diff --git a/testing/ci-local.sh b/testing/ci-local.sh
index 5bd9c99..21650b8 100755
--- a/testing/ci-local.sh
+++ b/testing/ci-local.sh
@@ -617,29 +617,77 @@ run_chaos() {
     return $rc
 }
 
+# Claim a free /64 for this run's gateway-lan network (Mechanism B).
+#
+# gateway-lan cannot float like fips-net: the LAN clients' resolv.conf pins the
+# gateway's LAN address as their nameserver, which must be a literal known
+# before they start. So instead of a fixed fd02::/64 that two concurrent runs
+# both request (the second failing on "Pool overlaps"), claim-and-advance a
+# candidate /64 and let docker's create be the atomic arbiter. The claimed
+# prefix is threaded — via the exported FIPS_GW_LAN6_PREFIX — into the compose
+# ipv6_address pins, the generated resolv.conf, and gateway-test.sh, so every
+# LAN address moves with the claim. i=0 renders today's fd02::/64.
+#
+# Mirrors sidecar/scripts/test-sidecar.sh:alloc_network. Deliberately does NOT
+# discard stderr: only an address-pool conflict is worth advancing on; any other
+# failure is real and burning through 256 candidates would bury the reason.
+claim_gateway_lan6() {
+    local net="$1" i err
+    for (( i = 0; i < 256; i++ )); do
+        if err=$(docker network create --ipv6 \
+                --subnet "fd02:0:0:${i}::/64" \
+                --label "$CI_LABEL" --label "$CI_LABEL_RUN" \
+                "$net" 2>&1); then
+            export FIPS_GW_LAN6_PREFIX="fd02:0:0:${i}"
+            info "[gateway] Claimed $net on fd02:0:0:${i}::/64"
+            return 0
+        fi
+        case "$err" in
+            *"Pool overlaps"*|*"pool overlaps"*) continue ;;
+            *) fail "[gateway] docker network create: $err"; return 1 ;;
+        esac
+    done
+    fail "[gateway] no free /64 in fd02:0:0:0-255::/64 after 256 attempts"
+    return 1
+}
+
 # Run gateway integration test
 run_gateway() {
     local compose="testing/static/docker-compose.yml"
+    local ext="testing/static/docker-compose.gateway-external-net.yml"
     local rc=0
     export COMPOSE_PROJECT_NAME="$(ci_project static)"
+    export FIPS_GW_LAN_NET="fips-gateway-lan${FIPS_CI_NAME_SUFFIX:-}"
 
-    info "[gateway] Generating configs"
-    bash testing/static/scripts/generate-configs.sh gateway gateway-test || { record "gateway" 1; return; }
-    bash testing/static/scripts/gateway-test.sh inject-config || { record "gateway" 1; return; }
-
-    info "[gateway] Starting containers"
-    docker compose -f "$compose" --profile gateway up -d || { record "gateway" 1; return; }
-
-    info "[gateway] Running gateway test"
-    if bash testing/static/scripts/gateway-test.sh; then
-        rc=0
-    else
-        rc=1
-        info "[gateway] Collecting failure logs"
-        docker compose -f "$compose" --profile gateway logs --no-color 2>&1 | tail -100
+    # Claim the LAN /64 first: generate-configs writes resolv.conf from the
+    # claimed prefix, and inject-config renders the port-forward targets from
+    # it, so both must see FIPS_GW_LAN6_PREFIX before they run.
+    info "[gateway] Claiming LAN network"
+    if ! claim_gateway_lan6 "$FIPS_GW_LAN_NET"; then
+        record "gateway" 1
+        return
     fi
 
-    docker compose -f "$compose" --profile gateway down --volumes --remove-orphans 2>/dev/null
+    info "[gateway] Generating configs"
+    if bash testing/static/scripts/generate-configs.sh gateway gateway-test \
+        && bash testing/static/scripts/gateway-test.sh inject-config \
+        && { info "[gateway] Starting containers"; \
+             docker compose -f "$compose" -f "$ext" --profile gateway up -d; }; then
+        info "[gateway] Running gateway test"
+        if bash testing/static/scripts/gateway-test.sh; then
+            rc=0
+        else
+            rc=1
+            info "[gateway] Collecting failure logs"
+            docker compose -f "$compose" -f "$ext" --profile gateway logs --no-color 2>&1 | tail -100
+        fi
+    else
+        rc=1
+    fi
+
+    # compose down does not remove an external network, so drop it explicitly.
+    docker compose -f "$compose" -f "$ext" --profile gateway down --volumes --remove-orphans 2>/dev/null
+    docker network rm "$FIPS_GW_LAN_NET" >/dev/null 2>&1 || true
     record "gateway" $rc
 }
 
diff --git a/testing/static/docker-compose.gateway-external-net.yml b/testing/static/docker-compose.gateway-external-net.yml
new file mode 100644
index 0000000..fa1d40d
--- /dev/null
+++ b/testing/static/docker-compose.gateway-external-net.yml
@@ -0,0 +1,15 @@
+# Override: attach gateway-lan to a pre-created external network instead of
+# letting compose create it from the base file's fd02::/64 pin.
+#
+# Applied only by ci-local.sh's run_gateway, which claims a free /64 per run
+# (claim_gateway_lan6) and exports FIPS_GW_LAN_NET / FIPS_GW_LAN6_PREFIX before
+# `up`. This is what makes two concurrent local gateway runs collision-safe:
+# each claims a distinct /64, so neither requests a fixed range the other holds.
+#
+# The GitHub matrix and any standalone `docker compose up` do NOT apply this
+# overlay; they use the base file's normal gateway-lan network unchanged. This
+# mirrors sidecar/docker-compose.external-net.yml.
+networks:
+  gateway-lan:
+    external: true
+    name: ${FIPS_GW_LAN_NET:-fips-gateway-lan}
diff --git a/testing/static/docker-compose.yml b/testing/static/docker-compose.yml
index 0e94542..602c037 100644
--- a/testing/static/docker-compose.yml
+++ b/testing/static/docker-compose.yml
@@ -8,6 +8,10 @@ networks:
     driver: bridge
     labels:
       - "com.corganlabs.fips-ci=1"
+  # IPv4 is dropped so docker auto-assigns it (nothing reads a LAN IPv4 — the
+  # whole gateway LAN path is IPv6). The fd02::/64 pin stays for the standalone /
+  # GitHub path; concurrent local runs replace this network with a per-run
+  # claimed /64 via docker-compose.gateway-external-net.yml (see run_gateway).
   gateway-lan:
     driver: bridge
     labels:
@@ -15,7 +19,6 @@ networks:
     enable_ipv6: true
     ipam:
       config:
-        - subnet: 172.20.1.0/24
         - subnet: fd02::/64
 
 x-fips-common: &fips-common
@@ -475,8 +478,7 @@ services:
     networks:
       fips-net:
       gateway-lan:
-        ipv4_address: 172.20.1.10
-        ipv6_address: fd02::10
+        ipv6_address: ${FIPS_GW_LAN6_PREFIX:-fd02}::10
 
   gw-server:
     <<: *fips-common
@@ -513,11 +515,10 @@ services:
     sysctls:
       - net.ipv6.conf.all.disable_ipv6=0
     volumes:
-      - ./configs/gateway-resolv.conf:/etc/resolv.conf:ro
+      - ./generated-configs${FIPS_CI_NAME_SUFFIX:-}/gateway/resolv.conf:/etc/resolv.conf:ro
     networks:
       gateway-lan:
-        ipv4_address: 172.20.1.20
-        ipv6_address: fd02::20
+        ipv6_address: ${FIPS_GW_LAN6_PREFIX:-fd02}::20
     restart: "no"
     env_file:
       - ./generated-configs${FIPS_CI_NAME_SUFFIX:-}/npubs.env
@@ -535,11 +536,10 @@ services:
     sysctls:
       - net.ipv6.conf.all.disable_ipv6=0
     volumes:
-      - ./configs/gateway-resolv.conf:/etc/resolv.conf:ro
+      - ./generated-configs${FIPS_CI_NAME_SUFFIX:-}/gateway/resolv.conf:/etc/resolv.conf:ro
     networks:
       gateway-lan:
-        ipv4_address: 172.20.1.21
-        ipv6_address: fd02::21
+        ipv6_address: ${FIPS_GW_LAN6_PREFIX:-fd02}::21
     restart: "no"
     env_file:
       - ./generated-configs${FIPS_CI_NAME_SUFFIX:-}/npubs.env
diff --git a/testing/static/scripts/gateway-test.sh b/testing/static/scripts/gateway-test.sh
index 16fc9c2..9cb8a60 100755
--- a/testing/static/scripts/gateway-test.sh
+++ b/testing/static/scripts/gateway-test.sh
@@ -26,6 +26,16 @@ SERVER2="fips-gw-server-2${FIPS_CI_NAME_SUFFIX:-}"
 CLIENT="fips-gw-client${FIPS_CI_NAME_SUFFIX:-}"
 CLIENT2="fips-gw-client-2${FIPS_CI_NAME_SUFFIX:-}"
 
+# LAN-side IPv6 addressing. run_gateway claims a per-run /64 and exports
+# FIPS_GW_LAN6_PREFIX; unset (standalone / GitHub) these render the base
+# compose's fd02:: addresses, byte-identical to before. GW_DNS is the gateway's
+# LAN address (nameserver + route next-hop); GW_CLIENT_LAN is gw-client's LAN
+# address (inbound port-forward target). fd01::/112 (the virtual pool) is NOT
+# claimed and stays literal below.
+GW_LAN6_PREFIX="${FIPS_GW_LAN6_PREFIX:-fd02}"
+GW_DNS="${GW_LAN6_PREFIX}::10"
+GW_CLIENT_LAN="${GW_LAN6_PREFIX}::20"
+
 # ── inject-config subcommand ─────────────────────────────────────────────
 
 inject_gateway_config() {
@@ -58,21 +68,21 @@ cfg['gateway'] = {
         {
             'listen_port': 18080,
             'proto': 'tcp',
-            'target': '[fd02::20]:8080',
+            'target': '[${GW_CLIENT_LAN}]:8080',
         },
         # 6B: second TCP forward — exercises multiple simultaneous TCP
         # rules sharing the same LAN backend on a different listen port.
         {
             'listen_port': 18082,
             'proto': 'tcp',
-            'target': '[fd02::20]:8081',
+            'target': '[${GW_CLIENT_LAN}]:8081',
         },
         # 6A: UDP forward — exercises the runtime UDP DNAT path (rule
         # shape + conntrack handling) end-to-end.
         {
             'listen_port': 18081,
             'proto': 'udp',
-            'target': '[fd02::20]:8081',
+            'target': '[${GW_CLIENT_LAN}]:8081',
         },
     ],
 }
@@ -130,7 +140,7 @@ for i in $(seq 1 30); do
     # Try resolving the server's npub via the gateway DNS from the client.
     # Match fd01:: specifically (the pool prefix) to avoid false-positive
     # matches on error messages containing fd02::10.
-    local_result=$(docker exec "$CLIENT" dig +short AAAA "${NPUB_B}.fips" @fd02::10 2>/dev/null || true)
+    local_result=$(docker exec "$CLIENT" dig +short AAAA "${NPUB_B}.fips" @${GW_DNS} 2>/dev/null || true)
     if echo "$local_result" | grep -q "^fd01::"; then
         echo "  Gateway DNS responding after ${i}s"
         DNS_READY=true
@@ -146,23 +156,23 @@ fi
 # Phase 3: Client network setup — route virtual IP pool via gateway
 echo ""
 echo "Phase 3: Client network setup"
-docker exec "$CLIENT" ip -6 route add fd01::/112 via fd02::10 2>/dev/null || true
-echo "  Added route fd01::/112 via fd02::10 on $CLIENT"
-docker exec "$CLIENT2" ip -6 route add fd01::/112 via fd02::10 2>/dev/null || true
-echo "  Added route fd01::/112 via fd02::10 on $CLIENT2"
+docker exec "$CLIENT" ip -6 route add fd01::/112 via ${GW_DNS} 2>/dev/null || true
+echo "  Added route fd01::/112 via ${GW_DNS} on $CLIENT"
+docker exec "$CLIENT2" ip -6 route add fd01::/112 via ${GW_DNS} 2>/dev/null || true
+echo "  Added route fd01::/112 via ${GW_DNS} on $CLIENT2"
 
 # Phase 4: DNS resolution test — resolve server npub from both clients,
 # exercising concurrent multi-client mappings.
 echo ""
 echo "Phase 4: DNS resolution"
-VIRTUAL_IP=$(docker exec "$CLIENT" dig +short AAAA "${NPUB_B}.fips" @fd02::10 2>/dev/null | head -1)
+VIRTUAL_IP=$(docker exec "$CLIENT" dig +short AAAA "${NPUB_B}.fips" @${GW_DNS} 2>/dev/null | head -1)
 if [ -n "$VIRTUAL_IP" ] && echo "$VIRTUAL_IP" | grep -q "fd01"; then
     check "Resolve ${NPUB_B:0:20}...fips on $CLIENT → $VIRTUAL_IP" 0
 else
     check "Resolve ${NPUB_B:0:20}...fips on $CLIENT (got: '$VIRTUAL_IP')" 1
 fi
 
-VIRTUAL_IP_2=$(docker exec "$CLIENT2" dig +short AAAA "${NPUB_C}.fips" @fd02::10 2>/dev/null | head -1)
+VIRTUAL_IP_2=$(docker exec "$CLIENT2" dig +short AAAA "${NPUB_C}.fips" @${GW_DNS} 2>/dev/null | head -1)
 if [ -n "$VIRTUAL_IP_2" ] && echo "$VIRTUAL_IP_2" | grep -q "fd01"; then
     check "Resolve ${NPUB_C:0:20}...fips on $CLIENT2 → $VIRTUAL_IP_2" 0
 else
@@ -357,7 +367,7 @@ else
     # 8080 backend serves "inbound-forward-ok" (no -2 suffix) — distinct
     # from the 8081 backend so a misrouted response would be detectable.
     if echo "$FWD_RESPONSE" | grep -qE '^inbound-forward-ok$'; then
-        check "Inbound HTTP via TCP forward 18080 → [fd02::20]:8080" 0
+        check "Inbound HTTP via TCP forward 18080 → [${GW_CLIENT_LAN}]:8080" 0
     else
         check "Inbound HTTP via TCP forward 18080 (response: '${FWD_RESPONSE:0:80}')" 1
     fi
@@ -365,7 +375,7 @@ else
     FWD_RESPONSE_2=$(docker exec "$SERVER" curl -6 -s --max-time 10 \
         "http://[${GW_MESH_IP}]:18082/" 2>&1) || true
     if echo "$FWD_RESPONSE_2" | grep -q "inbound-forward-ok-2"; then
-        check "Inbound HTTP via TCP forward 18082 → [fd02::20]:8081 (6B)" 0
+        check "Inbound HTTP via TCP forward 18082 → [${GW_CLIENT_LAN}]:8081 (6B)" 0
     else
         check "Inbound HTTP via TCP forward 18082 (response: '${FWD_RESPONSE_2:0:80}')" 1
     fi
@@ -384,7 +394,7 @@ except Exception as e:
     sys.stdout.write('ERR: ' + str(e))
 " 2>&1) || true
     if echo "$UDP_RESPONSE" | grep -q "udp-forward-ok:ping-via-udp-fwd"; then
-        check "Inbound UDP via forward 18081 → [fd02::20]:8081 (6A)" 0
+        check "Inbound UDP via forward 18081 → [${GW_CLIENT_LAN}]:8081 (6A)" 0
     else
         check "Inbound UDP via forward 18081 (response: '${UDP_RESPONSE:0:80}')" 1
     fi
@@ -444,8 +454,8 @@ docker exec "$GATEWAY" pkill -f "^fips --config" 2>/dev/null || true
 sleep 2
 
 # Gateway upstream timeout is 5s, so dig must wait longer than that.
-SERVFAIL_RESULT=$(docker exec "$CLIENT" dig +short +tries=1 +time=8 AAAA "test-servfail.fips" @fd02::10 2>&1 || true)
-SERVFAIL_STATUS=$(docker exec "$CLIENT" dig +tries=1 +time=8 AAAA "test-servfail.fips" @fd02::10 2>&1 | grep -c "SERVFAIL" || true)
+SERVFAIL_RESULT=$(docker exec "$CLIENT" dig +short +tries=1 +time=8 AAAA "test-servfail.fips" @${GW_DNS} 2>&1 || true)
+SERVFAIL_STATUS=$(docker exec "$CLIENT" dig +tries=1 +time=8 AAAA "test-servfail.fips" @${GW_DNS} 2>&1 | grep -c "SERVFAIL" || true)
 if [ "$SERVFAIL_STATUS" -ge 1 ]; then
     check "SERVFAIL when daemon DNS is down" 0
 else
diff --git a/testing/static/scripts/generate-configs.sh b/testing/static/scripts/generate-configs.sh
index a1d1a04..529ec12 100755
--- a/testing/static/scripts/generate-configs.sh
+++ b/testing/static/scripts/generate-configs.sh
@@ -274,6 +274,16 @@ generate_topology() {
         echo "${var_name}=$(get_key RESOLVED_NPUB "$node_id")" >> "$env_file"
     done
     echo "  ✓ Generated $env_file"
+
+    # Phase 4 (gateway only): write the LAN-client resolv.conf. Its nameserver
+    # is the gateway's LAN address, which must be a literal known before the
+    # client starts. run_gateway claims a per-run /64 and exports
+    # FIPS_GW_LAN6_PREFIX before calling this; unset (standalone / GitHub) it
+    # renders the base compose's fd02::10.
+    if [ "$topology_name" = "gateway" ]; then
+        echo "nameserver ${FIPS_GW_LAN6_PREFIX:-fd02}::10" > "$output_dir/resolv.conf"
+        echo "  ✓ Generated $output_dir/resolv.conf"
+    fi
 }
 
 main() {