|
|
Posted by Oli Filth on 02/27/06 22:42
Sandman said the following on 27/02/2006 20:27:
> In article <mYIMf.60881$mf2.26167@newsfe6-win.ntli.net>,
> Oli Filth <catch@olifilth.co.uk> wrote:
>
>> Sandman said the following on 27/02/2006 19:59:
>>> So, I have this list of valid IP scopes, in the form below. How do I
>>> match if $_SERVER[REMOTE_ADDR] is covered in any of these scopes?
>>>
>>> 193.11.120.0/21
>>> 193.11.128.0/24
>>> 193.11.129.0/24
>>> 193.11.130.0/24
>>> 193.11.131.0/24
>>>
>>>
>>> First, I don't really know how to interprete the "/24" ending. I am
>>> guessing that "193.11.131.0/24" means "193.11.131.X to 193.11.131.Y"
>>> or something, but what? When that's translated to something useful,
>>> how do I match IP numbers reliably?
>> The /XX represents the length of the subnet mask in bits, so your hunch
>> is pretty much correct.
>>
>> An IP address matches a given network IP address if:
>>
>> (Address ^ Mask) == (NetAddress ^ Mask)
>>
>> where Mask = 11111...0000, the number of ones given by the /XX.
>
> Ok, but how do I calculate it? Bits you say, but how do I translate 24
> bits to addresses? For instance, the first line above, what addresses
> does it cover?
>
An IP address is a 32-bit quantity, which can be represented as A.B.C.D,
where the actual 32-bit value is given by:
Y = (A * 2^24) + (B * 2^16) + (C * 2^8) + D
[I'm using ^ to represent "to the power of" in this case.]
So convert your test address and network address to this form, either
directly or by using the ip2long() function.
Then form your subnet mask as a 32-bit value.
[HINT: (2^X - 1) = (1000....000 - 1) = 111....111]
Then test the equality of the expression I originally posted, noting
that I got it wrong, and it should be:
(Address & Mask) == (NetAddress & Mask)
--
Oli
Navigation:
[Reply to this message]
|