Posted by Oli Filth on 02/27/06 22:05

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.


--
Oli

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