diff --git a/spring-cloud-netflix-eureka-server/src/main/java/org/springframework/cloud/netflix/eureka/server/CloudJacksonJson.java b/spring-cloud-netflix-eureka-server/src/main/java/org/springframework/cloud/netflix/eureka/server/CloudJacksonJson.java
index d5a26ace..d0eb04c1 100644
--- a/spring-cloud-netflix-eureka-server/src/main/java/org/springframework/cloud/netflix/eureka/server/CloudJacksonJson.java
+++ b/spring-cloud-netflix-eureka-server/src/main/java/org/springframework/cloud/netflix/eureka/server/CloudJacksonJson.java
@@ -123,12 +123,21 @@ public class CloudJacksonJson extends LegacyJacksonJson {
 				SerializerProvider provider) throws IOException {
 
 			if (info.getInstanceId() == null && info.getMetadata() != null) {
-				String instanceId = info.getMetadata().get("instanceId");
+				String instanceId = calculateInstanceId(info);
 				info = new InstanceInfo.Builder(info).setInstanceId(instanceId).build();
 			}
 
 			super.serialize(info, jgen, provider);
 		}
+
+		private String calculateInstanceId(InstanceInfo info) {
+			String instanceId = info.getMetadata().get("instanceId");
+			String hostName = info.getHostName();
+			if (instanceId != null && !instanceId.startsWith(hostName)) {
+				instanceId = hostName + ":" + instanceId;
+			}
+			return instanceId == null ? hostName : instanceId;
+		}
 	}
 
 	static class CloudInstanceInfoDeserializer extends InstanceInfoDeserializer {