diff --git a/TODO.md b/TODO.md
index 5ed9538..e80be3a 100644
--- a/TODO.md
+++ b/TODO.md
@@ -33,6 +33,9 @@ tested.
 
 # Completed Steps
 
+- 2026-10-27: the AS page lists its IPv4 and its IPv6 prefixes by mask length,
+  shortest first, and prefixes of one mask length by address. They used to be
+  listed by address (closes #153)
 - 2026-10-27: on `/` and `/status`, a spinner beside the status turns one frame
   each time the stats show more messages than the time before. A page that has
   not seen more messages for three refreshes shows "stale" and the time since
diff --git a/internal/server/handlers.go b/internal/server/handlers.go
index 156a47a..7c20bc1 100644
--- a/internal/server/handlers.go
+++ b/internal/server/handlers.go
@@ -1,7 +1,7 @@
 package server
 
 import (
-	"bytes"
+	"cmp"
 	"context"
 	"encoding/json"
 	"errors"
@@ -9,6 +9,7 @@ import (
 	"net/http"
 	"net/netip"
 	"runtime"
+	"slices"
 	"sort"
 	"strconv"
 	"strings"
@@ -1028,24 +1029,15 @@ func splitRoutesByIPVersion(
 	return ipv4Routes, ipv6Routes
 }
 
-// sortRoutesByNetwork sorts routes by network address, then by mask length.
-func sortRoutesByNetwork(routes []database.LiveRoute) {
-	sort.Slice(routes, func(i, j int) bool {
-		// Parse the prefixes to compare network addresses
-		ipI, netI, _ := net.ParseCIDR(routes[i].Prefix)
-		ipJ, netJ, _ := net.ParseCIDR(routes[j].Prefix)
+// sortRoutesByMaskLength sorts routes by mask length, shortest first, and
+// routes with the same mask length by network address.
+func sortRoutesByMaskLength(routes []database.LiveRoute) {
+	slices.SortFunc(routes, func(a, b database.LiveRoute) int {
+		prefixA, _ := netip.ParsePrefix(a.Prefix)
+		prefixB, _ := netip.ParsePrefix(b.Prefix)
 
-		// Compare by network address first
-		cmp := bytes.Compare(ipI.To16(), ipJ.To16())
-		if cmp != 0 {
-			return cmp < 0
-		}
-
-		// If network addresses are equal, compare by mask length
-		onesI, _ := netI.Mask.Size()
-		onesJ, _ := netJ.Mask.Size()
-
-		return onesI < onesJ
+		return cmp.Or(cmp.Compare(prefixA.Bits(), prefixB.Bits()),
+			prefixA.Addr().Compare(prefixB.Addr()))
 	})
 }
 
@@ -1212,9 +1204,8 @@ func (s *Server) handleASDetail() http.HandlerFunc {
 		// Group prefixes by IP version
 		ipv4Prefixes, ipv6Prefixes := splitRoutesByIPVersion(prefixes)
 
-		// Sort prefixes by network address
-		sortRoutesByNetwork(ipv4Prefixes)
-		sortRoutesByNetwork(ipv6Prefixes)
+		sortRoutesByMaskLength(ipv4Prefixes)
+		sortRoutesByMaskLength(ipv6Prefixes)
 
 		// Prepare template data
 		data := struct {
diff --git a/internal/server/handlers_internal_test.go b/internal/server/handlers_internal_test.go
index 620276a..78a8f83 100644
--- a/internal/server/handlers_internal_test.go
+++ b/internal/server/handlers_internal_test.go
@@ -1586,6 +1586,61 @@ func TestASDetailPageShowsBuiltInAS(t *testing.T) {
 	}
 }
 
+// TestSortRoutesByMaskLength expects the AS page's prefix lists in mask length
+// order, shortest first, and in address order within one mask length. Before
+// the fix they were in address order, so 10.1.0.0/16 came before 11.0.0.0/8
+// (https://git.eeqj.de/sneak/routewatch/issues/153).
+func TestSortRoutesByMaskLength(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name     string
+		prefixes []string
+		want     []string
+	}{
+		{
+			name: "IPv4",
+			prefixes: []string{
+				"100.64.1.0/24", "11.0.0.0/8", "10.1.0.0/16",
+				"20.0.1.0/24", "172.16.0.0/12", "9.0.0.0/8",
+			},
+			want: []string{
+				"9.0.0.0/8", "11.0.0.0/8", "172.16.0.0/12",
+				"10.1.0.0/16", "20.0.1.0/24", "100.64.1.0/24",
+			},
+		},
+		{
+			name: "IPv6",
+			prefixes: []string{
+				"2001:db8:1::/48", "2001:db8:8000::/33", "2001:db8:0:1::/64",
+				"2001:db8::/48", "2001:db8::/33",
+			},
+			want: []string{
+				"2001:db8::/33", "2001:db8:8000::/33", "2001:db8::/48",
+				"2001:db8:1::/48", "2001:db8:0:1::/64",
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		routes := make([]database.LiveRoute, 0, len(tt.prefixes))
+		for _, prefix := range tt.prefixes {
+			routes = append(routes, database.LiveRoute{Prefix: prefix})
+		}
+
+		sortRoutesByMaskLength(routes)
+
+		got := make([]string, 0, len(routes))
+		for _, route := range routes {
+			got = append(got, route.Prefix)
+		}
+
+		if !slices.Equal(got, tt.want) {
+			t.Errorf("%s: order %v, want %v", tt.name, got, tt.want)
+		}
+	}
+}
+
 // TestNoOriginHasNoASAnswers stores a route with no known origin, which has
 // origin AS 0 (https://git.eeqj.de/sneak/routewatch/issues/75). It expects
 // /as/0 and /api/v1/as/0 to answer 404.