diff --git a/lib/compile/formats.js b/lib/compile/formats.js
index d06792aa4..44895b0b4 100644
--- a/lib/compile/formats.js
+++ b/lib/compile/formats.js
@@ -5,7 +5,7 @@ var util = require('./util');
 var DATE = /^(\d\d\d\d)-(\d\d)-(\d\d)$/;
 var DAYS = [0,31,28,31,30,31,30,31,31,30,31,30,31];
 var TIME = /^(\d\d):(\d\d):(\d\d)(\.\d+)?(z|[+-]\d\d(?::?\d\d)?)?$/i;
-var HOSTNAME = /^[a-z0-9](?:[a-z0-9-]{0,61}[a-z0-9])?(?:\.[a-z0-9](?:[-0-9a-z]{0,61}[0-9a-z])?)*$/i;
+var HOSTNAME = /^(?=.{1,253}\.?$)[a-z0-9](?:[a-z0-9-]{0,61}[a-z0-9])?(?:\.[a-z0-9](?:[-0-9a-z]{0,61}[0-9a-z])?)*\.?$/i;
 var URI = /^(?:[a-z][a-z0-9+\-.]*:)(?:\/?\/(?:(?:[a-z0-9\-._~!$&'()*+,;=:]|%[0-9a-f]{2})*@)?(?:\[(?:(?:(?:(?:[0-9a-f]{1,4}:){6}|::(?:[0-9a-f]{1,4}:){5}|(?:[0-9a-f]{1,4})?::(?:[0-9a-f]{1,4}:){4}|(?:(?:[0-9a-f]{1,4}:){0,1}[0-9a-f]{1,4})?::(?:[0-9a-f]{1,4}:){3}|(?:(?:[0-9a-f]{1,4}:){0,2}[0-9a-f]{1,4})?::(?:[0-9a-f]{1,4}:){2}|(?:(?:[0-9a-f]{1,4}:){0,3}[0-9a-f]{1,4})?::[0-9a-f]{1,4}:|(?:(?:[0-9a-f]{1,4}:){0,4}[0-9a-f]{1,4})?::)(?:[0-9a-f]{1,4}:[0-9a-f]{1,4}|(?:(?:25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(?:25[0-5]|2[0-4]\d|[01]?\d\d?))|(?:(?:[0-9a-f]{1,4}:){0,5}[0-9a-f]{1,4})?::[0-9a-f]{1,4}|(?:(?:[0-9a-f]{1,4}:){0,6}[0-9a-f]{1,4})?::)|[Vv][0-9a-f]+\.[a-z0-9\-._~!$&'()*+,;=:]+)\]|(?:(?:25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(?:25[0-5]|2[0-4]\d|[01]?\d\d?)|(?:[a-z0-9\-._~!$&'()*+,;=]|%[0-9a-f]{2})*)(?::\d*)?(?:\/(?:[a-z0-9\-._~!$&'()*+,;=:@]|%[0-9a-f]{2})*)*|\/(?:(?:[a-z0-9\-._~!$&'()*+,;=:@]|%[0-9a-f]{2})+(?:\/(?:[a-z0-9\-._~!$&'()*+,;=:@]|%[0-9a-f]{2})*)*)?|(?:[a-z0-9\-._~!$&'()*+,;=:@]|%[0-9a-f]{2})+(?:\/(?:[a-z0-9\-._~!$&'()*+,;=:@]|%[0-9a-f]{2})*)*)(?:\?(?:[a-z0-9\-._~!$&'()*+,;=:@/?]|%[0-9a-f]{2})*)?(?:#(?:[a-z0-9\-._~!$&'()*+,;=:@/?]|%[0-9a-f]{2})*)?$/i;
 var URIREF = /^(?:[a-z][a-z0-9+\-.]*:)?(?:\/?\/(?:(?:[a-z0-9\-._~!$&'()*+,;=:]|%[0-9a-f]{2})*@)?(?:\[(?:(?:(?:(?:[0-9a-f]{1,4}:){6}|::(?:[0-9a-f]{1,4}:){5}|(?:[0-9a-f]{1,4})?::(?:[0-9a-f]{1,4}:){4}|(?:(?:[0-9a-f]{1,4}:){0,1}[0-9a-f]{1,4})?::(?:[0-9a-f]{1,4}:){3}|(?:(?:[0-9a-f]{1,4}:){0,2}[0-9a-f]{1,4})?::(?:[0-9a-f]{1,4}:){2}|(?:(?:[0-9a-f]{1,4}:){0,3}[0-9a-f]{1,4})?::[0-9a-f]{1,4}:|(?:(?:[0-9a-f]{1,4}:){0,4}[0-9a-f]{1,4})?::)(?:[0-9a-f]{1,4}:[0-9a-f]{1,4}|(?:(?:25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(?:25[0-5]|2[0-4]\d|[01]?\d\d?))|(?:(?:[0-9a-f]{1,4}:){0,5}[0-9a-f]{1,4})?::[0-9a-f]{1,4}|(?:(?:[0-9a-f]{1,4}:){0,6}[0-9a-f]{1,4})?::)|[Vv][0-9a-f]+\.[a-z0-9\-._~!$&'()*+,;=:]+)\]|(?:(?:25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(?:25[0-5]|2[0-4]\d|[01]?\d\d?)|(?:[a-z0-9\-._~!$&'"()*+,;=]|%[0-9a-f]{2})*)(?::\d*)?(?:\/(?:[a-z0-9\-._~!$&'"()*+,;=:@]|%[0-9a-f]{2})*)*|\/(?:(?:[a-z0-9\-._~!$&'"()*+,;=:@]|%[0-9a-f]{2})+(?:\/(?:[a-z0-9\-._~!$&'"()*+,;=:@]|%[0-9a-f]{2})*)*)?|(?:[a-z0-9\-._~!$&'"()*+,;=:@]|%[0-9a-f]{2})+(?:\/(?:[a-z0-9\-._~!$&'"()*+,;=:@]|%[0-9a-f]{2})*)*)?(?:\?(?:[a-z0-9\-._~!$&'"()*+,;=:@/?]|%[0-9a-f]{2})*)?(?:#(?:[a-z0-9\-._~!$&'"()*+,;=:@/?]|%[0-9a-f]{2})*)?$/i;
 // uri-template: https://tools.ietf.org/html/rfc6570
@@ -70,7 +70,7 @@ formats.full = {
   'uri-template': URITEMPLATE,
   url: URL,
   email: /^[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?$/i,
-  hostname: hostname,
+  hostname: HOSTNAME,
   ipv4: /^(?:(?:25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(?:25[0-5]|2[0-4]\d|[01]?\d\d?)$/,
   ipv6: /^\s*(?:(?:(?:[0-9a-f]{1,4}:){7}(?:[0-9a-f]{1,4}|:))|(?:(?:[0-9a-f]{1,4}:){6}(?::[0-9a-f]{1,4}|(?:(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)(?:\.(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)){3})|:))|(?:(?:[0-9a-f]{1,4}:){5}(?:(?:(?::[0-9a-f]{1,4}){1,2})|:(?:(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)(?:\.(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)){3})|:))|(?:(?:[0-9a-f]{1,4}:){4}(?:(?:(?::[0-9a-f]{1,4}){1,3})|(?:(?::[0-9a-f]{1,4})?:(?:(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)(?:\.(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)){3}))|:))|(?:(?:[0-9a-f]{1,4}:){3}(?:(?:(?::[0-9a-f]{1,4}){1,4})|(?:(?::[0-9a-f]{1,4}){0,2}:(?:(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)(?:\.(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)){3}))|:))|(?:(?:[0-9a-f]{1,4}:){2}(?:(?:(?::[0-9a-f]{1,4}){1,5})|(?:(?::[0-9a-f]{1,4}){0,3}:(?:(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)(?:\.(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)){3}))|:))|(?:(?:[0-9a-f]{1,4}:){1}(?:(?:(?::[0-9a-f]{1,4}){1,6})|(?:(?::[0-9a-f]{1,4}){0,4}:(?:(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)(?:\.(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)){3}))|:))|(?::(?:(?:(?::[0-9a-f]{1,4}){1,7})|(?:(?::[0-9a-f]{1,4}){0,5}:(?:(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)(?:\.(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)){3}))|:)))(?:%.+)?\s*$/i,
   regex: regex,
@@ -123,13 +123,6 @@ function date_time(str) {
 }
 
 
-function hostname(str) {
-  // https://tools.ietf.org/html/rfc1034#section-3.5
-  // https://tools.ietf.org/html/rfc1123#section-2
-  return str.length <= 255 && HOSTNAME.test(str);
-}
-
-
 var NOT_URI_FRAGMENT = /\/|:/;
 function uri(str) {
   // http://jmrware.com/articles/2009/uri_regexp/URI_regex.html + optional protocol + required "."
diff --git a/spec/tests/rules/format.json b/spec/tests/rules/format.json
index 226608bd4..c316a6ee5 100644
--- a/spec/tests/rules/format.json
+++ b/spec/tests/rules/format.json
@@ -82,10 +82,55 @@
         "data": "123.example.com",
         "valid": true
       },
+      {
+        "description": "valid hostname - trailing dot",
+        "data": "123.example.com.",
+        "valid": true
+      },
+      {
+        "description": "valid hostname - single label",
+        "data": "localhost",
+        "valid": true
+      },
+      {
+        "description": "valid hostname - single label with trailing dot",
+        "data": "localhost.",
+        "valid": true
+      },
       {
         "description": "valid hostname #312",
         "data": "lead-routing-qa.lvuucj.0001.use1.cache.amazonaws.com",
         "valid": true
+      },
+      {
+        "description": "valid hostname - maximum length label (63 chars)",
+        "data": "abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijk.example.com",
+        "valid": true
+      },
+      {
+        "description": "invalid hostname - label too long (64 chars)",
+        "data": "abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijkl.example.com",
+        "valid": false
+      },
+      {
+        "description": "valid hostname - maximum length hostname (255 octets)",
+        "data": "abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxy.example.com",
+        "valid": true
+      },
+      {
+        "description": "valid hostname - maximum length hostname (255 octets) with trailing dot",
+        "data": "abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxy.example.com.",
+        "valid": true
+      },
+      {
+        "description": "invalid hostname - hostname too long (256 octets)",
+        "data": "abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.example.com",
+        "valid": false
+      },
+      {
+        "description": "invalid hostname - hostname too long (256 octets) with trailing dot",
+        "data": "abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.example.com.",
+        "valid": false
       }
     ]
   },