>А какого типа hostaddr и mask?
>у меня просто входные данные типа
>char *ad="10.0.0.1";
>как это должно выглядеть?

int is_host_in_net(unsigned long net, unsigned long mask, unsigned long host){  
        long res,res1,res2;                                                    
        res1=net & mask;                                                        
        res2=host & mask;                                                      
        res=res1 - res2;                                                        
        if(res == 0)                                                            
                return 0;                                                      
        return 1;                                                              
}                                                                              
                                                                                
int is_host_in_net2(unsigned long net, unsigned long mask, char *host){        
        struct in_addr snet;                                                    
        inet_aton(host, &snet);                                                
        return is_host_in_net(net,mask,snet.s_addr);                            
}