/* Interrupt Vector Definitions*/
#define INT0      __vector_1
#define T1OVF     __vector_6
#define SIGNAL    __attribute__ ((signal))

/* AVR IO Registers Definitions */
#define PORTB  (*(volatile unsigned char *)(0x18 + 0x20))  /* Data register, Port B */
#define DDRB   (*(volatile unsigned char *)(0x17 + 0x20))  /* Data Direction Register, Port B */
#define DDRD   (*(volatile unsigned char *)(0x11 + 0x20))  /* Data Direction Register, Port D */
#define PIND   (*(volatile unsigned char *)(0x10 + 0x20))  /* Input Pins, Port D */
#define SREG   (*(volatile unsigned char *)(0x3F + 0x20))  /* Status Register */
#define GIMSK  (*(volatile unsigned char *)(0x3B + 0x20))  /* General Interrupt Mask Register */
#define TIMSK  (*(volatile unsigned char *)(0x39 + 0x20))  /* Timer/Counter Interrupt Mask Register */
#define MCUCR  (*(volatile unsigned char *)(0x35 + 0x20))  /* MCU General Control Register */
#define TCCR1B (*(volatile unsigned char *)(0x2E + 0x20))  /* Timer/Counter1 Control Register B */

//******************************* Aus dem Internet, soll funktionieren *****************
void INT0 (void) SIGNAL; 
void T1OVF () SIGNAL;

int main(void)
{
    DDRB  =  0xff;     // Set PORTB to output
    PORTB =  0xff;     // Turn off LEDs
    SREG  =  0x80;     // Enable Interrupts
    GIMSK =  0x40;     // Enable External Interrupt 0
    TIMSK |= 0x80;     // Enable Timer1 Overflow Interrupt
    MCUCR =  0x02;     // Interrupt 0 generated by falling edge

    while(1);

    return 0;
}

/* IRQ 0 Interrupt Handler */
void INT0 (void) 
{
    if(MCUCR == 0x03) {		// If IRQ0 is rising edge (button released)
	MCUCR  = 0x02;		// Set IRQ0 to falling edge
	PORTB  = 0xff;
	TCCR1B = 0x00;		// Disable TIMER1 
    } else {			// IRQ1 is falling edge (button pressed)
	MCUCR  = 0x03;
	TCCR1B = 0x04;		// Start TIMER1 @ CLK/256
    }
}

/* Timer1 Interrupt Handler */
void T1OVF () {
    PORTB = 0xaa;
}

//******************************************************
Mein Code mit den Fehlermeldungen:

#define GATEWAY_PTR		(*(volatile unsigned char *)(I2CHIP_BASE + 0x80))
#define SUBNET_MASK_PTR	(*(volatile unsigned char *)(I2CHIP_BASE + 0x84))
#define SRC_HA_PTR	    (*(volatile unsigned char *)(I2CHIP_BASE + 0x88))
#define SRC_IP_PTR		(*(volatile unsigned char *)(I2CHIP_BASE + 0x8E))


unsigned char mac[6]= {0x22,0xa0,0xb1,0xc2,0xd3,0xe4} ;
	unsigned char ip[4]= {192,168,0,3};
	unsigned char gw[4]= {192,168,1,1};
	unsigned char sn[4]= {255,255,255,0};
    char i;
//setMACAddr(ip);     // Setup MAC
    for (i=0; i<6; i++)
       (SRC_HA_PTR +i) = mac[i];	//warning: array subscript has type `char', error: invalid lvalue in assignment
	printf("\nMAC fertig!");
//setIP(ip);      // Setup source IP
    for (i=0; i<4; i++)
        (SRC_IP_PTR + i) = ip[i];	//warning: array subscript has type `char', error: invalid lvalue in assignment
	printf("\nIP fertig!");
//setgateway(ip);     // Setup gateway address
    for (i=0; i<4; i++)
        (GATEWAY_PTR + i) = gw[i];	//warning: array subscript has type `char', error: invalid lvalue in assignment
	printf("\nGW fertig!");
//setsubmask(ip);     // Setup subnet mask 
    for (i=0; i<4; i++)
        (SUBNET_MASK_PTR + i) = sn[i];	//warning: array subscript has type `char', error: invalid lvalue in assignment
	printf("\nSN fertig!");