Re: Matching IP scopes.

Sandman said the following on 27/02/2006 20:27:
In article <mYIMf.60881$mf2.26167@xxxxxxxxxxxxxxxxxxxx>,
Oli Filth <catch@xxxxxxxxxxxxxx> 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
.

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