Question

I have a variable that contains a 4 byte, network-order IPv4 address (this was created using pack and the integer representation). I have another variable, also a 4 byte network-order, subnet. I’m trying to add them together and add one to get the first IP in the subnet.

To get the ASCII representation, I can do inet_ntoa($ip&$netmask) to get the base address, but it’s an error to do inet_ntoa((($ip&$netmask)+1); I get a message like:

 Argument '\n\r&\0' isn't numeric in addition (+) at test.pm line 95. 

So what’s happening, the best as I can tell, is it’s looking at the 4 bytes, and seeing that the 4 bytes don’t represent a numeric string, and then refusing to add 1.

Another way of putting it: What I want it to do is add 1 to the least significant byte, which I know is the 4th byte? That is, I want to take the string \n\r&\0 and end up with the string \n\r&\1. What’s the simplest way of doing that?

Is there a way to do this without having to unpack and re-pack the variable?

Answer 1

What’s happening is that you make a byte string with $ip&$netmask, and then try to treat it as a number. This is not going to work, as such. What you have to feed to inet_ntoa is.

pack('N', unpack('N', $ip&$netmask) + 1) 

I don’t think there is a simpler way to do it.