Convert IP addresses from decimal to IPv4 and format output using vectoring

Hello,

I want to convert a vector of IP addresses in decimal format to the IPv4 format.

So far I have implemented this using a for loop and the output is a cell array of chars.

function result = dec2IP(decimalIP)

% Derive the octets
dec = double(decimalIP);
byte1=floor(dec/power(256,3));
mod1=mod(dec, power(256,3));
byte2=floor(mod1/power(256,2));
mod2=mod(mod1, power(256,2));
byte3=floor(mod2/power(256,1));
mod3=mod(mod2, power(256,1));
byte4=floor(mod3/power(256,0));

% Convert to char array  
for i=1:length(decimalIP)
  result(i) = {[int2str(byte1(i)), '.', int2str(byte2(i)), '.', int2str(byte3(i)), '.', int2str(byte4(i))]};
end

end

Is there a way to implement this using vectoring instead of a loop?

Thanks, George

Answer 1

[Ashish Gudla]

You can probably convert the IP address from decimal to IPv4 format using vectorization however its not very intuitive. I used a different logic to convert.

Each value in IPv4 format can be obtained by applying bit operations on the decimal number as follows.

Right shift the decimal value 24 bits and apply “bitand” operator to the resulting number and 255. Similarly for the other values, right shift by 16 ,8 and 0 bits.

1st value :

bitand(bitshift(IPVector,-24), 255)

2nd value :

bitand(bitshift(IPVector,-16), 255)

3rd value :

bitand(bitshift(IPVector,-8), 255)

4th value :

bitand(bitshift(IPVector,0), 255)

See doc page of bitand( bitand doc ) and bitshift ( bitshift doc ) for usage.

Now you can simply concatenate all the 4 values with a “.” In between them to form a IPv4 format address using the following command:

strcat(
num2str(bitand(bitshift(IPVector,-24), 255)) ,'.',
num2str(bitand(bitshift(IPVector,-16), 255)) ,'.',
num2str(bitand(bitshift(IPVector,-8), 255))  ,'.',
num2str(bitand(bitshift(IPVector,0), 255))  
)

Example:

>> IPVector = [3232235778 ; 3232235780 ; 3232235890]
IPVector =
   1.0e+09 *
    3.2322
    3.2322
    3.2322
>> IPVector_new = strcat(num2str(bitand(bitshift(IPVector,-24), 255)),'.',num2str(bitand(bitshift(IPVector,-16), 255)) ,'.',num2str(bitand(bitshift(IPVector,-8), 255)) ,'.',num2str(bitand(bitshift(IPVector,0), 255))  )
IPVector_new =
192.168.1.  2
192.168.1.  4
192.168.1.114

>> IPVector_new = strrep(cellstr(IPVector_new),' ','')
IPVector_new = 
    '192.168.1.2'
    '192.168.1.4'
    '192.168.1.114'

Answer 2

[J-G van der Toorn]

I was thinking about this inline function:

 ipstr = @(ip)sprintf('%d.%d.%d.%d',arrayfun(@(n)floor(mod(ip,2^(n*8))/2^((n-1)*8)),4:-1:1));

But with bitshift, it's also possible:

 ipstr=@(ip)sprintf('%d.%d.%d.%d',arrayfun(@(n)bitand(bitshift(ip,-32+n*8), 255),1:4));

and even shorter. There might be an even shorter notation on Cody, or it will be there soon.