我需要帮助来了解GetIpAddrTable和接口广播地址

Question

I'm trying to use GetIpAddrTable ( https://docs.microsoft.com/en-us/windows/win32/api/iphlpapi/nf-iphlpapi-getipaddrtable ) to gather the interface addresses but I'm struggling to understand the response for dwBCastAddr .

For my network the broadcast address for the interface should be 192.168.1.255 but all that dwBCastAddr returns is 1 and I have no idea what I'm supposed to do with that.

The documentation for the data structure ( https://docs.microsoft.com/en-us/windows/win32/api/ipmib/ns-ipmib-mib_ipaddrrow_w2k ) says the following:

dwBCastAddr

Type: DWORD

The broadcast address in network byte order. A broadcast address is typically >the IPv4 address with the host portion set to either all zeros or all ones.

The proper value for this member is not returned by the GetIpAddrTable function.

What are they trying to say here?

This is the line used to get the broadcast address:

IPAddr.S_un.S_addr = (u_long)pIPAddrTable->table[i].dwBCastAddr;

I'm just trying to figure out how to get 192.168.1.255 out of this.

Any help would be appreciated.

Answer 1

The proper value for this member is not returned by the GetIpAddrTable function.

It's saying that if you are using GetIpAddrTable , the value of dwBCastAddr is wrong and useless. So don't use it.

Instead you can calculate the broadcast address yourself from the IP address and subnet mask like so:

 IPAddr.S_un.S_addr=  pIPAddrTable->table[i].dwAddr | ~pIPAddrTable->table[i].dwMask;
 printf("\tBroadCast[%d]:    \t%s\n", i, inet_ntoa(IPAddr));

See How can I determine network and broadcast address from the IP address and subnet mask? for more details.