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Date:      Fri, 14 Nov 2008 16:42:49 +0000
From:      Matthew Seaman <m.seaman@infracaninophile.co.uk>
To:        Vincent Hoffman <vince@unsane.co.uk>
Cc:        FreeBSD Questions <freebsd-questions@freebsd.org>, Gary Hartl <ghartl@gmail.com>
Subject:   Re: inet hosts question
Message-ID:  <491DAA89.80808@infracaninophile.co.uk>
In-Reply-To: <491D9699.5000103@unsane.co.uk>
References:  <000001c94666$5eb02360$1c106a20$@com> <491D9699.5000103@unsane.co.uk>

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Vincent Hoffman wrote:
> Gary Hartl wrote:

>> I thought I could do it by using the /class ie /32 for class c but i c=
an't
>> remember what the class delegation is for that size of pool, I think i=
t is a
>> class B.

> 192.168.0.0/16 for your example.
> and yes this is a class B (not all /16s are though.)
>=20
> 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.

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=20
Universities probably could justify a Class B allocation, but I don't=20
think any single institution or body has ever put enough machines onto=20
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=20
address space coverage. As the whole 'network class' thing was an early=20
attempt to just shave a few bytes of RAM in internet routers by not=20
having to store explicit netmasks -- an economy that was rapidly made=20
obsolete by the falling cost and increasing capacity of hardware --=20
class based allocation is now completely obsolete and we live in a=20
fully CIDR world.

Except that is, for the 'Class D' and 'Class E' (Multicast and=20
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

--=20
Dr Matthew J Seaman MA, D.Phil.                   7 Priory Courtyard
                                                  Flat 3
PGP: http://www.infracaninophile.co.uk/pgpkey     Ramsgate
                                                  Kent, CT11 9PW


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