From: Josef 'Jupp' Schugt <jupp@...>
Date: 2003-07-03T05:19:02+09:00
Subject: Re: IP arithmetic

Saluton!

* Brian Candler; 2003-07-02, 10:46 UTC:
>     if ip_match("1.2.3.4", "1.2.3.0/24")
>       ... etc
>     end
> 
> and the efficient way of doing this would be to do bitwise operations on the
> numeric form.

Like the following?

def ipmatch(ip, net)

  unless ip  =~ /^(((1?\d|2[0-4])?\d|25[0-5])\.){3}((1?\d|2[0-4])?\d|25[0-5])$/ or
	 net =~ /^(((1?\d|2[0-4])?\d|25[0-5])\.){3}((1?\d|2[0-4])?\d|25[0-5])\/([12]?\d|3[0-2])$/
    raise ArgumentError
  end

  /^(\d+)\.(\d+)\.(\d+)\.(\d+)$/.match(ip)
  ipbin = $4.to_i + 256 * ($3.to_i + 256 * ($2.to_i + 256 * $1.to_i))
  /(\d+)\.(\d+)\.(\d+)\.(\d+)\/(\d+)/.match(net)
  rbin = $4.to_i + 256 * ($3.to_i + 256 * ($2.to_i + 256 * $1.to_i))
  s = 32 - $5.to_i
  (ipbin ^ rbin) & (~0 >> s << s) == 0
end

Some comments: The lengthy regex ensure that the IP consists of four
numbers in the 0.255 range and that the net consists of the same
followed by a slash followed by a number in the range 0..32 where
zero is the whole IPv4 Internet.

The first match assigns each number to an individual string; the next
line convert's form base 256 to base 10 using Horner's scheme (quite
efficient).

Next two lines do the same for net but additionally assign the number
of bits to an additional variable.

~0 >> s << s

generates a number of all but the last s bits equal one.

The whole meaning of

(ipbin ^ rbin) & (~0 >> s << s) == 0

is:

Find all bits that ipbin and rbin differ in, ignore all that are
allowed to be different and see if there are any left.

Port from C :->

Gis,

Josef 'Jupp' Schugt
-- 
Someone even submitted a fingerprint for Debian Linux running on the
Microsoft Xbox. You have to love that irony :).
                               -- Fyodor on nmap-hackers@insecure.org