diff --git a/ethereum.links.action.yml b/ethereum.links.action.yml
index fbcc203..5210db9 100644
--- a/ethereum.links.action.yml
+++ b/ethereum.links.action.yml
@@ -3,3 +3,9 @@ entity.ethereum_server.add_server_form:
   title: 'Add Ethereum node'
   appears_on:
     - ethereum.settings
+
+entity.ethereum_address.add_form:
+  route_name: entity.ethereum_address.add_form
+  title: 'Add address'
+  appears_on:
+    - entity.ethereum_address.collection
diff --git a/ethereum.links.menu.yml b/ethereum.links.menu.yml
index 62a38fa..2b077a7 100644
--- a/ethereum.links.menu.yml
+++ b/ethereum.links.menu.yml
@@ -14,3 +14,9 @@ ethereum.settings:
   parent: ethereum.admin_index
   title: 'Ethereum Network Configuration'
   description: 'Configure how Drupal connects to Ethereum.'
+
+entity.ethereum_address.collection:
+  title: 'Ethereum addresses'
+  parent: ethereum.admin_index
+  description: 'Create and manage Ethereum addresses.'
+  route_name: entity.ethereum_address.collection
diff --git a/ethereum.links.task.yml b/ethereum.links.task.yml
new file mode 100644
index 0000000..21c5a6f
--- /dev/null
+++ b/ethereum.links.task.yml
@@ -0,0 +1,4 @@
+entity.ethereum_address.collection:
+  title: List
+  route_name: entity.ethereum_address.collection
+  base_route: entity.ethereum_address.collection
diff --git a/ethereum.module b/ethereum.module
index 8a8b5ec..389219b 100644
--- a/ethereum.module
+++ b/ethereum.module
@@ -51,6 +51,15 @@ function ethereum_field_formatter_info_alter(array &$info) {
   $info['basic_string']['field_types'][] = 'ethereum_address';
 }
 
+/**
+ * Implements hook_views_data_alter().
+ */
+function ethereum_views_data_alter(array &$data) {
+  // Provide a nice list of options for the 'network' filter.
+  $data['ethereum_address']['network']['filter']['id'] = 'in_operator';
+  $data['ethereum_address']['network']['filter']['options callback'] = 'Drupal\ethereum\Controller\EthereumController::getNetworksAsOptions';
+}
+
 /**
  * Implements hook_js_settings_alter().
  *
diff --git a/src/Entity/EthereumAddress.php b/src/Entity/EthereumAddress.php
new file mode 100644
index 0000000..90aabc5
--- /dev/null
+++ b/src/Entity/EthereumAddress.php
@@ -0,0 +1,168 @@
+<?php
+
+namespace Drupal\ethereum\Entity;
+
+use Drupal\Core\Entity\ContentEntityBase;
+use Drupal\Core\Entity\EntityTypeInterface;
+use Drupal\Core\Field\BaseFieldDefinition;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\ethereum\EthereumAddressInterface;
+
+/**
+ * Defines the ethereum_address entity class.
+ *
+ * @ContentEntityType(
+ *   id = "ethereum_address",
+ *   label = @Translation("Ethereum address"),
+ *   label_collection = @Translation("Ethereum addresses"),
+ *   label_singular = @Translation("address"),
+ *   label_plural = @Translation("addresses"),
+ *   label_count = @PluralTranslation(
+ *     singular = "@count address",
+ *     plural = "@count addresses"
+ *   ),
+ *   handlers = {
+ *     "storage_schema" = "Drupal\ethereum\EthereumAddressStorageSchema",
+ *     "view_builder" = "Drupal\Core\Entity\EntityViewBuilder",
+ *     "views_data" = "Drupal\views\EntityViewsData",
+ *     "form" = {
+ *       "default" = "Drupal\Core\Entity\ContentEntityForm",
+ *       "delete" = "Drupal\Core\Entity\ContentEntityDeleteForm"
+ *     },
+ *     "route_provider" = {
+ *       "html" = "Drupal\Core\Entity\Routing\AdminHtmlRouteProvider",
+ *     },
+ *     "list_builder" = "Drupal\ethereum\EthereumAddressListBuilder",
+ *   },
+ *   admin_permission = "administer site configuration",
+ *   base_table = "ethereum_address",
+ *   entity_keys = {
+ *     "id" = "id",
+ *     "uuid" = "uuid",
+ *     "label" = "address",
+ *   },
+ *   field_ui_base_route = "entity.ethereum_address.collection",
+ *   links = {
+ *     "canonical" = "/admin/config/ethereum/addresses/address/{ethereum_address}",
+ *     "add-form" = "/admin/config/ethereum/addresses/add",
+ *     "edit-form" = "/admin/config/ethereum/addresses/address/{ethereum_address}/edit",
+ *     "delete-form" = "/admin/config/ethereum/addresses/address/{ethereum_address}/delete",
+ *     "collection" = "/admin/config/ethereum/addresses",
+ *   },
+ *   constraints = {
+ *     "EthereumAddressNetwork" = {}
+ *   },
+ * )
+ */
+class EthereumAddress extends ContentEntityBase implements EthereumAddressInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getAddress() {
+    return $this->get('address')->value;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setAddress($address) {
+    $this->set('address', $address);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getNetworkId() {
+    return $this->get('network')->value;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setNetworkID($network_id) {
+    $this->set('network', $network_id);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isContract() {
+    return (bool) $this->get('contract')->value;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setContract($is_contract) {
+    $this->set('contract', $is_contract);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
+    $fields = parent::baseFieldDefinitions($entity_type);
+
+    // Ensure that we can store as many addresses as possible.
+    $fields['id']->setSetting('size', 'big');
+
+    $fields['address'] = BaseFieldDefinition::create('ethereum_address')
+      ->setLabel(new TranslatableMarkup('Address'))
+      ->setRequired(TRUE)
+      ->setReadOnly(TRUE)
+      ->setDisplayOptions('view', [
+        'label' => 'hidden',
+        'type' => 'ethereum_address',
+      ])
+      ->setDisplayOptions('form', [
+        'type' => 'ethereum_address',
+      ])
+      ->setDisplayConfigurable('view', TRUE)
+      ->setDisplayConfigurable('form', TRUE);;
+
+    $fields['network'] = BaseFieldDefinition::create('list_string')
+      ->setLabel(new TranslatableMarkup('Network'))
+      ->setDescription(new TranslatableMarkup('The Ethereum network on which this address exists.'))
+      ->setRequired(TRUE)
+      ->setReadOnly(TRUE)
+      ->setDefaultValueCallback('\Drupal\ethereum\Entity\EthereumAddress::getCurrentNetworkId')
+      ->setSetting('allowed_values_function', '\Drupal\ethereum\Controller\EthereumController::getNetworksAsOptions')
+      ->setDisplayOptions('form', [
+        'type' => 'options_select',
+      ])
+      ->setDisplayConfigurable('view', TRUE)
+      ->setDisplayConfigurable('form', TRUE);
+
+
+    $fields['contract'] = BaseFieldDefinition::create('boolean')
+      ->setLabel(new TranslatableMarkup('Is contract'))
+      ->setDefaultValue(FALSE)
+      ->setReadOnly(TRUE)
+      ->setDisplayOptions('form', [
+        'type' => 'boolean_checkbox',
+      ])
+      ->setDisplayConfigurable('view', TRUE)
+      ->setDisplayConfigurable('form', TRUE);
+
+    return $fields;
+  }
+
+  /**
+   * Default value callback for the 'network' base field.
+   *
+   * @see ::baseFieldDefinitions()
+   *
+   * @return array
+   *   An array of default values.
+   */
+  public static function getCurrentNetworkId() {
+    $current_server = \Drupal::config('ethereum.settings')->get('current_server');
+    $server = EthereumServer::load($current_server);
+    return [$server->getNetworkId()];
+  }
+
+}
diff --git a/src/EthereumAddressInterface.php b/src/EthereumAddressInterface.php
new file mode 100644
index 0000000..dea51f8
--- /dev/null
+++ b/src/EthereumAddressInterface.php
@@ -0,0 +1,66 @@
+<?php
+
+namespace Drupal\ethereum;
+
+use Drupal\Core\Entity\ContentEntityInterface;
+
+/**
+ * Provides an interface defining an ethereum_address entity.
+ */
+interface EthereumAddressInterface extends ContentEntityInterface {
+
+  /**
+   * Gets the Ethereum address.
+   *
+   * @return string
+   *   The Ethereum address.
+   */
+  public function getAddress();
+
+  /**
+   * Sets the address.
+   *
+   * @param string $address
+   *   The address.
+   *
+   * @return $this
+   */
+  public function setAddress($address);
+
+  /**
+   * Gets the address' network ID.
+   *
+   * @return string
+   *   The Ethereum network ID where this address exists.
+   */
+  public function getNetworkId();
+
+  /**
+   * Sets the address' Ethereum network ID.
+   *
+   * @param string $network_id
+   *   The address' network ID.
+   *
+   * @return $this
+   */
+  public function setNetworkID($network_id);
+
+  /**
+   * Returns whether the address is a contract.
+   *
+   * @return bool
+   *   TRUE if the address is a contract.
+   */
+  public function isContract();
+
+  /**
+   * Sets whether the address is a contract.
+   *
+   * @param bool $is_contract
+   *   TRUE to mark this address as a contract, FALSE otherwise.
+   *
+   * @return $this
+   */
+  public function setContract($is_contract);
+
+}
diff --git a/src/EthereumAddressListBuilder.php b/src/EthereumAddressListBuilder.php
new file mode 100644
index 0000000..cb734ef
--- /dev/null
+++ b/src/EthereumAddressListBuilder.php
@@ -0,0 +1,36 @@
+<?php
+
+namespace Drupal\ethereum;
+
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\Entity\EntityListBuilder;
+use Drupal\ethereum\Controller\EthereumController;
+
+/**
+ * Defines a class to build a listing of ethereum_address entities.
+ */
+class EthereumAddressListBuilder extends EntityListBuilder {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildHeader() {
+    $header = [
+      'address' => $this->t('Address'),
+      'network' => $this->t('Network'),
+    ];
+    return $header + parent::buildHeader();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildRow(EntityInterface $entity) {
+    /** @var \Drupal\ethereum\EthereumAddressInterface $entity */
+    $networks = EthereumController::getNetworks();
+    $row['address'] = $entity->getAddress();
+    $row['network'] = $networks[$entity->getNetworkId()]['label'];
+    return $row + parent::buildRow($entity);
+  }
+
+}
diff --git a/src/EthereumAddressStorageSchema.php b/src/EthereumAddressStorageSchema.php
new file mode 100644
index 0000000..9d951c2
--- /dev/null
+++ b/src/EthereumAddressStorageSchema.php
@@ -0,0 +1,51 @@
+<?php
+
+namespace Drupal\ethereum;
+
+use Drupal\Core\Entity\ContentEntityTypeInterface;
+use Drupal\Core\Entity\Sql\SqlContentEntityStorageSchema;
+use Drupal\Core\Field\FieldStorageDefinitionInterface;
+
+/**
+ * Defines the ethereum_address schema handler.
+ */
+class EthereumAddressStorageSchema extends SqlContentEntityStorageSchema {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getEntitySchema(ContentEntityTypeInterface $entity_type, $reset = FALSE) {
+    $schema = parent::getEntitySchema($entity_type, $reset);
+
+    // Add a UNIQUE index on the 'address' and 'network' fields because two
+    // identical addresses can not live on the same network.
+    $schema[$this->storage->getBaseTable()]['unique keys'] += [
+      'ethereum_address__address_network' => ['address', 'network'],
+    ];
+
+    return $schema;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getSharedTableFieldSchema(FieldStorageDefinitionInterface $storage_definition, $table_name, array $column_mapping) {
+    $schema = parent::getSharedTableFieldSchema($storage_definition, $table_name, $column_mapping);
+    $field_name = $storage_definition->getName();
+
+    if ($table_name == $this->storage->getBaseTable()) {
+      switch ($field_name) {
+        case 'address':
+          $schema['fields'][$field_name]['not null'] = TRUE;
+          break;
+        case 'network':
+          $schema['fields'][$field_name]['not null'] = TRUE;
+          $schema['fields'][$field_name]['length'] = 10;
+          break;
+      }
+    }
+
+    return $schema;
+  }
+
+}
diff --git a/src/Plugin/EntityReferenceSelection/EthereumAddressSelection.php b/src/Plugin/EntityReferenceSelection/EthereumAddressSelection.php
new file mode 100644
index 0000000..18f7b39
--- /dev/null
+++ b/src/Plugin/EntityReferenceSelection/EthereumAddressSelection.php
@@ -0,0 +1,66 @@
+<?php
+
+namespace Drupal\ethereum\Plugin\EntityReferenceSelection;
+
+use Drupal\Core\Entity\Plugin\EntityReferenceSelection\DefaultSelection;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\ethereum\Entity\EthereumServer;
+
+/**
+ * Provides specific selection logic for the ethereum_address entity type.
+ *
+ * @EntityReferenceSelection(
+ *   id = "default:ethereum_address",
+ *   label = @Translation("Ethereum address selection"),
+ *   entity_types = {"ethereum_address"},
+ *   group = "default",
+ *   weight = 1
+ * )
+ */
+class EthereumAddressSelection extends DefaultSelection {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function defaultConfiguration() {
+    return [
+      'restrict_to_current_network' => TRUE,
+    ] + parent::defaultConfiguration();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
+    $configuration = $this->getConfiguration();
+
+    $form['restrict_to_current_network'] = [
+      '#type' => 'checkbox',
+      '#title' => $this->t('Filter the available addresses by the current network'),
+      '#default_value' => $configuration['restrict_to_current_network'],
+    ];
+    $form += parent::buildConfigurationForm($form, $form_state);
+
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function buildEntityQuery($match = NULL, $match_operator = 'CONTAINS') {
+    $query = parent::buildEntityQuery($match, $match_operator);
+
+    $configuration = $this->getConfiguration();
+
+    // Filter by the current network if needed.
+    if ($configuration['restrict_to_current_network']) {
+      $current_server = \Drupal::config('ethereum.settings')->get('current_server');
+      $server = EthereumServer::load($current_server);
+
+      $query->condition('network', $server->getNetworkId(), '=');
+    }
+
+    return $query;
+  }
+
+}
diff --git a/src/Plugin/Field/FieldFormatter/EthereumAddressFormatter.php b/src/Plugin/Field/FieldFormatter/EthereumAddressFormatter.php
new file mode 100644
index 0000000..aeda39c
--- /dev/null
+++ b/src/Plugin/Field/FieldFormatter/EthereumAddressFormatter.php
@@ -0,0 +1,173 @@
+<?php
+
+namespace Drupal\ethereum\Plugin\Field\FieldFormatter;
+
+use Drupal\Core\Field\Annotation\FieldFormatter;
+use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\Core\Field\Plugin\Field\FieldFormatter\BasicStringFormatter;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Url;
+use Drupal\ethereum\Controller\EthereumController;
+use Drupal\ethereum\Entity\EthereumServer;
+use Drupal\ethereum\Plugin\Field\FieldType\EthereumAddressItem;
+
+/**
+ * Plugin implementation of the 'ethereum_address' formatter.
+ *
+ * @FieldFormatter(
+ *   id = "ethereum_address",
+ *   label = @Translation("Ethereum address"),
+ *   field_types = {
+ *     "ethereum_address"
+ *   }
+ * )
+ */
+class EthereumAddressFormatter extends BasicStringFormatter {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function defaultSettings() {
+    return [
+      'link_to_network' => '',
+      'rel' => '',
+      'target' => '',
+    ] + parent::defaultSettings();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function settingsForm(array $form, FormStateInterface $form_state) {
+    $elements = parent::settingsForm($form, $form_state);
+
+    $elements['link_to_network'] = [
+      '#type' => 'checkbox',
+      '#title' => $this->t('Link to the network address page'),
+      '#default_value' => $this->getSetting('link_to_network'),
+    ];
+    $elements['rel'] = [
+      '#type' => 'checkbox',
+      '#title' => $this->t('Add rel="nofollow" to the link'),
+      '#return_value' => 'nofollow',
+      '#default_value' => $this->getSetting('rel'),
+      '#states' => array(
+        'visible' => array(
+          array(
+            'input[name="fields[address][settings_edit_form][settings][link_to_network]"]' => array('checked' => TRUE),
+          ),
+        ),
+      ),
+    ];
+    $elements['target'] = [
+      '#type' => 'checkbox',
+      '#title' => $this->t('Open link in new window'),
+      '#return_value' => '_blank',
+      '#default_value' => $this->getSetting('target'),
+      '#states' => array(
+        'visible' => array(
+          array(
+            'input[name="fields[address][settings_edit_form][settings][link_to_network]"]' => array('checked' => TRUE),
+          ),
+        ),
+      ),
+    ];
+
+    return $elements;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function settingsSummary() {
+    $summary = [];
+
+    $settings = $this->getSettings();
+
+    if (!empty($settings['link_to_network'])) {
+      $summary[] = $this->t('Link to the network address page');
+    }
+    if (!empty($settings['rel'])) {
+      $summary[] = $this->t('Add rel="@rel"', ['@rel' => $settings['rel']]);
+    }
+    if (!empty($settings['target'])) {
+      $summary[] = $this->t('Open link in new window');
+    }
+
+    return $summary;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function viewElements(FieldItemListInterface $items, $langcode) {
+    $elements = [];
+    $entity = $items->getEntity();
+    $settings = $this->getSettings();
+
+    if (empty($settings['link_to_network'])) {
+      return parent::viewElements($items, $langcode);
+    }
+
+    $current_server = \Drupal::config('ethereum.settings')->get('current_server');
+    $server = EthereumServer::load($current_server);
+    $networks = EthereumController::getNetworks();
+
+    if ($entity->getEntityTypeId() === 'ethereum_address') {
+      $network = $networks[$entity->get('network')->first()->value];
+    }
+    else {
+      $network = $networks[$server->getNetworkId()];
+    }
+
+    foreach ($items as $delta => $item) {
+      $url = $this->buildUrl($item, $network);
+
+      $elements[$delta] = [
+        '#type' => 'link',
+        '#title' => $item->value,
+        '#url' => $url,
+        '#options' => $url->getOptions(),
+      ];
+    }
+
+    return $elements;
+  }
+
+  /**
+   * Builds the \Drupal\Core\Url object for a field item.
+   *
+   * @param \Drupal\ethereum\Plugin\Field\FieldType\EthereumAddressItem $item
+   *   The  field item being rendered.
+   * @param array $network
+   *   An associative array with network information.
+   *
+   * @return \Drupal\Core\Url
+   *   A Url object.
+   */
+  protected function buildUrl(EthereumAddressItem $item, array $network) {
+    if (!empty($network['link_to_address'])) {
+      $network_link = str_replace('@address', $item->value, $network['link_to_address']);
+      $url = Url::fromUri($network_link);
+    }
+    else {
+      $url = Url::fromRoute('<none>');
+    }
+
+    $settings = $this->getSettings();
+    $options = $url->getOptions();
+
+    // Add optional 'rel' attribute to link options.
+    if (!empty($settings['rel'])) {
+      $options['attributes']['rel'] = $settings['rel'];
+    }
+    // Add optional 'target' attribute to link options.
+    if (!empty($settings['target'])) {
+      $options['attributes']['target'] = $settings['target'];
+    }
+    $url->setOptions($options);
+
+    return $url;
+  }
+
+}
diff --git a/src/Plugin/Field/FieldType/EthereumAddressItem.php b/src/Plugin/Field/FieldType/EthereumAddressItem.php
index 8303009..e724764 100644
--- a/src/Plugin/Field/FieldType/EthereumAddressItem.php
+++ b/src/Plugin/Field/FieldType/EthereumAddressItem.php
@@ -14,7 +14,7 @@
  *   label = @Translation("Ethereum address"),
  *   description = @Translation("Provides a field for Ethereum addresses."),
  *   default_widget = "ethereum_address",
- *   default_formatter = "basic_string"
+ *   default_formatter = "ethereum_address"
  * )
  */
 class EthereumAddressItem extends StringItem {
diff --git a/src/Plugin/Validation/Constraint/EthereumAddressNetworkConstraint.php b/src/Plugin/Validation/Constraint/EthereumAddressNetworkConstraint.php
new file mode 100644
index 0000000..ac01c25
--- /dev/null
+++ b/src/Plugin/Validation/Constraint/EthereumAddressNetworkConstraint.php
@@ -0,0 +1,32 @@
+<?php
+
+namespace Drupal\ethereum\Plugin\Validation\Constraint;
+
+use Drupal\Core\Entity\Plugin\Validation\Constraint\CompositeConstraintBase;
+
+/**
+ * Supports validating Ethereum addresses on a network.
+ *
+ * @Constraint(
+ *   id = "EthereumAddressNetwork",
+ *   label = @Translation("Ethereum address network", context = "Validation"),
+ *   type = "entity:ethereum_address"
+ * )
+ */
+class EthereumAddressNetworkConstraint extends CompositeConstraintBase {
+
+  /**
+   * Message shown when the address already exists on the given network.
+   *
+   * @var string
+   */
+  public $message = 'This address already exists on the %network_label network.';
+
+  /**
+   * {@inheritdoc}
+   */
+  public function coversFields() {
+    return ['address', 'network'];
+  }
+
+}
diff --git a/src/Plugin/Validation/Constraint/EthereumAddressNetworkConstraintValidator.php b/src/Plugin/Validation/Constraint/EthereumAddressNetworkConstraintValidator.php
new file mode 100644
index 0000000..fcea004
--- /dev/null
+++ b/src/Plugin/Validation/Constraint/EthereumAddressNetworkConstraintValidator.php
@@ -0,0 +1,67 @@
+<?php
+
+namespace Drupal\ethereum\Plugin\Validation\Constraint;
+
+use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
+use Drupal\Core\Entity\EntityStorageInterface;
+use Drupal\ethereum\Controller\EthereumController;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidator;
+
+/**
+ * Validates the EthereumAddressNetwork constraint.
+ */
+class EthereumAddressNetworkConstraintValidator extends ConstraintValidator implements ContainerInjectionInterface {
+
+  /**
+   * Validator 2.5 and upwards compatible execution context.
+   *
+   * @var \Symfony\Component\Validator\Context\ExecutionContextInterface
+   */
+  protected $context;
+
+  /**
+   * The ethereum_address storage handler.
+   *
+   * @var \Drupal\Core\Entity\EntityStorageInterface
+   */
+  protected $ethereumAddressStorage;
+
+  /**
+   * Constructs a new EthereumAddressNetworkConstraintValidator.
+   *
+   * @param \Drupal\Core\Entity\EntityStorageInterface $ethereum_address_storage
+   *   The ethereum_address storage handler.
+   */
+  public function __construct(EntityStorageInterface $ethereum_address_storage) {
+    $this->ethereumAddressStorage = $ethereum_address_storage;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static($container->get('entity_type.manager')->getStorage('ethereum_address'));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validate($entity, Constraint $constraint) {
+    /** @var \Drupal\ethereum\EthereumAddressInterface $entity */
+    $network_id = $entity->getNetworkId();
+    $existing_address = $this->ethereumAddressStorage->loadByProperties([
+      'address' => $entity->getAddress(),
+      'network' => $network_id,
+    ]);
+
+    if ($existing_address) {
+      $networks = EthereumController::getNetworks();
+      $this->context->buildViolation($constraint->message, ['%network_label' => $networks[$network_id]['label']])
+        ->atPath('address')
+        ->addViolation();
+    }
+  }
+
+}