convert ip address

[patlv23]

is there a way to convert ip address returned by gethostbyname (hex?) to dot notation (12.12.12.12) and vice versa? i'm using unix c. thanks in advance

 

[victorv]

Hi,
the value is a longword ( you can print it in Hex,Dec,Bin).

When you see 192.168.10.1, printed in Hex comes:

C0.a8.0a.01

If you don't writes dots comes C0a80a01: if you translate
it by a desktop calculator comes 3232238081 in dec.

Well, this does not answer to your programming question,
but it is useful to understand what you managing.

In programs you can use many ways to obtain results:

1) look on your platform and maby you find a routine.
2) use a struct/union
3) write iterative code

The struct you can use is:

#define u_char unsigned char
#define u_short unsigned short
#define u_long unsigned long

struct in_addr
{
union
{
struct { u_char s_b1,s_b2,s_b3,s_b4; } S_un_b;
struct { u_short s_w1,s_w2; } S_un_w;
u_long S_addr;
} ;
} S_un;


S_un.S_addr = gethostbyname( "fred" ) ;
printf( "%d.%d.%d.%d\n",
S_un.S_un_b.s_b4,
S_un.S_un_b.s_b3,
S_un.S_un_b.s_b2,
S_un.S_un_b.s_b1 ) ;

Be careful: in some platform b1..b4 may be changed.


Another approach is to shift bytes to go in both directs.


unsigned char ipc[4]={192,168,10,1};
unsigned long ipl=0, mask;
int i,j;

for( i=0 ; i<4 ; i++ )
{
printf( &quot;%d%c&quot;, ipc, i==3 ? '\n' : '.' ) ;
}

for( i=0 ; i<4 ; i++ )
{
j=3-i;
ipl+= ipc << (8*j) ;
}
printf( &quot;ipl=%u\n&quot;, ipl ) ;

THE reverse..

unsigned long x,y;

for( i=0 ; i<4 ; i++ )
{
j=3-i;
mask= 0xFF << (8*j) ;
y= ipl & mask ;
x = y >> 8*j ;
ipc= x ;
printf( &quot;%08x %08x %08x %3d\n&quot;, mask, y,x, ipc) ;
}

x and y vars are used to understand better the mechanism.


BYE

 

[danielhozac]

You should be able to use the inet_ntoa function. It returns the IP dot-formatted. See man inet_ntoa for more info. //Daniel