Broadcast address validation - broadcast

Question

How to validate broadcast IP address?
IP Addr: 192.168.36.226
Netmask: 255.255.255.0
Broadcast: 192.168.36.255
whether it is possible to provide any broadcast address (suppose 1.2.3.4). If yes what will be the impact on the communication to other network.

Answer 1

The IP address and netmask determine the broadcast address. You cannot use just any broadcast address, or anything requiring broadcast traffic will not work.
The netmask specifies the size of the subnet. A netmask of 255.255.255.0 is commonly represented as /24, meaning that the first 24 bits of the IP address specify the network ID, where the last 8 bits are for the individual host. 192.168.0.100/24 is a host on a network with an ID of 192.168.0.0 and a broadcast address of 192.168.0.255. The highest value available in the network is the broadcast address.
10.1.2.3/8's broadcast address is 10.255.255.255. Its decimal subnet mask is 255.0.0.0.

Answer 2

As Brad mentioned, if you have the netmask, then it's very easy to compute the broadcast address. You simply swap all the bits and do an OR with the main IP address.
Typing a mask such as 255.255.255.0 is now deprecated. Instead, you are expected to use a number of bits on the IP. For example: 192.168.36.226/24.
The number, 24, indicates the number of 1s starting in the most significant bit. This is the same principle with IPv6 which uses 128 bits instead. So those numbers are often much larger with IPv6.
One way to compute the mask for an IPv4, is to use -1 and 32 - size (which is our case is 32 - 24 = 8):
size = 24; // somehow you get this number...
...
unsigned int mask = -1;
shift = 32 - size;
mask <<= shift; // C++ operator, in C write: mask = mask << shift;
Now you can compute the broadcast address by flipping all the bits of the mask and ORing that with the IP:
broadcast_bits = ~mask;
broadcast_ip = ip | broadcast_bits;
I think that IPv6 does the same thing, but with 128 bits. Also IPv6 does not offer Multicast (IPs that match 224.0.0.0/4).
If your question was about finding the broadcast address as defined in the Interface of an Ethernet or similar network card, then the above code is not what you are interested in. Instead, you want to list the interfaces, list each IP address defined on each interface (on some systems you could have up to 65536 IPs on one interface). That list includes the broadcast IP address and the main IP address and a mask. In other words, you can do:
if(my_ip & mask == interface->ip & mask)
{
if(my_ip == interface->broadcast_ip)
{
return interface;
}
}
return nullptr;
The list of interfaces is accessed with the following:
struct ifaddrs * ifa_start(nullptr);
getifaddrs(&ifa_start);
for(struct ifaddrs * ifa(ifa_start); ifa != nullptr; ifa = ifa->ifa_next)
{
if((ifa->ifa_flags & IFF_BROADCAST) != 0
&& ifa->ifa_broadaddr != nullptr)
{
// found the broadcast address of that interface
}
}
With that information, you should be able to find the address and the mask used above. Note that all interfaces may not support the broadcasting (unlikely, I think).
For a complete implementation, see libaddr (C++). The interface implementation is found in libaddr/iface.cpp.