diff --git a/packages/host/app/services/network.ts b/packages/host/app/services/network.ts
index c452419d57..08fc828f7d 100644
--- a/packages/host/app/services/network.ts
+++ b/packages/host/app/services/network.ts
@@ -5,7 +5,6 @@ import {
   VirtualNetwork,
   authorizationMiddleware,
   baseRealm,
-  registerCardReferencePrefix,
   fetcher,
 } from '@cardstack/runtime-common';
 
@@ -54,33 +53,33 @@ export default class NetworkService extends Service {
     let resolvedBaseRealmURL = new URL(
       withTrailingSlash(config.resolvedBaseRealmURL),
     );
+    // URL mapping kept for the fake https://cardstack.com/base/ → real URL.
+    // addRealmMapping registers the @cardstack/base/ scoped prefix.
     virtualNetwork.addURLMapping(new URL(baseRealm.url), resolvedBaseRealmURL);
+    virtualNetwork.addRealmMapping(
+      '@cardstack/base/',
+      resolvedBaseRealmURL.href,
+    );
     shimExternals(virtualNetwork);
     virtualNetwork.addImportMap('@cardstack/boxel-icons/', (rest) => {
       return `${config.iconsURL}/@cardstack/boxel-icons/v1/icons/${rest}.js`;
     });
     if (config.resolvedCatalogRealmURL) {
-      let catalogURL = withTrailingSlash(config.resolvedCatalogRealmURL);
-      registerCardReferencePrefix('@cardstack/catalog/', catalogURL);
-      virtualNetwork.addImportMap(
+      virtualNetwork.addRealmMapping(
         '@cardstack/catalog/',
-        (rest) => new URL(rest, catalogURL).href,
+        withTrailingSlash(config.resolvedCatalogRealmURL),
       );
     }
     if (config.resolvedSkillsRealmURL) {
-      let skillsURL = withTrailingSlash(config.resolvedSkillsRealmURL);
-      registerCardReferencePrefix('@cardstack/skills/', skillsURL);
-      virtualNetwork.addImportMap(
+      virtualNetwork.addRealmMapping(
         '@cardstack/skills/',
-        (rest) => new URL(rest, skillsURL).href,
+        withTrailingSlash(config.resolvedSkillsRealmURL),
       );
     }
     if (config.resolvedOpenRouterRealmURL) {
-      let openRouterURL = withTrailingSlash(config.resolvedOpenRouterRealmURL);
-      registerCardReferencePrefix('@cardstack/openrouter/', openRouterURL);
-      virtualNetwork.addImportMap(
+      virtualNetwork.addRealmMapping(
         '@cardstack/openrouter/',
-        (rest) => new URL(rest, openRouterURL).href,
+        withTrailingSlash(config.resolvedOpenRouterRealmURL),
       );
     }
     return virtualNetwork;
diff --git a/packages/realm-server/main.ts b/packages/realm-server/main.ts
index f065e41a2d..3aaadb6e58 100644
--- a/packages/realm-server/main.ts
+++ b/packages/realm-server/main.ts
@@ -9,7 +9,6 @@ import {
   CachingDefinitionLookup,
   DEFAULT_CARD_SIZE_LIMIT_BYTES,
   DEFAULT_FILE_SIZE_LIMIT_BYTES,
-  registerCardReferencePrefix,
 } from '@cardstack/runtime-common';
 import { NodeAdapter } from './node-realm';
 import yargs from 'yargs';
@@ -237,9 +236,7 @@ for (let i = 0; i < fromUrls.length; i++) {
     virtualNetwork.addURLMapping(fromURL, to);
     urlMappings.push([fromURL, to]);
   } else {
-    // Non-URL prefix like @cardstack/catalog/
-    registerCardReferencePrefix(from, to.href);
-    virtualNetwork.addImportMap(from, (rest) => new URL(rest, to).href);
+    virtualNetwork.addRealmMapping(from, to.href);
     urlMappings.push([to, to]); // use toUrl for both in hrefs
   }
 }
diff --git a/packages/realm-server/tests/card-reference-resolver-test.ts b/packages/realm-server/tests/card-reference-resolver-test.ts
index caa6cc48f6..4a7641341e 100644
--- a/packages/realm-server/tests/card-reference-resolver-test.ts
+++ b/packages/realm-server/tests/card-reference-resolver-test.ts
@@ -6,6 +6,7 @@ import {
   resolveCardReference,
   resolveRRI,
   RealmPaths,
+  VirtualNetwork,
 } from '@cardstack/runtime-common';
 import type {
   SingleCardDocument,
@@ -618,4 +619,133 @@ module(basename(__filename), function () {
       });
     });
   });
+
+  module('VirtualNetwork.addRealmMapping', function (hooks) {
+    let vn: VirtualNetwork;
+    let prefix = '@test/realm/';
+    let target = 'http://localhost:9000/realm/';
+
+    hooks.beforeEach(function () {
+      vn = new VirtualNetwork();
+      vn.addRealmMapping(prefix, target);
+    });
+
+    hooks.afterEach(function () {
+      unregisterCardReferencePrefix(prefix);
+    });
+
+    test('populates importMap so resolveImport works', function (assert) {
+      let result = vn.resolveImport('@test/realm/card-api');
+      assert.strictEqual(result, 'http://localhost:9000/realm/card-api');
+    });
+
+    test('populates global prefixMappings so resolveCardReference works', function (assert) {
+      let result = resolveCardReference('@test/realm/Foo', undefined);
+      assert.strictEqual(result, 'http://localhost:9000/realm/Foo');
+    });
+
+    test('normalizes trailing slashes', function (assert) {
+      unregisterCardReferencePrefix(prefix);
+      let vn2 = new VirtualNetwork();
+      // No trailing slashes
+      vn2.addRealmMapping('@test/other', 'http://localhost:9000/other');
+      let result = vn2.resolveImport('@test/other/card');
+      assert.strictEqual(result, 'http://localhost:9000/other/card');
+      unregisterCardReferencePrefix('@test/other/');
+    });
+
+    test('overwrites cleanly when called twice with same prefix', function (assert) {
+      let newTarget = 'http://localhost:8000/realm/';
+      vn.addRealmMapping(prefix, newTarget);
+      let result = vn.resolveImport('@test/realm/card-api');
+      assert.strictEqual(result, 'http://localhost:8000/realm/card-api');
+    });
+
+    test('knownRealms returns registered realm identifiers', function (assert) {
+      let realms = vn.knownRealms();
+      assert.true(
+        realms.includes('@test/realm/' as any),
+        'contains the registered realm',
+      );
+    });
+
+    test('knownRealms reflects multiple registrations', function (assert) {
+      vn.addRealmMapping('@test/other/', 'http://localhost:9000/other/');
+      let realms = vn.knownRealms();
+      assert.strictEqual(realms.length, 2);
+      assert.true(realms.includes('@test/realm/' as any));
+      assert.true(realms.includes('@test/other/' as any));
+      unregisterCardReferencePrefix('@test/other/');
+    });
+  });
+
+  module('VirtualNetwork.fetch with RRI', function (hooks) {
+    let vn: VirtualNetwork;
+    let prefix = '@test/fetch-realm/';
+    let target = 'http://localhost:9000/fetch-realm/';
+
+    hooks.beforeEach(function () {
+      vn = new VirtualNetwork();
+      vn.addRealmMapping(prefix, target);
+    });
+
+    hooks.afterEach(function () {
+      unregisterCardReferencePrefix(prefix);
+    });
+
+    test('resolves scoped RRI to real URL and fetches', async function (assert) {
+      let interceptedUrl: string | undefined;
+      vn.mount(async (req: Request) => {
+        interceptedUrl = req.url;
+        return new Response('ok', { status: 200 });
+      });
+
+      let response = await vn.fetch('@test/fetch-realm/card-api');
+      assert.strictEqual(response.status, 200);
+      assert.strictEqual(
+        interceptedUrl,
+        'http://localhost:9000/fetch-realm/card-api',
+      );
+    });
+
+    test('passes through normal URLs unchanged', async function (assert) {
+      let interceptedUrl: string | undefined;
+      vn.mount(async (req: Request) => {
+        interceptedUrl = req.url;
+        return new Response('ok', { status: 200 });
+      });
+
+      await vn.fetch('http://localhost:9000/fetch-realm/card-api');
+      assert.strictEqual(
+        interceptedUrl,
+        'http://localhost:9000/fetch-realm/card-api',
+      );
+    });
+
+    test('passes RequestInit through when fetching with RRI', async function (assert) {
+      let interceptedMethod: string | undefined;
+      vn.mount(async (req: Request) => {
+        interceptedMethod = req.method;
+        return new Response('ok', { status: 200 });
+      });
+
+      await vn.fetch('@test/fetch-realm/card-api', { method: 'POST' });
+      assert.strictEqual(interceptedMethod, 'POST');
+    });
+
+    test('@cardstack/base/card-api resolves through full fetch chain', async function (assert) {
+      let baseVN = new VirtualNetwork();
+      baseVN.addRealmMapping('@cardstack/base/', 'http://localhost:4201/base/');
+      let interceptedUrl: string | undefined;
+      baseVN.mount(async (req: Request) => {
+        interceptedUrl = req.url;
+        return new Response('ok', { status: 200 });
+      });
+
+      let response = await baseVN.fetch('@cardstack/base/card-api');
+      assert.strictEqual(response.status, 200);
+      assert.strictEqual(interceptedUrl, 'http://localhost:4201/base/card-api');
+      unregisterCardReferencePrefix('@cardstack/base/');
+    });
+  });
 });
diff --git a/packages/realm-server/worker.ts b/packages/realm-server/worker.ts
index 86d654b046..41f7d7f60e 100644
--- a/packages/realm-server/worker.ts
+++ b/packages/realm-server/worker.ts
@@ -6,7 +6,6 @@ import {
   isUrlLike,
   logger,
   IndexWriter,
-  registerCardReferencePrefix,
   type StatusArgs,
   type IndexingProgressEvent,
 } from '@cardstack/runtime-common';
@@ -107,8 +106,7 @@ for (let i = 0; i < fromUrls.length; i++) {
   if (isUrlLike(from)) {
     virtualNetwork.addURLMapping(new URL(from), to);
   } else {
-    registerCardReferencePrefix(from, to.href);
-    virtualNetwork.addImportMap(from, (rest) => new URL(rest, to).href);
+    virtualNetwork.addRealmMapping(from, to.href);
   }
 }
 let autoMigrate = migrateDB || undefined;
diff --git a/packages/runtime-common/virtual-network.ts b/packages/runtime-common/virtual-network.ts
index 6254224cdb..5ba5bfd29e 100644
--- a/packages/runtime-common/virtual-network.ts
+++ b/packages/runtime-common/virtual-network.ts
@@ -1,5 +1,9 @@
-import { RealmPaths } from './paths';
+import { RealmPaths, ensureTrailingSlash } from './paths';
 import { baseRealm, isNode } from './index';
+import {
+  registerCardReferencePrefix,
+  type RealmIdentifier,
+} from './card-reference-resolver';
 import type { ModuleDescriptor } from './package-shim-handler';
 import {
   PackageShimHandler,
@@ -19,6 +23,7 @@ export class VirtualNetwork {
   private handlers: Handler[] = [];
   private urlMappings: [string, string][] = [];
   private importMap: Map<string, (rest: string) => string> = new Map();
+  private realmMappings = new Map<string, string>();
 
   constructor(nativeFetch = createEnvironmentAwareFetch()) {
     this.nativeFetch = nativeFetch;
@@ -66,6 +71,24 @@ export class VirtualNetwork {
     this.importMap.set(prefix, handler);
   }
 
+  addRealmMapping(realmIdentifier: string, targetURL: string): void {
+    let normalizedId = ensureTrailingSlash(realmIdentifier);
+    let normalizedTarget = ensureTrailingSlash(targetURL);
+    this.realmMappings.set(normalizedId, normalizedTarget);
+
+    // Backward compat bridge: populate both existing registration systems
+    // so that resolveImport and resolveCardReference continue to work
+    this.addImportMap(
+      normalizedId,
+      (rest) => new URL(rest, normalizedTarget).href,
+    );
+    registerCardReferencePrefix(normalizedId, normalizedTarget);
+  }
+
+  knownRealms(): RealmIdentifier[] {
+    return [...this.realmMappings.keys()] as RealmIdentifier[];
+  }
+
   private nativeFetch: typeof globalThis.fetch;
 
   private resolveURLMapping(
@@ -118,6 +141,15 @@ export class VirtualNetwork {
     urlOrRequest: string | URL | Request,
     init?: RequestInit,
   ) => {
+    // Resolve RRI strings to real URLs before creating the Request,
+    // since new Request('@cardstack/base/...') would throw (not a valid URL).
+    if (typeof urlOrRequest === 'string') {
+      let resolved = this.resolveRRIToURL(urlOrRequest);
+      if (resolved) {
+        urlOrRequest = resolved;
+      }
+    }
+
     let request =
       urlOrRequest instanceof Request
         ? urlOrRequest
@@ -135,6 +167,15 @@ export class VirtualNetwork {
     return response;
   };
 
+  private resolveRRIToURL(rri: string): string | undefined {
+    for (let [prefix, target] of this.realmMappings) {
+      if (rri.startsWith(prefix)) {
+        return new URL(rri.slice(prefix.length), target).href;
+      }
+    }
+    return undefined;
+  }
+
   private async runFetch(request: Request, init?: RequestInit) {
     let handlers: FetcherMiddlewareHandler[] = this.handlers.map((h) => {
       return async (request, next) => {