TLDR

For years a combination of gaps in policy, disjointed standards development, hoarding behaviour, and administrative laxity, led to substantial wastage of IPv4 address space that persists to this day, in part because multiple independent website tenants sharing an IP address was (and often still is) difficult.

Glossary

DNS: Domain Name System, how computers discover (or resolve) each other's numeric IP addresses from symbolic hierarchical names.

HTTP WG: HTTP Working Group, the standards committee(s) responsible for defining the application-layer protocols by which a web browser talks to a web server. Under the auspices of the IETF.

IETF: Internet Engineering Task Force, the standards organization for the Internet. Most famous for being the entity that publishes TCP/IP and the cherries on top.

LIR: Local Internet Registry, an entity that applies to be the holder of block-allocated IP address space. Typically ISPs and hosting companies that assign it for their use or onward customer use.

RIPE: The European peak body, a Regional Internet Registry (RIR), responsible for (amongst many other things) allocating IP space to the LIRs, ultimately under license from the global steward IANA. Sibling of ARIN, LACNIC, AFRINIC, APNIC (North & South America, Africa, Asia-Pacific respectively).

SNI: Server Name Identification. Before SNI was introduced to SSL/TLS, the only way to tell which HTTPS origin was being requested, was by server IP address.

SRV: A specific type of record in the DNS that allows lookup of a service (e.g. IMAP mail, XMPP messaging, LDAP directories) for a given domain name, to return a set of hostname(s) that actually provide that service, and the ports on which it is available. This enables multi-tenant services on shared IP addresses and thereby conserves IP space.

/19 network prefix: A large chunk of IPv4 address space, corresponding to 8192 unique IPv4 addresses. Refers to the first 19 bits of the block of (32 bit) IPv4 addresses being allocated. Back in the 90s a /19 was a common granularity of allocation from RIR to LIR and could be granted with a low bar to justification. Mathematically, 2^(32-19) = 8192.

Address record: A specific type of record in the DNS that maps a hostname to an IP address. Technically referred to as an "A record" for IPv4 addresses, or an "AAAA" record for IPv6 addresses which are literally 4x the binary length.

Allocation, Assignment, and Announcement: IP address space is allocated in blocks by RIRs to LIRs, who then assign smaller parcels of it to specific purposes such as retail hosting and internet access, and announce it at internet exchanges and to their peers for actual traffic exchange. Very often, the quantity of assignment failed to justify a large allocation, leaving space unassigned i.e. unused, and in some cases unannounced, but still hoarded, with incentives for hoarding becoming perversely stronger as the addresses ran out and the policy screws started to tighten ca.2005-2011.

Apex: in "example.com" it is the "example.com" rather than "www.example.com". For marketing purposes, almost every entity wants their zone apex to be directly reachable as a website, which means placing an address record at the apex. This excludes using the apex address record for any other service but HTTP, which I have always regarded as downright antisocial. Since alias records (a.k.a CNAME records) aren't permitted at the apex, it also makes DNS management harder when you have frequent changes to make, as in the highly dynamic world of cloud-based services, or even just want to point your website at a third party hosting service without having to edit your zones whenever their IP addresses change.

Normative: A formal declaration in one technical standard (e.g. SMTP) that it depends in whole or in part on another standard (e.g. DNS) for full specification. It is unusual for a TCP/IP-based IETF protocol standard to omit DNS as normative or otherwise specify how they use the DNS for discovery, but HTTP has always been super vague about it.

Origin: Fancy word for your actual website. Or in the presence of complications like reverse proxies/load balancers/CDNs etc, the front of your service stack from the point of view of a web browser.