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Date:      Fri, 14 Nov 2008 16:50:38 +0000
From:      Vincent Hoffman <vince@unsane.co.uk>
To:        Matthew Seaman <m.seaman@infracaninophile.co.uk>
Cc:        FreeBSD Questions <freebsd-questions@freebsd.org>, Gary Hartl <ghartl@gmail.com>
Subject:   Re: inet hosts question
Message-ID:  <491DAC5E.6070004@unsane.co.uk>
In-Reply-To: <491DAA89.80808@infracaninophile.co.uk>
References:  <000001c94666$5eb02360$1c106a20$@com>	<491D9699.5000103@unsane.co.uk> <491DAA89.80808@infracaninophile.co.uk>

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Matthew Seaman wrote:
> Vincent Hoffman wrote:
>> Gary Hartl wrote:
>
>>> I thought I could do it by using the /class ie /32 for class c but i
>>> can't
>>> remember what the class delegation is for that size of pool, I think
>>> it is a
>>> class B.
>
>> 192.168.0.0/16 for your example.
>> and yes this is a class B (not all /16s are though.)
>>
>> the /x notation is called CIDR (classless interdomain routing.)
>> http://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing
>
> Class C surely?  192.168.0.0/16 is the RFC1918 Class C reserved
> range of 256 /24 networks.
>

Doh yes indeed. no idea why i said B. not the stupidest thing i've said
today either ;)

Vince

> Yes, Class B networks were /16s, but the A, B, C... classification
> is derived from the number of leading 1's in the binary representation
> of the first octet of the address, not the netmask.  Thus
>
> Binary:                 Decimal:    Class:  Used for:
> -----------------------------------------------------------------
> 0000 0000 -- 0111 1111  (0   - 127) Class A /8 Networks
> 1000 0000 -- 1011 1111  (128 - 191) Class B /16 Networks
> 1100 0000 -- 1101 1111  (192 - 223) Class C /24 Networks
> 1110 0000 -- 1110 1111  (224 - 239) Class D Multicast
> 1111 0111 -- 1111 1111  (240 - 255) Class E Reserved, experimental
>
> Hence the first /half/ of the address space was reserved for class A
> network allocations (16777214 hosts per net) and half of the rest was
> reserved for class B allocations (65534 hosts per net).  Some large
> Universities probably could justify a Class B allocation, but I don't
> think any single institution or body has ever put enough machines onto
> the Internet to justify having a whole Class A network to themselves
> according to modern criterea.
>
> Needless to say, this was incredibly wasteful scheme in terms of
> address space coverage. As the whole 'network class' thing was an
> early attempt to just shave a few bytes of RAM in internet routers by
> not having to store explicit netmasks -- an economy that was rapidly
> made obsolete by the falling cost and increasing capacity of hardware
> -- class based allocation is now completely obsolete and we live in a
> fully CIDR world.
>
> Except that is, for the 'Class D' and 'Class E' (Multicast and
> Experimental) ranges which still exist.  It's also why the loopback
> interface is given a /8 netmask -- 127.0.0.1 is a Class A address
> by this scheme.
>
>     Cheers,
>
>     Matthew
>




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