IP address or not?

Question

Your network scanning tool is annoyingly picky about input, and immediately crashes if you feed it an IPv4 address that contains improper characters or isn't properly formatted.

An IPv4 address is a 32-bit numeric address written as four numbers separated by periods. Each number can be zero to 255.

We need to write a tool to pre-validate the input to avoid those crashes, and our specific tool is picky: A valid format will look like a.b.c.d where a, b, c and d:

• Can be a 0 or a natural number with no leading zeros.
• Should be between 0 - 255 (inclusive).
• Should not contain special symbols like +, -, ,, and others.
• Should be decimal (base 10)

Input: A string

Output: Truthy or Falsey value (arbitrary values also accepted)

Test Cases:

Input            |  Output  |  Reason
                 |          |
- 1.160.10.240   |  true    |
- 192.001.32.47  |  false   |  (leading zeros present)
- 1.2.3.         |  false   |  (only three digits)
- 1.2.3          |  false   |  (only three digits)
- 0.00.10.255    |  false   |  (leading zeros present)
- 1.2.$.4        |  false   |  (only three digits and a special symbol present)
- 255.160.0.34   |  true    |
- .1.1.1         |  false   |  (only three digits)
- 1..1.1.1       |  false   |  (more than three periods)
- 1.1.1.-0       |  false   |  (special symbol present)
- .1.1.+1        |  false   |  (special symbol present)
- 1 1 1 1        |  false   |  (no periods)
- 1              |  false   |  (only one digit)
- 10.300.4.0     |  false   |  (value over 255)
- 10.4F.10.99    |  false   |  (invalid characters)
- fruit loops    |  false   |  (umm...)
- 1.2.3.4.5      |  false   |  (too many periods/numbers)
- 0.0.0.0        |  true    |
- 0.0 0.0.       |  false   |  (periods misplaced)
- 1.23..4        |  false   |  (a typo of 1.2.3.4)
- 1:1:1:1:1:1:1:1|  false   |  (an IPv6 address, not IPv4)

This is code-golf, so fewest bytes will win!

Note for the users - if you want to add some more test-cases, you're welcomed (by suggesting an edit). But, please make sure that the test-cases don't repeat themselves! Thanks

Answer 1

X86_64 machine code: 18 16 bytes

Edit: This answer doesn't quite work, as

1. I am using inet_pton from the standard C libraries, which means I need the extern. I didn't include the extern in my byte count though.
2. I used the red zone as a result for the actual address, but called a function which also could've used the red zone. It fortunately doesn't on my machine, but some odd standard library build might use it which could cause undefined behavior.

And yeah, the whole thing is pretty much being done by an already written function

Anyway, this is what I got: 48 89 fe 6a 02 5f 48 8d 54 24 80 e9 00 00 00 00

Assembly:

section .text
    extern inet_pton
    global ipIsValid

ipIsValid:
    mov rsi, rdi
    ;mov rdi, 2 ; change to 10 for ipv6
    push 2
    pop rdi ; thank you peter
    lea rdx, [rsp - 128]
    jmp inet_pton

Explanation:

Take a look at inet_pton(3). It takes a string IP address and puts it in a buffer you can use with struct sockaddr. It takes 3 arguments: the address family (AF_INET (ipv4), 2, or AF_INET6 (ipv6), 10), the ip address's string, and a pointer to the output. It returns 1 on success, 0 for an invalid address, or -1 for when the address family is neither AF_INET or AF_INET6 (which will never occur because I'm passing a constant to it).

So I simply move the string to the register for the second argument, set the first register to 2, and set the third register to the red zone (128 bytes below the stack pointer) since I don't care about the result. Then I can simply jmp to inet_pton and let that return straight to the caller!

I spun up this quick test program to test your cases:

#include <stdio.h>
#include <arpa/inet.h>
#include <netinet/ip.h>

extern int ipIsValid(char *);

int main(){
    char *addresses[] = {
        "1.160.10.240",
        "192.001.32.47",
        "1.2.3.",
        "1.2.3",
        "0.00.10.255",
        "1.2.$.4",
        "255.160.0.34",
        ".1.1.1",
        "1..1.1.1",
        "1.1.1.-0",
        ".1.1.+1",
        "1 1 1 1",
        "1",
        "10.300.4.0",
        "10.4F.10.99",
        "fruit loops",
        "1.2.3.4.5",
        NULL
    };

    for(size_t i = 0; addresses[i] != NULL; ++i){
        printf("Address %s:\t%s\n", addresses[i],
            ipIsValid(addresses[i]) ? "true" : "false");
    }
    return 0;
}

Assemble with nasm -felf64 assembly.asm, compile with gcc -no-pie test.c assembly.o, and you'll get:

Address 1.160.10.240:   true
Address 192.001.32.47:  false
Address 1.2.3.: false
Address 1.2.3:  false
Address 0.00.10.255:    false
Address 1.2.$.4:    false
Address 255.160.0.34:   true
Address .1.1.1: false
Address 1..1.1.1:   false
Address 1.1.1.-0:   false
Address .1.1.+1:    false
Address 1 1 1 1:    false
Address 1:  false
Address 10.300.4.0: false
Address 10.4F.10.99:    false
Address fruit loops:    false
Address 1.2.3.4.5:  false

I could make this much smaller if the caller was supposed to pass AF_INET or AF_INET6 to the function

Answer 2

Java (JDK), 63 bytes

s->("."+s).matches("(\\.(25[0-5]|(2[0-4]|1\\d|[1-9])?\\d)){4}")

Try it online!

Credits

• -1 byte thanks to Kevin Cruijssen
• l4m2 for showing the previously-failing case .1.1.1.1.

Answer 3

JavaScript (Node.js), 43 bytes

x=>x.split`.`.map(t=>[t&255]==t&&[])==`,,,`

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JavaScript (Node.js), 46 bytes

x=>x.split`.`.every(t=>k--&&[t&255]==t,k=4)*!k

Try it online!

used Arnauld's part

JavaScript (Node.js), 54 53 51 bytes

x=>x.split`.`.every(t=>k--*0+t<256&[~~t]==t,k=4)*!k

Try it online!

-2B for 0+t<256, -1B from Patrick Stephansen, +1B to avoid input 1.1.1.1e-80

RegExp solution 5854 bytes

s=>/^((2(?!5?[6-9])|1|(?!0\d))\d\d?\.?\b){4}$/.test(s)

Thank Deadcode for 3 bytes

Answer 4

PHP, 39 36 bytes

<?=+!!filter_var($argv[1],275,5**9);

Try it online!

275 resembles the constant FILTER_VALIDATE_IP

5**9 is being used instead of the constant FILTER_FLAG_IPV4. This is sufficient, because 5**9 & FILTER_FLAG_IPV4 is truthy, which is exactly what PHP does in the background, as Benoit Esnard pointed out.

Here, filter_var returns the first argument, if it's a valid IPv4 address, or false if it's not. With +!!, we produce the output required by the challenge.

Answer 5

PHP, 36 Bytes

echo(ip2long($argv[1])===false?0:1);

ip2long is a well-known built-in function.

Answer 6

Perl 6, 22 21 20 bytes

-1 byte thanks to Phil H.

{?/^@(^256)**4%\.$/}

Try it online!

Explanation

{                  }  # Anonymous Block
  /               /   # Regex match
   ^             $    # Anchor to start/end
    @(    )           # Interpolate
      ^256            #   range 0..255,
                      #   effectively like (0|1|2|...|255)
           **4        # Repeated four times
              %\.     # Separated by dot
 ?                    # Convert match result to Bool

Answer 7

05AB1E, 26 24 23 22 23 bytes

'.¡©g4Q₅Ý®å`®1šDïþJsJQP

-1 byte thanks to @Emigna.
+1 byte for bugfixing test case 1.1.1.1E1 incorrectly returning a truthy result.

Try it online or verify all test cases.

Explanation:

'.¡              '# Split the (implicit) input by "."
   ©              # Save it in the register (without popping)
    g4Q           # Check that there are exactly 4 numbers
    ₅Ý®å          # Check for each of the numbers that they are in the range [0,255],
        `         # and push the result for each number separated onto the stack
    ®1šDïþJsJQ    # Check that each number does NOT start with a "0" (excluding 0s itself),
                  # and that they consist of digits only
              P   # Check if all values on the stack are truthy (and output implicitly)

Answer 8

PowerShell, 59 51 49 bytes

-8 bytes, thanks @AdmBorkBork

-2 bytes, true or false allowed by author

try{"$args"-eq[IPAddress]::Parse($args)}catch{!1}

Test script:

$f = {

try{"$args"-eq[IPAddress]::Parse($args)}catch{!1}

}

@(
    ,("1.160.10.240" , $true)
    ,("192.001.32.47" , $false)
    ,("1.2.3." , $false)
    ,("1.2.3" , $false)
    ,("0.00.10.255" , $false)
    ,("192.168.1.1" , $true)
    ,("1.2.$.4" , $false)
    ,("255.160.0.34" , $true)
    ,(".1.1.1" , $false)
    ,("1..1.1.1" , $false)
    ,("1.1.1.-0" , $false)
    ,("1.1.1.+1" , $false)
    ,("1 1 1 1" , $false)
    ,("1"            ,$false)
    ,("10.300.4.0"   ,$false)
    ,("10.4F.10.99"  ,$false)
    ,("fruit loops"  ,$false)
    ,("1.2.3.4.5"    ,$false)

) | % {
    $s,$expected = $_
    $result = &$f $s
    "$($result-eq$expected): $result : $s"
}

Output:

True: True : 1.160.10.240
True: False : 192.001.32.47
True: False : 1.2.3.
True: False : 1.2.3
True: False : 0.00.10.255
True: True : 192.168.1.1
True: False : 1.2.$.4
True: True : 255.160.0.34
True: False : .1.1.1
True: False : 1..1.1.1
True: False : 1.1.1.-0
True: False : 1.1.1.+1
True: False : 1 1 1 1
True: False : 1
True: False : 10.300.4.0
True: False : 10.4F.10.99
True: False : fruit loops
True: False : 1.2.3.4.5

Explanation:

The script tries to parse an argument string, to construct a .NET object, IPAddress.

• return $true if object created and the argument string is equal to a string representation of the object (normalized address by object.toString())
• return $false otherwise

PowerShell, 59 56 54 bytes, 'don't use a .NET lib' alternative

-3 bytes, true or false allowed by author

-2 bytes, thanks to @Deadcode for the cool regexp.

".$args"-match'^(\.(2(?!5?[6-9])|1|(?!0\B))\d\d?){4}$'

Try it online!

Thanks @Olivier Grégoire for the original regular expression.

Answer 9

PHP 7+, 37 35 32 bytes

This uses the builtin function filter_var, to validate that it is an IPv4 address.

For it to work, you need to pass the key i over a GET request.

<?=filter_var($_GET[i],275,5**9);

Will output nothing (for a falsy result) or the IP (for a truthy result), depending on the result.

You can try this on: http://sandbox.onlinephpfunctions.com/code/639c22281ea3ba753cf7431281486d8e6e66f68e http://sandbox.onlinephpfunctions.com/code/ff6aaeb2b2d0e0ac43f48125de0549320bc071b4

---

This uses the following values directly:

• 275 = FILTER_VALIDATE_IP
• 1<<20 = 1048576 = FILTER_FLAG_IPV4
• 5**9 = 1953125 (which has the required bit as "1", for 1048576)

Thank you to Benoit Esnard for this tip that saved me 1 byte!

Thank you to Titus for reminding me of the changes to the challenge.

---

I've looked into using the function ip2long, but it works with non-complete IP addresses.

Non-complete IPv4 addresses are considered invalid in this challenge.

If they were allowed, this would be the final code (only for PHP 5.2.10):

<?=ip2long($_GET[i]);

Currently, it isn't explicit in the documentation that this will stop working (when passed an incomplete ip) with newer PHP versions.

After testing, confirmed that that was the case.

Thanks to nwellnhof for the tip!

Answer 10

C (gcc) / POSIX, 26 bytes

f(s){s=inet_pton(2,s,&s);}

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Works as 64-bit code on TIO but probably requires that sizeof(int) == sizeof(char*) on other platforms.

Answer 11

Python 3: 81 78 70 69 66 bytes

['%d.%d.%d.%d'%(*x.to_bytes(4,'big'),)for x in range(16**8)].count

Loop over all possible IPv4 addresses, get the string representation and compare it to the input. It uh... takes a while to run.

EDIT: Removed 3 bytes by switching from full program to anonymous function.

EDIT2: Removed 8 bytes with help from xnor

EDIT3: Removed 1 byte by using an unpacked map instead of list comprehension

EDIT4: Removed 3 bytes by using list comprehension instead of the ipaddress module

Answer 12

C# (Visual C# Interactive Compiler), 84 79 65 bytes

s=>s.Split('.').Sum(t=>byte.TryParse(t,out var b)&t==b+""?1:5)==4

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-5 and -14 bytes saved thanks to @dana!

# C# (Visual C# Interactive Compiler), 61 bytes

s=>s.Count(c=>c==46)==3&IPAddress.TryParse(s,out IPAddress i)

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This is a work in progress. The code use System.Net (+17 bytes if you count it). if you wonder why I count and parse:

The limitation with IPAddress.TryParse method is that it verifies if a string could be converted to IP address, thus if it is supplied with a string value like "5", it consider it as "0.0.0.5".

source

As @milk said in comment, it will indeed fail on leading zeroes. So, the 61 bytes one is not working.

Answer 13

Python 2, 85 82 81 bytes

-1 byte thanks to Kevin Cruijssen

from ipaddress import*
I=input()
try:r=I==str(IPv4Address(I))
except:r=0
print~~r

Try it online!

113 byte answer is deleted as it fails for 1.1.1.1e-80

Answer 14

Japt, 17 15 bytes

q.
ʶ4«Uk#ÿòs)Ê

Try it or run all test cases or verify additional test cases from challenge comments

---

Explanation

We split to an array on ., check that the length of that array is equal to 4 AND that the length when all elements in the range ["0","255"] are removed from it is falsey (0).

                 :Implicit input of string U
q.               :Split on "."
\n               :Reassign resulting array to U
Ê                :Length of U
 ¶4              :Equals 4?
   «             :&&!
    Uk           :Remove from U
      #ÿ         :  255
        ò        :  Range [0,255]
         s       :  Convert each to a string
          )      :End removal
           Ê     :Length of resulting array

Answer 15

sfk, 176 bytes

* was originally Bash + SFK but TIO has since added a proper SFK wrapper

xex -i "_[lstart][1.3 digits].[1.3 digits].[1.3 digits].[1.3 digits][lend]_[part2]\n[part4]\n[part6]\n[part8]_" +xed _[lstart]0[digit]_999_ +hex +linelen +filt -+1 -+2 +linelen

Try it online!

Answer 16

Mathematica, 39 31 bytes

Original version:

¬FailureQ[Interpreter["IPAddress"][#]]&

Modified version (thanks to Misha Lavrov)

 AtomQ@*Interpreter["IPAddress"]

which returns True if the input is a valid IP address (try it).

In case you insist on getting 1 and 0 instead, then an additional 7 bytes would be necessary:

Boole/@AtomQ@*Interpreter["IPAddress"]

Answer 17

Python 2, 93 89 67 53 bytes

[i==`int(i)&255`for i in input().split('.')]!=[1]*4>_

Try it online!

Thanks to Dennis for shaving another 14 bytes on the internal comparisons and exit code.

Special thanks to Jonathan Allan for shaving 22 bytes & a logic fix! Pesky try/except begone!

Taking properly formatted strings instead of raw bytes shaves off 4 bytes, thanks Jo King.

Answer 18

Python3 Bash* 60

*Also other shells. Any one for which the truthy/falsy test passes on a program exit code

read I
python3 -c "from ipaddress import*;IPv4Address('$I')"

Explanation

The trouble with a pure Python solutions is that a program crashing is considered indeterminate. We could use a "lot" of code to convert an exception into a proper truthy/fasly value. However, at some point the Python interpreter handles this uncaught exception and returns a non-zero exit code. For the low-low cost of changing languages to your favourite Unix shell, we can save quite a bit of code!

Of course, this is vulnerable to injection attacks... Inputs such as 1.1.1.1'); print('Doing Something Evil are an unmitigated threat!

Answer 19

JavaScript (ES6), 49 bytes

Returns a Boolean value.

s=>[0,1,2,3].map(i=>s.split`.`[i]&255).join`.`==s

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Answer 20

ECMAScript pure regex, 41 bytes

^((2(?!5?[6-9])|1|(?!0\B))\d\d?\.?\b){4}$

Try it online!
Try it on regex101

I think the logic in this regex speaks for itself, so I will merely pretty-print but not comment it:

^
(
    (
        2(?!5?[6-9])
    |
        1
    |
        (?!0\B)
    )
    \d\d?
    \.?\b
){4}
$

This can be used to shave 2 bytes off the following other answers:

• Olivier Grégoire (Java (JDK))
• l4m2 (JavaScript (Node.js))
• mazzy (PowerShell)
• Kevin Cruijssen (Retina)

Here is an alternative version that allows leading zeros, but does so consistently (octets may be represented by a maximum of 3 decimal digits):

^((2(?!5?[6-9])|1|0?)\d\d?\.?\b){4}$

Or allow any number of leading zeros:

^(0*(2(?!5?[6-9])|1?)\d\d?\.?\b){4}$

Answer 21

Red, 106 bytes

func[s][if error? try[t: load s][return off]if 4 <> length? t[return off]s =
form as-ipv4 t/1 t/2 t/3 t/4]

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Returnd true or false

Explanation:

f: func [ s ] [
    if error? try [                  ; checks if the execution of the next block result in an error
        t: load s                    ; loading a string separated by '.' gives a tuple   
    ] [                              ; each part of which must be in the range 0..255
        return off                   ; if there's an error, return 'false' 
    ]
    if 4 <> length? t [              ; if the tuple doesn't have exactly 4 parts
        return off                   ; return 'false'  
    ]
    s = form as-ipv4 t/1 t/2 t/3 t/4 ; is the input equal to its parts converted to an IP adress
]

Answer 22

Charcoal, 45 21 bytes

Ｉ∧⁼№θ.³¬Φ⪪θ.¬№Ｅ²⁵⁶Ｉλι

Try it online! Link is to verbose version of code. Edit: Saved 24 bytes by porting @Shaggy's Japt answer. Explanation:

    θ                   Input string
   №                    Count occurrences of
     .                  Literal `.`
  ⁼                     Equal to
      ³                 Literal 3
 ∧                      Logical And
       ¬                Logical Not
          θ             Input string
         ⪪              Split on
           .            Literal `.`
        Φ               Filter by
            ¬           Logical Not
               ²⁵⁶      Literal 256
              Ｅ         Map over implicit range
                   λ    Map value
                  Ｉ     Cast to string
             №          Count occurrences of
                    ι   Filter value
Ｉ                       Cast to string
                        Implicitly print

Answer 23

Python 3, 109 93 bytes

import re
lambda x:bool(re.match(r'^((25[0-5]|2[0-4]\d|1\d\d|[1-9]\d|\d)(\.(?!$)|$)){4}$',x))

Explanation

Each octet can be 0 - 255 :

• starts with 25 and having 0-5 as last digit
• start with 2, has 0-4 as second digit and any digit at the end
• starts with 1, and 00 - 99 as rest digits
• has only 2 digits - 1-9 being the first one and any digit thereafter
• or just a single digit

An octet can end with a (.) or just end, with the condition that it cannot do both , the negative lookahead (?!$) takes care of this case

Thanks @Zachary for making me realize I can discard spaces (since it is code golf)
Thanks @DLosc for the improvements and making me realize my mistake, its been corrected now.

Answer 24

Retina, 46 44 bytes

^
.
^(\.(25[0-5]|(2[0-4]|1\d|[1-9])?\d)){4}$

Port of @OlivierGrégoire's Java answer, so make sure to upvote him!
-2 bytes thanks to @Neil.

Try it online.

Explanation:

^
.                           # Prepend a dot "." before the (implicit) input
^...$                       # Check if the entire string matches the following regex
                            # exactly, resulting in 1/0 as truthy/falsey:
 (                          #  Open a capture group
  \.                        #   A dot "."
    (25[0-5]                #   Followed by a number in the range [250,255]
    |(2[0-4]|         ) \d) #   or by a number in the range [200,249]
    |(      |1\d|     ) \d) #   or by a number in the range [100,199]
    |(          |[1-9]) \d) #   or by a number in the range [10,99]
    |(                )?\d) #   or by a number in the range [0,9]
 )                          #  Close capture group
  {4}                       #  This capture group should match 4 times after each other

Answer 25

Stax, 14 bytes

∞n·Θ3ª&JH‼∙*~Γ

Run and debug it

Unpacked, ungolfed, and commented, it looks like this.

VB      constant 256
r       [0 .. 255]
'|*     coerce and string-join with "|"; i.e. "0|1|2|3 ... 254|255"
:{      parenthesize to "(0|1|2|3 ... 254|255)"
]4*     make 4-length array of number pattern
.\.*    string join with "\\."; this forms the complete regex
|Q      is the input a complete match for the regex?

Run this one

Answer 26

Bash, 30 bytes

ipcalc -c `cat`;echo $(($?^1))

Try it online!

Answer 27

Retina, 42 41 bytes

~(K`

255*
["^(("|'|]")\.?\b){4}$"L$`
$.`

Try it online! Based on a previous version of @nwellnhof's Perl 6 answer, but 1 byte saved by stealing the \.?\b trick from @Deadcode's answer. Explanation:

K`

Clear the work area.

255*

Insert 255 characters.

["^(("|'|]")\.?\b){4}$"L$`
$.`

Generate the range 0..255 separated with |s, prefixed with ^((, and suffixed with )\.?\b){4}$, thus building the regular expression ^((0|1|...255)\.?\b){4}$.

~(

Evaluate that on the original input.

Answer 28

Jelly, 11 bytes

⁹ḶṾ€ṗ4j€”.ċ

A monadic link accepting a list of characters which yields \$1\$ if it's a valid address and \$0\$ otherwise. Builds a list of all \$256^4=4294967296\$ addresses and then counts the number of occurrences of the input therein.

Here's similar @ Try it online! that uses \$16\$ (⁴) rather than \$256\$ (⁹), since the method is so inefficient!

How?

⁹ḶṾ€ṗ4j€”.ċ - Link: list of characters, S
⁹           - literal 256
 Ḷ          - lowered range = [0,1,2,...,254,255]
  Ṿ€        - unevaluate €ach = ['0','1',...,['2','5','4'],['2','5','5']]
    ṗ4      - 4th Cartesian power = ALL 256^4 lists of 4 of them
            -               (e.g.: ['0',['2','5','5'],'9',['1','0']])
        ”.  - literal '.' character
      j€    - join for €ach (e.g. ['0','.','2','5','5','.','9','.','1','0'] = "0.255.9.10")
          ċ - count occurrences of right (S) in left (that big list)

Answer 29

C (gcc), 287 197 bytes

#import<regex.h>
g;f(char*s){regex_t r;g=regcomp(&r,"^(([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\\.){3}([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])$",1);g=regexec(&r,s,0,0,0);return !g;}

Try it online!

Thanks to Logern for the big save !

Answer 30

Pip, 25 16 bytes

a~=X,256RL4J"\."

Takes the candidate IP address as a command-line argument. Try it online! or Verify all test cases

Explanation

Regex solution, essentially a port of recursive's Stax answer.

                  a is 1st cmdline arg (implicit)
    ,256          Range(256), i.e. [0 1 2 ... 255]
   X              To regex: creates a regex that matches any item from that list
                  i.e. essentially `(0|1|2|...|255)`
        RL4       Create a list with 4 copies of that regex
           J"\."  Join on this string
 ~=               Regex full-match
a                 against the input

Answer 31

Perl 5 -pF/\./, 47 bytes

$\=@F==4&&!/[^0-9.]/;$\&&=!/^0./&&$_<256for@F}{

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Answer 32

R, 59 bytes

Passes special cases not in test with periods at start/end

function(s)length(a<-scan(,t=s,s,,,"."))==4&all(a%in%0:255)

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Answer 33

JavaScript, 89 bytes

(_,r=`(${[...Array(256).keys()].join`|`})`)=>RegExp(`^${(r+'\\.').repeat(3)+r}$`).test(_)

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Create RegExp capture groups from indexes of an array having length 256 for range 0-255 joined with | and followed by escaped . character (^(0|1...|255)\.(0|1...|255)\.(0|1...|255)\.(0|1...|255)$) repeated 3 times closing with joined array followed by $ to match end of string, return true or false result of input passed to RegExp.prototype.test().

Answer 34

Ruby, 123 107 bytes

Try it online!

a=gets.chomp
e=a.split'.'
(p 0;exit)if(e.map(&:to_i).join'.')!=a
p(e.all?{|i|(0..255)===i.to_i}&&e.size==4)

This could definitely be shorter, probably with a regex of some sort, but I wanted to do it without a regex.

Thanks to MegaTom for Array#all?

0 or false is falsy; true is truthy.

Explanation:

The input is taken from stdin. It's split into an array of the numbers (as strings) on the periods.

If the array isn't the same as it is with the elements converted to numbers and back to strings, the program outputs 0 and exits. This eliminates cases with leading zeros and illegal characters.

The array is converted to an array of true/false values representing whether the value is between 0 and 255, inclusive.

true is outputted if the array is only true values, meaning that all the numbers were between 0 and 255, and only if it has exactly 4 elements, meaning that there were 4 original numbers. false is outputted otherwise, meaning invalid numbers (higher than 255) or too many numbers.

Answer 35

Batch, 53 bytes

@(ping -4 %1&echo g 255.255.255.255 w)|find/c"g %1 w"

Disconnect your Internet connection first to avoid the standard loophole.

I can't find information on how to avoid ^".

Answer 36

APL(NARS) 50 chars, 100 bytes

{'...'≢⍵∼⎕D:0⋄4≠≢w←⍵⊂⍨⍵≠'.':0⋄w≢⍕¨m←⍎¨w:0⋄256>⌈/m}

test:

  p←{'...'≢⍵∼⎕D:0⋄4≠≢w←⍵⊂⍨⍵≠'.':0⋄w≢⍕¨m←⍎¨w:0⋄256>⌈/m}
  {⍞←'[',⍵,'=',(p ⍵),']'⋄⍬}¨a 
[1.160.10.240= 1 ][192.001.32.47= 0 ][1.2.3.= 0 ][0.00.10.255= 0 ][1.2.$.4= 0 ][255.160.0.34= 1 ][.1.1.1= 0 ][1..1.1.1= 0 ][1.1.1.-0= 0 ][.1.1.+1= 0 ][1 1 1 1= 0 ][1= 0 ][10.300.4.0= 0 ][10.4F.10.99= 0 ][fruit loops= 0 ][1.2.3.4.5= 0 ][0.0.0.0= 1 ][0.0 0.0.= 0 ][1.23..4= 0 ][1:1:1:1:1:1:1:1= 0 ]  

Answer 37

PHP, 27 26 24 Bytes

<?=+!!ip2long($argv[1]);

Based on @rexkogitans and @nwellnhof solutions Fails for 0.0.0.0 because PHP internal function 0.0.0.0 is not considered a valid IP address Try it!