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This is because you are doing a using namespace std, which I believe makes the code use std::swap, rather than the swap you defined.

Try this:

#include<iostream>
#include <stdlib.h>
  
using std::cout;
using std::endl;
  
struct node
{
    int a;
    int b;
};
  
typedef struct node Node;
  
void swap(void *a,void *b)
{
    void *temp;
    temp=a;  
    a=b; 
    b=temp;
}
   
int main()
{
    Node *a1,*b1;
    a1=(Node*)malloc(sizeof(Node));
    b1=(Node*)malloc(sizeof(Node));
    a1->a=10;
    a1->b=20;
    b1->a=30;
    b1->b=40;
    cout<<a1->a<<" "<<a1->b<<endl;
    cout<<b1->a<<" "<<b1->b<<endl;
    cout << a1 << " " << b1 << endl;
    swap(a1,b1);
    cout << a1 << " " << b1 << endl;
    cout<<a1->a<<" "<<a1->b<<endl;
    cout<<b1->a<<" "<<b1->b;
}

This code is not working you forgot to add header file for malloc. You will have to add
#include<malloc.h>
in the starting of code.
after that code is working fine. Complete running code at my terminal.

#include<iostream>
#include<malloc.h>
using namespace std;
struct node{
int a,b;
};
typedef struct node Node;
void swap(void *a,void *b){
void *temp;
temp=a;
a=b;
b=temp;
}
int main() {
Node *a1,*b1;
a1=(Node*)malloc(sizeof(Node));
b1=(Node*)malloc(sizeof(Node));
a1->a=10;
a1->b=20;
b1->a=30;
b1->b=40;
cout<<a1->a<<" "<<a1->b<<endl;
cout<<b1->a<<" "<<b1->b<<endl;
swap(a1,b1);
cout<<a1->a<<" "<<a1->b<<endl;
cout<<b1->a<<" "<<b1->b;
return 0;
}

Your swap code doesn't work. Actually, the parameter passed through the function is passed by value, instead of refence, even if it's a pointer. As a result, what your code does is just to swap the two temporary pointers, which are destroyed when they're out of function scope.

I think because of void pointer. No Type casting done.

What worked for me
1.Explicitly calling swap using swap((void *)a,(void *)b);
2.Using memcpy to physically change contents of the objects.
Code below. Tested on c++ in codepad.com

//Swapping objects using void pointers
#include <iostream>
#include <stdlib.h>
using namespace std;

struct node
{
 int a,b;
};
typedef struct node Node;

void swap(void *a, void *b, int flag=1)
{
 cout<<"\n-In written function";
//CORRECT CODE
 if (flag==1)
 {
  //Physically replaces data while pointers point to same location
   cout<<"\n-Switch Values physically";
   void *temp = new Node;
   memcpy(temp,a,sizeof(Node));
   memcpy(a,b,sizeof(Node));
   memcpy(b,temp,sizeof(Node));
 }
//INCORRECT CODE
 else if (flag==2)
 {
   //Exchanges what pointers are pointing to
   cout<<"\n-Does nothing in code";
   void *temp;
   temp =a;
   a=b;
   b=temp;
 }
 else 
 {
   cout<<"\n-Wrong choice nothing done";
 }
}

int main()
{
  
  Node P,Q;
  Node *p1 = &P;
  Node *p2 = &Q;

  p1->a = 10; p1->b = 20;
  p2->a = 30; p2->b = 40;

  cout<<"\n Before Pointer p1: "<<((Node *)p1)->a<<((Node *)p1)->b;
  cout<<"\n Before Objects P : "<<P.a<<P.b;
  cout<<"\n In built swap called. Not the one we wrote. Changes what the pointers point to. Uses template to accomplish. Wise.";
  swap(p1,p2); //Calls build in function from std. No String in output.
  cout<<"\n After Pointer p1: "<<((Node *)p1)->a<<((Node *)p1)->b;
  cout<<"\n After Object P  : "<<P.a<<P.b;

  swap(p1,p2); //swaps back
  //Correct Run
  cout<<"\n\n Before Pointer p1: "<<((Node *)p1)->a<<((Node *)p1)->b;
  cout<<  "\n Before Object P  : "<<P.a<<P.b;
  swap((void *)p1,(void *)p2,1); //Explicit call to our function
  cout<<"\n After Pointer p1: "<<((Node *)p1)->a<<((Node *)p1)->b;
  cout<<"\n After Object P  : "<<P.a<<P.b;
  
  swap((void *)p1,(void *)p2,1); //swaps back
 //Incorrect Run
  cout<<"\n\n Before Pointer p1: "<<((Node *)p1)->a<<((Node *)p1)->b;
  cout  <<"\n Before Object P  : "<<P.a<<P.b;
  swap((void *)p1,(void *)p2,2); //Explicit call to our function
  cout<<"\n After Pointer p1: "<<((Node *)p1)->a<<((Node *)p1)->b;
  cout<<"\n After Object P  : "<<P.a<<P.b;

  return 0;
}

Output
//IN BUILT SWAP FUNCTION CALLED INSTEAD OF THE ONE ABOVE
Before Pointer p1: 1020
Before Objects P : 1020
In built swap called. Not the one we wrote. Changes what the pointers point to. Uses template to accomplish.Wise.
After Pointer p1: 3040
After Object P : 1020
//CORRECT RESULT
Before Pointer p1: 1020
Before Object P : 1020
-In written function
-Switch Values physically
After Pointer p1: 3040
After Object P : 3040

//SWAP BACK IGNORE
-In written function
-Switch Values physically

//INCORRECT RESULT
Before Pointer p1: 1020
Before Object P : 1020
-In written function
-Does nothing in code
After Pointer p1: 1020
After Object P : 1020

Do let me know if you have any comments.

Perfect. Interesting problem, it used std's swap and gives proper output. But actual intension of writing swap function is not being used.

Thanks for posting this problem.

C++ std::swap function gets called, so values are swapped because std::swap function actually swaps these pointers.

I think this is just the issue of right using pointers. Just change the relative code to:
void swap(void **a,void **b){
void *temp;
temp=*a;
*a=*b;
*b=temp;
}

And then use.
swap(&a1,&b1);

In your swap function pass parameter by ref or in C pass pointer to pointer to get the correct result.