diff --git a/decnet/mutator/ops.py b/decnet/mutator/ops.py
index 0029d00e..245bc94a 100644
--- a/decnet/mutator/ops.py
+++ b/decnet/mutator/ops.py
@@ -118,7 +118,7 @@ async def apply_add_lan(
 
     if subnet is None:
         reserved = await reserved_subnets(repo)
-        alloc = SubnetAllocator(base_prefix="172.20", reserved=reserved)
+        alloc = SubnetAllocator(base_prefix="172.16.0.0/12", reserved=reserved)
         subnet = alloc.next_free()
 
     await repo.add_lan(
diff --git a/decnet/topology/allocator.py b/decnet/topology/allocator.py
index 2749688b..7600c730 100644
--- a/decnet/topology/allocator.py
+++ b/decnet/topology/allocator.py
@@ -69,31 +69,47 @@ class IPAllocator:
 
 
 class SubnetAllocator:
-    """Hands out ``/24`` subnets under a base prefix (e.g. ``172.20``)."""
+    """Hands out ``/24`` subnets inside a parent network.
 
-    _MAX_INDEX = 256  # 172.20.0/24 .. 172.20.255/24
+    Accepted ``base_prefix`` forms:
+
+    * Full CIDR:  ``"172.16.0.0/12"`` → 4096 ``/24`` slots
+    * Legacy two-octet shorthand: ``"172.20"`` → auto-lifted to
+      ``"172.20.0.0/16"`` (256 slots), for backward compat with
+      configs written before mass-scale topologies were a thing.
+
+    The parent must be at most ``/24`` wide (i.e. its prefix length
+    must be ≤ 24); a ``/24`` base yields exactly one slot, anything
+    larger yields more.
+    """
 
     def __init__(
         self,
         base_prefix: str,
         reserved: Iterable[str] = (),
     ) -> None:
-        self._base = base_prefix.rstrip(".")
+        parent = _parse_base(base_prefix)
+        if parent.prefixlen > 24:
+            raise ValueError(
+                f"subnet base {parent.with_prefixlen} is narrower than /24; "
+                "cannot carve /24 children out of it"
+            )
+        self._parent = parent
+        # A generator over all /24 subnets of the parent. ipaddress
+        # yields them in order, so the allocator preserves the legacy
+        # "sequential-third-octet" behaviour for /16 bases. For /12
+        # bases you get second.third-octet sweep.
+        self._iter = parent.subnets(new_prefix=24) if parent.prefixlen < 24 else iter([parent])
         self._reserved: set[str] = {s for s in reserved}
-        self._cursor = 0
-
-    def _candidate(self, idx: int) -> str:
-        return f"{self._base}.{idx}.0/24"
 
     def next_free(self) -> str:
-        while self._cursor < self._MAX_INDEX:
-            subnet = self._candidate(self._cursor)
-            self._cursor += 1
+        for net in self._iter:
+            subnet = net.with_prefixlen
             if subnet not in self._reserved:
                 self._reserved.add(subnet)
                 return subnet
         raise AllocatorExhausted(
-            f"no free /24s left under {self._base}.0.0/16"
+            f"no free /24s left under {self._parent.with_prefixlen}"
         )
 
     def reserve(self, subnet: str) -> None:
@@ -103,6 +119,21 @@ class SubnetAllocator:
         return subnet not in self._reserved
 
 
+def _parse_base(base_prefix: str) -> IPv4Network:
+    """Accept either ``'a.b.c.d/n'`` or legacy ``'a.b'`` shorthand."""
+    stripped = base_prefix.strip().rstrip(".")
+    if "/" in stripped:
+        return IPv4Network(stripped, strict=False)
+    octets = stripped.split(".")
+    if len(octets) == 2:
+        return IPv4Network(f"{stripped}.0.0/16", strict=False)
+    if len(octets) == 4:
+        return IPv4Network(f"{stripped}/24", strict=False)
+    raise ValueError(
+        f"unrecognised subnet base {base_prefix!r}; expected 'x.y' or CIDR"
+    )
+
+
 # Topology statuses whose LANs still claim subnets.  torn_down is the
 # only state that releases its networks back to the pool.
 _SUBNET_CLAIMING_STATES: frozenset[str] = frozenset(
diff --git a/decnet/topology/config.py b/decnet/topology/config.py
index 1f2098ec..373dc8ea 100644
--- a/decnet/topology/config.py
+++ b/decnet/topology/config.py
@@ -30,8 +30,16 @@ class TopologyConfig(BaseModel):
     # from its LAN to a non-parent, non-child LAN.  0.0 yields a tree.
     cross_edge_probability: float = Field(default=0.0, ge=0.0, le=1.0)
 
-    # IP allocation base.  LANs get sequential /24s starting here.
-    subnet_base_prefix: str = Field(default="172.20", pattern=r"^\d{1,3}\.\d{1,3}$")
+    # IP allocation base.  LANs get sequential /24s carved out of this
+    # network.  Accepts either a full CIDR (e.g. ``172.16.0.0/12`` for
+    # 4096 slots) or the legacy two-octet shorthand ``172.20`` which
+    # auto-lifts to ``172.20.0.0/16`` (256 slots).  Default is a /12
+    # so mass-scale topologies (depth/branching trees with >256 LANs)
+    # don't exhaust the pool on first generation.
+    subnet_base_prefix: str = Field(
+        default="172.16.0.0/12",
+        pattern=r"^\d{1,3}\.\d{1,3}(\.\d{1,3}\.\d{1,3}/\d{1,2})?$",
+    )
 
     # Service selection — reuses decnet.fleet.build_deckies' randomizer.
     randomize_services: bool = Field(default=True)
diff --git a/decnet/web/router/topology/api_catalog.py b/decnet/web/router/topology/api_catalog.py
index 858640e8..74f42ee7 100644
--- a/decnet/web/router/topology/api_catalog.py
+++ b/decnet/web/router/topology/api_catalog.py
@@ -85,7 +85,10 @@ async def api_list_archetypes(
 )
 @_traced("api.topology.catalog.next_subnet")
 async def api_next_subnet(
-    base: str = Query(default="172.20", pattern=r"^\d{1,3}\.\d{1,3}$"),
+    base: str = Query(
+        default="172.16.0.0/12",
+        pattern=r"^\d{1,3}\.\d{1,3}(\.\d{1,3}\.\d{1,3}/\d{1,2})?$",
+    ),
     _viewer: dict = Depends(require_viewer),
 ) -> NextSubnetResponse:
     reserved = await reserved_subnets(repo)
diff --git a/tests/topology/test_allocator.py b/tests/topology/test_allocator.py
index 472a81a4..c0e17ebd 100644
--- a/tests/topology/test_allocator.py
+++ b/tests/topology/test_allocator.py
@@ -86,6 +86,49 @@ def test_subnet_allocator_exhaustion_raises():
         s.next_free()
 
 
+def test_subnet_allocator_accepts_cidr_base():
+    """Full-CIDR base form is equivalent to the legacy two-octet form."""
+    s = SubnetAllocator("172.20.0.0/16")
+    assert s.next_free() == "172.20.0.0/24"
+    assert s.next_free() == "172.20.1.0/24"
+
+
+def test_subnet_allocator_slash12_yields_more_than_256_slots():
+    """The whole point of widening: a /12 base must outlast a single /16."""
+    s = SubnetAllocator("172.16.0.0/12")
+    # Burn the first 256 /24s.  With a /16 base this is exhaustion; with
+    # /12 we should roll into 172.17.x.x without raising.
+    for _ in range(256):
+        s.next_free()
+    nxt = s.next_free()
+    assert nxt.startswith("172.17.")
+    assert nxt.endswith(".0/24")
+
+
+def test_subnet_allocator_slash12_total_capacity_is_4096():
+    s = SubnetAllocator("172.16.0.0/12")
+    count = 0
+    try:
+        while True:
+            s.next_free()
+            count += 1
+    except AllocatorExhausted:
+        pass
+    assert count == 4096
+
+
+def test_subnet_allocator_rejects_narrower_than_slash24():
+    with pytest.raises(ValueError, match="narrower than /24"):
+        SubnetAllocator("192.168.1.0/25")
+
+
+def test_subnet_allocator_exhausted_message_uses_parent_cidr():
+    s = SubnetAllocator("172.20.0.0/24")  # exactly one slot
+    s.next_free()
+    with pytest.raises(AllocatorExhausted, match="172.20.0.0/24"):
+        s.next_free()
+
+
 # --------------------------------------------------------------------- reserved_subnets