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