Registered Class C addresses

Howard C. Berkowitz hcb at
Fri Dec 18 17:58:22 UTC 1998

At 11:56 AM -0500 12/17/98, Peter Polasek wrote:
>I apologize in advance if this is an ISP 101 question.  Is there
>any reasonable method for acquiring a registered Class C address
>these days?

ISPs have to start somewhere!  Let me suggest rephrasing the question a
bit.  No one can get class A, B, or C addresses any longer, because the
concept of "classful" addresses is dead.  Address space allocation is on a
CIDR block basis, where a class C would be equivalent to a /24, a class B
to a /16, and a class A to an /8.

If you need the equivalent of two class C blocks, your requirement is for a
/23 CIDR block.   A starting point is Hank Nussbacher's CIDR FAQ at

We are opening a data center in England and need at
>least 2 registered Class C addresses IP for the production network.
>Last time I checked, subnetted class C registered addresses were
>still available (within the Class A range), but pure Class C addresses
>were not.  We very strongly prefer to avoid subnetting addresses because
>this becomes extremely difficult maintain when the network becomes large
>(as this will).

See RFC2050, the policy for address allocation.  I doubt that any address
registry would consider a request for address space that is not subnetted
because it is inconvenient to maintain.

Take a look at my tutorial at the last NANOG meeting, which is on the NANOG
web site at  I describe
some approaches to semi-automating ISP address space management.

For a somewhat broader look, I've just published a book that deals with
address justification and planning, _Designing Address Architectures for
Routing & Switching_ (Macmillan Technical Publishing ISBN #1-57870-059-0).
Major online and physical bookstores just started making it available,
although Amazon is correcting the title.



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