Determining if two IP adresses are on same subnet - is it leading or trailing 0s get dropped from IP address?

Question

I understand if two IP addresses are AND\'d with a subnet mask if the result is the same then they are on the same network. If the result is different then they are on diffe

Answer 1

IPv4 addreses are written as four ordinary decimal numbers (each fitting into a byte) separated by dots.

The ordinary decimal number 1 is "one", which can also be written 001. However 100 is a different number, namely "a hundred".

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The AND operation is always a bitwise AND, so to understand it you must first see how the dotted-decimal address and netmask corresponds to a raw binary 32-bit address:

        126   .    127   .     0    .    1
     01111110   01111111   00000000   00000001

        255   .    128   .     0    .    0
AND  11111111   10000000   00000000   00000000
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     01111110   00000000   00000000   00000000
        126   .      0   .     0    .    0

So 126.127.0.1 with netmask 255.128.0.0 is in subnet 126.0.0.0/9

In software one usually stores IPv4 addresses in a single 32-bit variable -- so 126.127.0.1 is 01111110011111110000000000000001 binary (which also encodes 2122252289 decimal, except that nobody ever cares what the decimal value of a 32-bit IP address is), and it is converted to dotted-decimal only when it needs to be shown to human users. This representation is what glglgl decribes as multiplying by 256 several times.

If you also have the netmask in a 32-bit variable (11111111100000000000000000000000 binary or 4286578688 decimal), you can AND them in a single machine instruction to get the network address 01111110000000000000000000000000.