diff --git a/create-a-container/migrations/20260506000000-transport-service-site-scoped-port-constraint.js b/create-a-container/migrations/20260506000000-transport-service-site-scoped-port-constraint.js
new file mode 100644
index 00000000..5174ae5e
--- /dev/null
+++ b/create-a-container/migrations/20260506000000-transport-service-site-scoped-port-constraint.js
@@ -0,0 +1,63 @@
+'use strict';
+
+module.exports = {
+  async up(queryInterface, Sequelize) {
+    // 1. Add siteId column (nullable initially for backfill)
+    await queryInterface.addColumn('TransportServices', 'siteId', {
+      type: Sequelize.INTEGER,
+      allowNull: true,
+      references: { model: 'Sites', key: 'id' },
+      onUpdate: 'CASCADE',
+      onDelete: 'RESTRICT'
+    });
+
+    // 2. Backfill siteId via Service -> Container
+    await queryInterface.sequelize.query(`
+      UPDATE "TransportServices" ts
+      SET "siteId" = c."siteId"
+      FROM "Services" s
+      JOIN "Containers" c ON s."containerId" = c.id
+      WHERE ts."serviceId" = s.id
+    `);
+
+    // 3. Make siteId NOT NULL after backfill
+    await queryInterface.changeColumn('TransportServices', 'siteId', {
+      type: Sequelize.INTEGER,
+      allowNull: false
+    });
+
+    // 4. Remove duplicate FK constraints introduced by changeColumn (Sequelize bug)
+    const fks = await queryInterface.getForeignKeyReferencesForTable('TransportServices');
+    const seen = new Set();
+    for (const fk of fks) {
+      const key = `${fk.columnName}->${fk.referencedTableName}.${fk.referencedColumnName}`;
+      if (seen.has(key)) {
+        await queryInterface.removeConstraint('TransportServices', fk.constraintName);
+      }
+      seen.add(key);
+    }
+
+    // 5. Drop the global (protocol, externalPort) unique index
+    await queryInterface.removeIndex('TransportServices', 'transport_services_unique_protocol_port');
+
+    // 6. Add site-scoped (siteId, protocol, externalPort) unique index
+    await queryInterface.addIndex('TransportServices', ['siteId', 'protocol', 'externalPort'], {
+      unique: true,
+      name: 'transport_services_unique_site_protocol_port'
+    });
+  },
+
+  async down(queryInterface, Sequelize) {
+    // Remove site-scoped index
+    await queryInterface.removeIndex('TransportServices', 'transport_services_unique_site_protocol_port');
+
+    // Restore global (protocol, externalPort) unique index
+    await queryInterface.addIndex('TransportServices', ['protocol', 'externalPort'], {
+      unique: true,
+      name: 'transport_services_unique_protocol_port'
+    });
+
+    // Remove siteId column
+    await queryInterface.removeColumn('TransportServices', 'siteId');
+  }
+};
diff --git a/create-a-container/models/transport-service.js b/create-a-container/models/transport-service.js
index 10397805..2c5d6bab 100644
--- a/create-a-container/models/transport-service.js
+++ b/create-a-container/models/transport-service.js
@@ -5,13 +5,16 @@ module.exports = (sequelize, DataTypes) => {
   class TransportService extends Model {
     static associate(models) {
       TransportService.belongsTo(models.Service, { foreignKey: 'serviceId', as: 'service' });
+      TransportService.belongsTo(models.Site, { foreignKey: 'siteId', as: 'site' });
     }
 
-    // Find the next available external port for the given protocol in the specified range
-    static async nextAvailablePortInRange(protocol, minPort, maxPort, transaction = null) {
+    // Find the next available external port for the given protocol in the specified range,
+    // scoped to a specific site.
+    static async nextAvailablePortInRange(protocol, minPort, maxPort, siteId, transaction = null) {
       const queryOptions = {
         where: {
           protocol: protocol,
+          siteId: siteId,
           externalPort: {
             [sequelize.Sequelize.Op.between]: [minPort, maxPort]
           }
@@ -19,7 +22,7 @@ module.exports = (sequelize, DataTypes) => {
         attributes: ['externalPort'],
         order: [['externalPort', 'ASC']]
       };
-      
+
       if (transaction) {
         queryOptions.transaction = transaction;
         queryOptions.lock = sequelize.Sequelize.Transaction.LOCK.UPDATE;
@@ -49,6 +52,14 @@ module.exports = (sequelize, DataTypes) => {
         key: 'id'
       }
     },
+    siteId: {
+      type: DataTypes.INTEGER,
+      allowNull: false,
+      references: {
+        model: 'Sites',
+        key: 'id'
+      }
+    },
     protocol: {
       type: DataTypes.ENUM('tcp', 'udp'),
       allowNull: false,
@@ -74,9 +85,9 @@ module.exports = (sequelize, DataTypes) => {
     modelName: 'TransportService',
     indexes: [
       {
-        name: 'transport_services_unique_protocol_port',
+        name: 'transport_services_unique_site_protocol_port',
         unique: true,
-        fields: ['protocol', 'externalPort']
+        fields: ['siteId', 'protocol', 'externalPort']
       }
     ]
   });
diff --git a/create-a-container/routers/containers.js b/create-a-container/routers/containers.js
index 9ace32f5..a113c49c 100644
--- a/create-a-container/routers/containers.js
+++ b/create-a-container/routers/containers.js
@@ -423,10 +423,11 @@ router.post('/', async (req, res) => {
           }, { transaction: t });
         } else {
           const minPort = 2000;
-          const maxPort = 65565;
-          const externalPort = await TransportService.nextAvailablePortInRange(protocol, minPort, maxPort, t);
+          const maxPort = 65535;
+          const externalPort = await TransportService.nextAvailablePortInRange(protocol, minPort, maxPort, siteId, t);
           await TransportService.create({
             serviceId: createdService.id,
+            siteId,
             protocol: protocol,
             externalPort
           }, { transaction: t });
@@ -615,8 +616,8 @@ router.put('/:id', requireAuth, async (req, res) => {
           } else if (serviceType === 'dns') {
              await DnsService.create({ serviceId: createdService.id, recordType: 'SRV', dnsName }, { transaction: t });
           } else {
-             const externalPort = await TransportService.nextAvailablePortInRange(protocol, 2000, 65565);
-             await TransportService.create({ serviceId: createdService.id, protocol, externalPort }, { transaction: t });
+              const externalPort = await TransportService.nextAvailablePortInRange(protocol, 2000, 65535, siteId, t);
+              await TransportService.create({ serviceId: createdService.id, siteId, protocol, externalPort }, { transaction: t });
           }
         }