求主网络，子网掩码

下面为自己写的代码，直接根据有类网络IP地址的定义进行解析。

/*!
 *\fn			UINT32 ucRipGetNetIP()
 *\brief		get network ip address by a ip address
 *\details	
 *\param[in]	UINT32 ipAddr		the ip address
 *\return		UINT32 
 *\retval		the network ip address
 *\author		NieYong <NieYong@tp-link.net>
 */
UINT32 ucRipGetNetIP(UINT32 ipAddr)
{
	UINT32 net_ipAddr = 0;
	char *byte;
	char *net_byte;

	byte = (char*)&ipAddr;
	net_byte = (char*)&net_ipAddr;

	/* get the network ip address  */
	if(0x00 == (0x80 & (*byte)))		/* 1~127 */
	{
		*(net_byte) = *(byte);
	}
	else if(0x00 == (0x40 & (*byte)))		/* 128~191 */
	{
		*(net_byte) = *(byte);
		*(net_byte + 1) = *(byte + 1);
	}
	else if(0x00 == (0x20 & (*byte)))		/* 192~223 */
	{
		*(net_byte) = *(byte);
		*(net_byte + 1) = *(byte + 1);
		*(net_byte + 2) = *(byte + 2);
	}
	else
	{
		*(net_byte) = *(byte);
		*(net_byte + 1) = *(byte + 1);
		*(net_byte + 2) = *(byte + 2);
		*(net_byte + 3) = *(byte + 3);
	}
	
	return net_ipAddr;
}

下面是使用in.h中的宏重写的版本。

/*!
 *\fn			UINT32 swRipNetAddr()
 *\brief		get network address part of ip address
 *\details	
 *\param[in]	UINT32 ipAddr		the ip address
 *\return		UINT32 
 *\retval		the network  address part
 *\author		NieYong <NieYong@tp-link.net>
 */
UINT32 swRipNetAddr(UINT32 ipAddr)
{
	/* get the network  address part  */
	if(IN_CLASSA(ipAddr))		/* 1~127 */
	{
		return (ipAddr & IN_CLASSA_NET);
	}
	else if(IN_CLASSB(ipAddr))		/* 128~191 */
	{
		return (ipAddr & IN_CLASSB_NET);
	}
	else if(IN_CLASSC(ipAddr))		/* 192~223 */
	{
		return (ipAddr & IN_CLASSC_NET);
	}
	else
	{
		return (ipAddr);
	}
}

在in.h文件中，宏的定义如下：

/*
 * Definitions of bits in internet address integers.
 * On subnets, the decomposition of addresses to host and net parts
 * is done according to subnet mask, not the masks here.
 */
#define	IN_CLASSA(i)		(((u_int32_t)(i) & 0x80000000) == 0)
#define	IN_CLASSA_NET		0xff000000
#define	IN_CLASSA_NSHIFT	24
#define	IN_CLASSA_HOST		0x00ffffff
#define	IN_CLASSA_MAX		128

#define	IN_CLASSB(i)		(((u_int32_t)(i) & 0xc0000000) == 0x80000000)
#define	IN_CLASSB_NET		0xffff0000
#define	IN_CLASSB_NSHIFT	16
#define	IN_CLASSB_HOST		0x0000ffff
#define	IN_CLASSB_MAX		65536

#define	IN_CLASSC(i)		(((u_int32_t)(i) & 0xe0000000) == 0xc0000000)
#define	IN_CLASSC_NET		0xffffff00
#define	IN_CLASSC_NSHIFT	8
#define	IN_CLASSC_HOST		0x000000ff

#define	IN_CLASSD(i)		(((u_int32_t)(i) & 0xf0000000) == 0xe0000000)
#define	IN_CLASSD_NET		0xf0000000	/* These ones aren't really */
#define	IN_CLASSD_NSHIFT	28		/* net and host fields, but */
#define	IN_CLASSD_HOST		0x0fffffff	/* routing needn't know.    */
#define	IN_MULTICAST(i)		IN_CLASSD(i)