If you want the full historical definition, blow the dust off RFC791, and open your hymnals to section 2.3.

"Addresses are fixed length of four octets (32 bits). An address

begins with a network number, followed by local address (called the

"rest" field). There are three formats or classes of internet

addresses: in class a, the high order bit is zero, the next 7 bits

are the network, and the last 24 bits are the local address; in

class b, the high order two bits are one-zero, the next 14 bits are

the network and the last 16 bits are the local address; in class c,

the high order three bits are one-one-zero, the next 21 bits are the

network and the last 8 bits are the local address."

This is depicted visually if that's your deal in RFC796.

Back in '81 this was totally fine, but times change, and CIDR / VLSM eventually made way more sense.

It's good to have at least a passing understanding of the old terminology simply because documentation for newer stuff likes to reference it, but don't get too hung up on it.

On Tue, Dec 18, 2018 at 6:47 AM Justin M. Streiner <streinerj@gmail.com> wrote:

On Mon, 17 Dec 2018, Joe wrote:

> Apologizes in advance for a simple question. I am finding conflicting
> definitions of Class networks. I was always under the impression that a
> class "A" network was a /8 a class "B" network was a /16 and a class "C"
> network was a /24. Recently, I was made aware that a class "A" was indeed a
> /8 and a class "B" was actually a /12 (172.16/172.31.255.255) while a class
> "C" is actually a /16.

As others have mentioned, IP address classes are no longer relevant,
beyond understanding how things were done in the past. Address classes
haven't been used for assignment or routing purposes for over 20 years,
but the term lives on because it keeps getting undeserved new life in
networking classes and training materials.

Classfull address assignment/routing was horribly inefficient for two main
reasons, both of which were corrected by a combination of CIDR and VLSM:

1. Assigning IP networks on byte boundaries (/8, /16, /24) was not
granular enough to allow networks to be assigned as close as possible to
actual need in many cases. If you only needed 25 addresses for a
particular network, you had to request or assign a /24 (legacy class C),
resulting in roughly 90% of those addresses being wasted.

2. Classfull routing was starting to bloat routing tables, both inside of
and between networks. If a network had a little over 8,000 IPv4 addresses
under its control, in the pre-CIDR days, that meant that they or their
upstream provider would need to announce routes for 32 individual and/or
contiguous /24s. In the post-CIDR world, under the the best
circumstances (all of their address space is contiguous and falls on an
appropriately maskable boundary like x.y.0.0 through x.y.31.0), that
network could announce a single /19. When scaled up to a full Internet
routing table, the possible efficiencies become much more obvious. The
network operator community has has to continue to grapple with routing
table bloat since then, but for different reasons.

Had CIDR, VLSM, and NAT/PAT not been implemented, we (collectively) would
have run out of IPv4 addresses many years before we actually did.

Thank you
jms

> Is this different depending on the IP segment, i.e. if it is part of a
> RC1918 group it is classed differently (maybe a course I missed?) Or aren't
> all IP's classed the same.
> I was always under the impression, /8 = A, /16 = B, /24=C, so rightly, or
> wrongly I've always seen 10.x.x.x as "A", and 192.168.x.x as "B", with
> 172.16/12 as one that just a VLSM between the two.
>
> Again, apologizes for the simple question, just can't seem to find a solid
> answer.
>
> Happy holidays all the same!
> -Joe
>