The Hexadecimal System
Example of comparing an IP against a bitmask in PHP:
$BlockedNetworks = array(ip2long("192.168.0.0"),
ip2long("10.0.0.0"));
$BlockRange = ip2long("255.255.0.0");
$VisitIP = ip2long($_SERVER['REMOTE_ADDR']);
foreach ($BlockedNetworks as $BN) {
if (($VisitIP & $BlockRange) == $BN)
exit(); // or redirect or whatever you want.
}
In the example above, any IP in the 192.168.x.x and 10.0.x.x ranges will be matched. The 255 numbers you see in masks represent
all 8 bits in an octet being turned on, so comparing against it will always reserve the octet bits without turning them off. Meaning
192.168.1.10 & 255.255.0.0 equals 192.168.0.0 because the last two octets will be turned off during comparison.
How Does It Work?
Bitwise AND(& operator). It's what network masks are all about. It's the bitwise operator used in programming if you need to turn off
a bit, by comparing to a off-bit/0.
1 & 1 = 1
1 & 0 = 0
0 & 1 = 0
Here's a turned off byte/octet:
[0 0 0 0 0 0 0 0] Which represents:
[128 64 32 16 8 4 2 1] in decimal values when individually turned on.
To get 192 we need to turn on 128 + 64. The octet now looks like this:
[1 1 0 0 0 0 0 0]
Now that we know how to convert binary into decimal, let's do the whole address:
[11000000].[10101000].[00000001].[00001010] = 192.168.1.10
Let's put it on top of the mask and compare bits one by one:
[11000000].[10101000].[00000001].[00001010] (192.168.1.10)
& [11111111].[11111111].[00000000].[00000000] (255.255.0.0)
= [11000000].[10101000].[00000000].[00000000] (192.168.0.0)
As you see, when comparing 1 AND 1 it will stay 1, while 0 turns it off. Hence why 192.168.1.10 will equal 192.168.0.0 when using
bitwise AND comparison. This should give a deeper understanding of network masks works in general and how they work in normal
networking.
A Quick Note on The Hexadecimal System
It's just a number system with a base of 16 instead of 2 and 10 (0-9 and a-f/A-F). E.g. a value like 0xABCD consists of a prefix (0x) to
tell the parser it's actually a hexadecimal value, and the value itself. Since A follows after 9, its value is 10 - and so on up until
15(F). So you got 10, 11, 12 and 13.
These are actually just easier ways of representing binary and decimal values since the base of
16 is bigger. A B C and D represents groups of 4 binary bits with a base of 2, and those bits represents the same decimal values with a
base of 10 as in the binary system. Knowing this, a convertion between them is always possible even if you choose to do it through
math or just value mapping/representation.
A Summary Illustration:
Hexidecimal: 0xABCD
= Binary: 0x[1010][1011][1100][1101] > [8+2=10][8+2+1=11][8+4=12][8+4+1=13].
= Decimal: 0x[32768+0+8192+0][2048+0+512+256][128+64+0+0][8+4+0+1] = 43981.