inet hosts question

Matthew Seaman m.seaman at
Fri Nov 14 08:43:03 PST 2008

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.

> for your example.
> and yes this is a class B (not all /16s are though.)
> the /x notation is called CIDR (classless interdomain routing.)

Class C surely? 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 
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 -- is a Class A address
by this scheme.



Dr Matthew J Seaman MA, D.Phil.                   7 Priory Courtyard
                                                  Flat 3
PGP:     Ramsgate
                                                  Kent, CT11 9PW

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