函数IsLAN可以直接使用，其中硬编码的计算来自下边第二段代码

BOOL IsLAN(const struct sockaddr FAR * lpTo)
{
    int ip = ((sockaddr_in*)lpTo)->sin_addr.s_addr;
    if (ip >= 0x0a000000 && ip <= 0x0affffff)      // A类地址
        return TRUE;
    else if (ip >= 0xac100000 && ip <= 0xac1fffff) // B类地址
        return TRUE;
    else if (ip >= 0xc0a80000 && ip <= 0xc0a8ffff) // C类地址
        return TRUE;
    else if (ip == 0x7f000001)                     // 127.0.0.1
        return TRUE;
    else
        return FALSE;
}

这个代码是我用来计算各类地址起始的测试

#include "stdafx.h"
#include "Winsock2.h"
#include <string>
#pragma comment(lib, "ws2_32.lib")

int _tmain(int argc, _TCHAR* argv[])
{
    /*--------------------------------------------------------------------------------------------
    局域网IP地址范围                 Little Endia         network byte order       decimal
    A: 10.0.0.0-10.255.255.255      0000000a ~ ffffff0a  0x0a000000 ~ 0x0affffff  10 ~ -246
    B: 172.16.0.0-172.31.255.255    000010ac ~ ffff1fac  0xac100000 ~ 0xac1fffff  4268 ~ -57428
    C: 192.168.0.0-192.168.255.255  0000a8c0 ~ ffffa8c0  0xc0a80000 ~ 0xc0a8ffff  43200 ~ -22336
    127.0.0.1                       0x0100007f           0x7f000001
    --------------------------------------------------------------------------------------------*/

    //sockaddr_in addr;
    //addr.sin_addr = (IN_ADDR)inet_addr("10.0.0.0");

    UINT aBegin = htonl(inet_addr("10.0.0.0"));
    UINT aEnd = htonl(inet_addr("10.255.255.255"));

    UINT bBegin = htonl(inet_addr("172.16.0.0"));
    UINT bEnd = htonl(inet_addr("172.31.255.255"));

    UINT cBegin = htonl(inet_addr("192.168.0.0"));
    UINT cEnd = htonl(inet_addr("192.168.255.255"));

    printf("%08x ~ %08x\n", aBegin, aEnd);
    printf("%08x ~ %08x\n", bBegin, bEnd);
    printf("%08x ~ %08x\n", cBegin, cEnd);

    if (argc == 2) {
        int ip = htonl(inet_addr(argv[1]));
        std::string type;
        if (ip >= aBegin && ip <= aEnd)
            type = "a类ip地址";
        else if (ip >= bBegin && ip <= bEnd)
            type = "b类ip地址";
        else if (ip >= cBegin && ip <= cEnd)
            type = "c类ip地址";
        else
            type = "外网ip地址";
        printf("%s : %08x\n", type.c_str(), ip);
    }

    printf("127.0.0.1 host    byte order: %08x\n", inet_addr("127.0.0.1"));
    printf("127.0.0.1 network byte order: %08x\n", htonl(inet_addr("127.0.0.1")));

    system("pause");


    return 0;
}