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I was inspecting my apache access logs(I use default combined log format) and I came a cross a wired entry - - [22/Oct/2012:18:15:20 +0200] "GET /some site resources HTTP/1.1" 404 514 "-" "facebookexternalhit/1.0 (+"

As u see, this query come from a facebook robot that extract objects from site when somebody post a link. What I find weird is the logged ip address :

Does anybody know how is that possible ?

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up vote 2 down vote accepted

That's a perfectly valid host IP address.

As you can see from a whois query on that address, it's part of a /19 subnet allocated to Facebook. They can subnet that allocation however they choose.

Thinking in terms of a "standard" /24 network, the .0 address would not be a host address, but it's perfectly valid in subnets larger than /24.

Please see this question for an exhaustive explanation of subnetting in the IPv4 world.

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Unless my logic is off, a x.x.x.0 host address would only be valid in a network of /23 or larger. – USD Matt Oct 22 '12 at 16:26
@USDMatt - yes, that's correct. I've clarified my answer. – EEAA Oct 22 '12 at 16:44

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