Section 6.1: tier applicability by segment type

The segment tiering from the Introduction rework belongs with
Applicability, which previously said only that hosts without
IPv6 are out of scope. The test is the one 1.1 sets up: does
lateral IPv4 exist on the segment, and can it be enumerated.

- Access and hosting: lateral IPv4 suppressed by design, so the
  subnet only ever located the gateway and removing it costs
  nothing that was in use.
- Enterprise and campus: a managed estate, so on-link IPv4 can
  be inventoried rather than guessed at, and the genuinely
  IPv4-only remnants are already statically administered and can
  be enclaved.
- Residential: out of scope. Consumer devices carry unauditable
  local-IPv4 assumptions and the home segment is the case where
  hosts really do address each other over IPv4.
- Segments where a default gateway is not meaningful: also out
  of scope, the IXP peering LAN being canonical. A mechanism for
  resolving the way off a segment has nothing to offer one that
  is nothing but exits.

Closes #12

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: remcovm

draft-ietf-intarea-ipv6-resolved-gateway.md

@@ -106,7 +106,7 @@ indefinitely.
 This document addresses that situation by separating two things
 that have long been treated as one.
 
-## Two Kinds of IPv4
+## Two Kinds of IPv4 {#two-kinds}
 
 The IPv4 that applications require is reachability to and from
 everything beyond the first hop: the public IPv4 Internet, and
@@ -714,6 +714,38 @@ Hosts without a functional IPv6 implementation on the relevant
 interface cannot perform Neighbor Discovery and are outside
 the scope of this document.
 
+Segment types differ in how well they suit this mechanism, and
+the distinction follows the analysis in {{two-kinds}}: what
+matters is whether IPv4 traffic lateral to the segment exists,
+and whether it can be enumerated.
+
+Access and hosting segments are the clearest case. Lateral IPv4
+traffic between subscribers or between tenants is generally
+suppressed by design, so the on-link IPv4 subnet serves only to
+locate the gateway. Removing it costs nothing that was in use.
+
+Enterprise and campus segments are a managed estate. The hosts
+and applications that genuinely require on-link IPv4 can be
+inventoried rather than guessed at, and where genuinely
+IPv4-only systems remain they are typically already statically
+administered. Such systems can be kept in a small dual-stack
+enclave under the management model they already have, while the
+rest of the segment converts.
+
+Residential segments are outside the scope of this document.
+Consumer devices and applications carry unauditable assumptions
+about local IPv4 reachability -- casting and display protocols,
+game and peripheral discovery, and similar -- and the home
+segment is the case in which hosts genuinely do address one
+another over IPv4 in ways an operator cannot enumerate.
+
+Segments on which a default gateway is not a meaningful notion
+are likewise out of scope. An Internet exchange peering LAN is
+the canonical example: there is no default gateway, every
+destination is a peer, and all traffic is lateral by design. A
+mechanism concerned with resolving the way off a segment has
+nothing to offer one that consists entirely of exits.
+
 ## Relationship to Other Transition Mechanisms
 
 This mechanism complements {{RFC8925}} (IPv6-Only Preferred