diff --git a/docs/configuration.md b/docs/configuration.md
index 8137e72..8ddf74a 100644
--- a/docs/configuration.md
+++ b/docs/configuration.md
@@ -179,13 +179,14 @@ Rules:
   outside `10000-29999` — only the overlap with the usable domain counts, and a
   fully-outside range contributes nothing.
 - Values are **auto-aligned to whole hundreds**: the low end rounds up to a
-  block start and the high end rounds down then extends to `+99`, so the
-  effective range always ends in `...99`, never `...00`. For example
-  `10001-29998` becomes `10100-29999`.
+  block start and the high end rounds down to a block end (`...99 <= B`), so the
+  effective range always ends in `...99`, never `...00`, and never spills past
+  the requested upper bound. For example `10001-29998` becomes `10100-29899`,
+  and `10000-15000` becomes `10000-14999`.
 - At least one whole 100-port block must remain (the value is rejected
   otherwise) — a range that cannot host even one container is an error.
 - Capacity follows the ranges: `10000-29999` allows **200** containers;
-  `10000-20000, 25000-30000` allows **151**; and so on. The interactive prompt
+  `10000-20000, 25000-30000` allows **150**; and so on. The interactive prompt
   echoes the computed capacity for confirmation.
 - A narrowed set reserves far fewer host user ports, so an install's
   port-occupancy check scans only the configured ranges — not the whole
diff --git a/scripts/00-ip-ask.sh b/scripts/00-ip-ask.sh
index e2dfb50..618fc24 100644
--- a/scripts/00-ip-ask.sh
+++ b/scripts/00-ip-ask.sh
@@ -315,8 +315,8 @@ PY
 # comma-separated) and the container capacity on one line ("canon|cap"). Exit 0
 # only when the value yields at least one usable 100-port block. Mirrors
 # cfg.ParseUserPorts: low end rounds up to a block start, high end rounds down
-# then +99 (never ends in ...00); ranges outside 10000-29999 are clamped, and
-# fully-outside ranges contribute nothing.
+# to a block end <= b (never ends in ...00, never exceeds user's b); ranges
+# outside 10000-29999 are clamped, and fully-outside ranges contribute nothing.
 normalize_user_ports(){
   python3 - "$1" <<'PY'
 import sys
@@ -337,7 +337,7 @@ def norm(s):
         if a > b:
             continue
         lo = ((a + 99) // 100) * 100
-        hi = (b // 100) * 100 + 99
+        hi = ((b + 1) // 100) * 100 - 1
         if lo > hi:
             continue
         out.append([lo, hi])
diff --git a/src/internal/cfg/cfg.go b/src/internal/cfg/cfg.go
index 0847f36..c2de403 100644
--- a/src/internal/cfg/cfg.go
+++ b/src/internal/cfg/cfg.go
@@ -386,9 +386,8 @@ func LegacySlotRangeToUserPorts(slotRange string) (string, error) {
 //   - Lo/hi may extend outside [UserPortBase, UserPortMax] — only the overlap
 //     with the usable domain counts (a range fully outside contributes nothing).
 //   - Each range is aligned inward to whole hundreds: the low end rounds UP to
-//     a block start, the high end rounds DOWN to a block start then extends to
-//     +99 (so the effective range always ends in ...99, never ...00). A range
-//     narrower than one block is dropped.
+//     a block start, the high end rounds DOWN to a block end (never exceeding
+//     b, ending in ...99). A range narrower than one block is dropped.
 //   - Overlapping/adjacent ranges are merged, so capacity is never double-counted.
 //   - At least one usable 100-port block must remain, or the value is rejected
 //     (a range that cannot host even one container is an error).
@@ -421,9 +420,9 @@ func ParseUserPorts(s string) ([]PortRange, error) {
 		if a > b {
 			continue
 		}
-		// Align inward: lo up to a block start, hi down to a block start then +99.
+		// Align inward: lo rounds UP to a block start, hi rounds DOWN to a block end <= b.
 		lo := ((a + PortsPerUser - 1) / PortsPerUser) * PortsPerUser
-		hi := (b/PortsPerUser)*PortsPerUser + PortsPerUser - 1
+		hi := ((b + 1) / PortsPerUser) * PortsPerUser - 1
 		if lo > hi {
 			continue // narrower than one block
 		}
@@ -448,7 +447,7 @@ func ParseUserPorts(s string) ([]PortRange, error) {
 }
 
 // CanonicalUserPorts renders parsed ranges as their canonical comma-separated
-// string form ("10000-20099, 25000-29999"). Returns the raw input unchanged if
+// string form ("10000-19999, 25000-29999"). Returns the raw input unchanged if
 // it does not parse.
 func CanonicalUserPorts(s string) string {
 	rs, err := ParseUserPorts(s)
diff --git a/src/internal/cfg/cfg_test.go b/src/internal/cfg/cfg_test.go
index 7e322d5..20c4bbb 100644
--- a/src/internal/cfg/cfg_test.go
+++ b/src/internal/cfg/cfg_test.go
@@ -266,17 +266,19 @@ func TestParseUserPorts(t *testing.T) {
 		canon string // canonical form; "" = must be rejected
 	}{
 		{"10000-29999", 200, "10000-29999"},                           // default full range
-		{"10000-20000", 101, "10000-20099"},                           // hi rounds down then +99 (never ends in 00)
-		{"10001-29998", 199, "10100-29999"},                           // inward-aligned both ends
+		{"10000-20000", 100, "10000-19999"},                           // hi rounds down to block end <= b (never exceeds b)
+		{"10000-15000", 50, "10000-14999"},                            // boundary test: 15000 rounds down to 14999, not 15099
+		{"10001-29998", 198, "10100-29899"},                           // inward-aligned both ends (29998 rounds down to 29899)
+		{"10001-29999", 199, "10100-29999"},                           // inward-aligned low end only
 		{"10001-10099", 0, ""},                                        // narrower than one block
 		{"10000-10099", 1, "10000-10099"},                             // exactly one block
-		{"10000-20000, 25000-30000", 151, "10000-20099, 25000-29999"}, // discontiguous
-		{"10000-15000, 14000-20000", 101, "10000-20099"},              // overlapping → merged
-		{"5000-15000", 51, "10000-15099"},                             // lo below domain clamps to 10000
+		{"10000-20000, 25000-30000", 150, "10000-19999, 25000-29999"}, // discontiguous
+		{"10000-15000, 14000-20000", 100, "10000-19999"},              // overlapping → merged
+		{"5000-15000", 50, "10000-14999"},                             // lo below domain clamps to 10000
 		{"20000-40000", 100, "20000-29999"},                           // hi above domain clamps to 29999
-		{"10000-10000", 1, "10000-10099"},                             // single port still yields its block
+		{"10000-10000", 0, ""},                                         // single port has no whole block
 		{"5000-6000", 0, ""},                                          // fully outside domain
-		{"10000-20000, 21000-21099", 102, "10000-20099, 21000-21099"}, // adjacent not merged (gap 100)
+		{"10000-20000, 21000-21099", 101, "10000-19999, 21000-21099"}, // adjacent not merged (gap 100)
 	}
 	for _, c := range cases {
 		rs, err := ParseUserPorts(c.in)
@@ -329,8 +331,8 @@ func TestUserPortCount(t *testing.T) {
 		t.Errorf("default UserPortCount() = %d, want 200", n)
 	}
 	cfg.Net.UserPorts = "10000-20000, 25000-30000"
-	if n := cfg.UserPortCount(); n != 151 {
-		t.Errorf("UserPortCount() = %d, want 151", n)
+	if n := cfg.UserPortCount(); n != 150 {
+		t.Errorf("UserPortCount() = %d, want 150", n)
 	}
 	cfg.Net.UserPorts = "garbage"
 	if n := cfg.UserPortCount(); n != 200 {
@@ -346,8 +348,8 @@ func TestFillAutoUserPortsEnv(t *testing.T) {
 	if err := c.FillAuto(); err != nil {
 		t.Fatalf("FillAuto: %v", err)
 	}
-	if c.Net.UserPorts != "10000-20099, 25000-29999" {
-		t.Errorf("user_ports = %q, want canonical 10000-20099, 25000-29999", c.Net.UserPorts)
+	if c.Net.UserPorts != "10000-19999, 25000-29999" {
+		t.Errorf("user_ports = %q, want canonical 10000-19999, 25000-29999", c.Net.UserPorts)
 	}
 
 	// A bad env value must fail loudly, not silently default.