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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This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enigBAEC01202EA545843180F478 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: quoted-printable 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 --------------enigBAEC01202EA545843180F478 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.9 (FreeBSD) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iEYEAREIAAYFAkkdqo8ACgkQ8Mjk52CukIxtngCfVgMF1YBvKRAaeHaQrtiCkTWk rKIAn2DqHp3cjIdzwM9nC9bHjmiF5eW+ =YNOA -----END PGP SIGNATURE----- --------------enigBAEC01202EA545843180F478--
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