1): 56 data bytes 64 bytes from 127.0.0.1: icmp_seq=0 ttl=64 time=0.060 ms
The underlying networking library is converting the octal IP address to the decimal format (and then to binary).
4. Hexadecimal notation#
"If something can be represented in octal, it probably can be represented in hexadecimal too."
Yes, you are correct. The numbers of the dotted-decimal IP address can be represented in the hexadecimal format too. So in hexadecimal notation
The dots are optional if you preceed the concatenated hex values with a
Try pinging that:
$ ping 0x7f000001 PING 0x7f000001 (127.0.0.1): 56 data bytes 64 bytes from 127.0.0.1: icmp_seq=0 ttl=64 time=0.072 ms
And, you can left-pad the hex value with any amount of any random hex values. So, the following also refer to
0xDEADBEEF7f000001 0xBADF00D7f000001 0xDEADC0DE7f000001 0xBADC0DE7f000001 0xBAAAaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa7f000001
5. Decimal notation a.k.a dword notation#
Dword is the non-dotted decimal representation of an IP address. In dword notation
Try pinging it:
$ ping 2130706433 PING 2130706433 (127.0.0.1): 56 data bytes 64 bytes from 127.0.0.1: icmp_seq=0 ttl=64 time=0.031 ms
6. Binary notation#
You must have bee wondering if IP addresses can be represented in binary notation; since they can be repersented in decimal, octal, and hexadecimal.
You are right, IP addresses can be represented in binary too.
127.0.0.1 in binary notation is:
Note, however, not all HTTP clients support IP address in binary format.
7. Mixed notation#
How about mixing what we have learnt so far? A different notation for one or more segments of the address? Just keep the left-most segment intact. Here are some examples:
00000000000000000000000000000000000000000000000000177.1 0x7f.1 127.0x1
With three of the four segments represented the same in dec, oct, and hex,
127.0.0.1 doesn't give us much room to play. Let's see how
18.104.22.168 (google.com) may be represented in the decimal-octal-hexadecimal-dword mixed notation:
172.14263982 0254.0xd9a6ae 0xac.000000000000000000331.0246.174 0331.14263982
8. IPv6 format#
Then we have IPv6. All of the following resolve to
0000000000000:0000:0000:0000:0000:00000000000000:0000:1 0000:0000:0000:0000:0000:0000:0000:0001 0:0:0:0:0:0:0:1 0:0:0:0::0:0:1
Remember zero compression and zero suppression?
9. URL-encoded IP address#
URL-encoded IP addresses are accepted as valid IP addresses in most browsers and HTTP clients. So, the following refers to
And the following refers to
So why does an IP address have some many forms?
The fact is, an IP address is actually a 32-bit number in IPv4 and 128-bit number in IPv6. The binary notation is the correct representation of an IP address. Every other notation is a simply a convenience (at various degrees) for humans interacting with machine standards; and all of them are eventually converted to the binary notation.
The various number systems and the various optimizations conventions are what make the weird phenomena of having an unlimited different formats of IP addresses possible.
Valid IP addresses necessarily need not look "valid". IP addresses can be represented in idefinitely different ways, therefore it cannot be determined if a value is an IP address or not.
If you have a regex for detecting IP addresses, it is broken by default. If this regex is used for access control, you have a vulnerability on your system.
How's your IP address detection algorithm feeling today?