Stupid Question maybe?

Tue Dec 18 11:45:25 UTC 2018

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
>