socket编程，发送UDP广播数据，抓包检查时，发现目的MAC地址不是全0xff，而是网关的MAC地址，哪位知道这是为什么？

程序如下：

    int socket_fd;
        struct sockaddr_in my_addr,user_addr;
        int so_broadcast=1;

            my_addr.sin_family=AF_INET;
            my_addr.sin_port=htons(SERVPORT);
            my_addr.sin_addr.s_addr=htonl(INADDR_ANY);
            bzero(&(my_addr.sin_zero), 8 );

            user_addr.sin_family=AF_INET;
            user_addr.sin_port=htons(CLIENTPORT);
            user_addr.sin_addr.s_addr=inet_addr("255.255.255.255");
            bzero(&(user_addr.sin_zero), 8 );
            if((socket_fd=(socket(AF_INET,SOCK_DGRAM,0)))==-1) 
                    {
                printf("socket:%s:%d\n", strerror( errno ), errno );
                exit(1);
            }
            setsockopt(socket_fd,SOL_SOCKET,SO_BROADCAST,&so_broadcast,sizeof(so_broadcast));
            if((bind(socket_fd,(struct sockaddr *)&my_addr, sizeof(struct sockaddr)))==-1) 
                    {
                    printf("bind:%s:%d\n", strerror( errno ), errno );
                    exit(1);
               }

            char buf[8]={3, 0, 4, 192, 168, 1, 101, 0};

           while( 1 )
           { 

            //sleep 1 second
            sleep( 1 );

            //send broadcast message to inform client the address of server                
            if( sendto(socket_fd,buf, 7 ,0,(struct sockaddr *)&user_addr,sizeof(user_addr) ) == -1)
            {
                    printf("sendto():%s:%d\n", strerror( errno ), errno );
            }

        }