diff --git a/cookiejar.gemspec b/cookiejar.gemspec
index 1c58a00..7413964 100644
--- a/cookiejar.gemspec
+++ b/cookiejar.gemspec
@@ -20,6 +20,7 @@ Gem::Specification.new do |s|
   s.rdoc_options  = ['--title', 'CookieJar -- Client-side HTTP Cookies']
   s.require_paths = ['lib']
 
+  s.add_dependency 'public_suffix', '~> 4.0'
   s.add_development_dependency 'rake', '>= 10.0', '< 13'
   s.add_development_dependency 'rspec-collection_matchers', '~> 1.0'
   s.add_development_dependency 'rspec', '~> 3.0'
diff --git a/lib/cookiejar/cookie_validation.rb b/lib/cookiejar/cookie_validation.rb
index a64c607..c7372aa 100644
--- a/lib/cookiejar/cookie_validation.rb
+++ b/lib/cookiejar/cookie_validation.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 require 'cgi'
 require 'uri'
+require 'public_suffix'
 
 module CookieJar
   # Represents a set of cookie validation errors
@@ -107,7 +108,7 @@ def self.hostname_reach(hostname)
       host = to_domain hostname
       host = host.downcase
       match = BASE_HOSTNAME.match host
-      match[1] if match
+      match[1] if match && (PublicSuffix.valid?(match[1]) || match[1] == 'local')
     end
 
     # Compute the base of a path, for default cookie path assignment
@@ -161,10 +162,11 @@ def self.compute_search_domains(request_uri)
     def self.compute_search_domains_for_host(host)
       host = effective_host host
       result = [host]
-      unless host =~ IPADDR
+      return result if host =~ IPADDR
+      loop do
         result << ".#{host}"
-        base = hostname_reach host
-        result << ".#{base}" if base
+        host = hostname_reach(host)
+        break unless host
       end
       result
     end
diff --git a/spec/cookie_validation_spec.rb b/spec/cookie_validation_spec.rb
index e4cae54..424ed0a 100644
--- a/spec/cookie_validation_spec.rb
+++ b/spec/cookie_validation_spec.rb
@@ -36,11 +36,6 @@
         Cookie.from_set_cookie 'http://www.foo.com/', 'foo=bar;domain=bar.com'
       end).to raise_error InvalidCookieError
     end
-    it 'should fail for domains more than one level up' do
-      expect(lambda do
-        Cookie.from_set_cookie 'http://x.y.z.com/', 'foo=bar;domain=z.com'
-      end).to raise_error InvalidCookieError
-    end
     it 'should fail for setting subdomain cookies' do
       expect(lambda do
         Cookie.from_set_cookie 'http://foo.com/', 'foo=bar;domain=auth.foo.com'
@@ -111,7 +106,7 @@
   describe '#compute_search_domains' do
     it 'should handle subdomains' do
       expect(CookieValidation.compute_search_domains('http://www.auth.foo.com/')).to eq(
-        ['www.auth.foo.com', '.www.auth.foo.com', '.auth.foo.com'])
+        ['www.auth.foo.com', '.www.auth.foo.com', '.auth.foo.com', '.foo.com'])
     end
     it 'should handle root domains' do
       expect(CookieValidation.compute_search_domains('http://foo.com/')).to eq(
@@ -129,6 +124,10 @@
       expect(CookieValidation.compute_search_domains('http://zero/')).to eq(
         ['zero.local', '.zero.local', '.local'])
     end
+    it 'should handle multi-part tlds' do
+      expect(CookieValidation.compute_search_domains('http://foo.co.nz/')).to eq(
+        ['foo.co.nz', '.foo.co.nz'])
+    end
   end
   describe '#determine_cookie_domain' do
     it 'should add a dot to the front of domains' do
@@ -190,9 +189,10 @@
     it 'should handle matching a superdomain' do
       expect(CookieValidation.domains_match('.foo.com', 'auth.foo.com')).to eq '.foo.com'
       expect(CookieValidation.domains_match('.y.z.foo.com', 'x.y.z.foo.com')).to eq '.y.z.foo.com'
+      expect(CookieValidation.domains_match('.z.foo.com', 'x.y.z.foo.com')).to eq '.z.foo.com'
     end
-    it 'should not match superdomains, or illegal domains' do
-      expect(CookieValidation.domains_match('.z.foo.com', 'x.y.z.foo.com')).to be_nil
+    it 'should not match tlds, or illegal domains' do
+      expect(CookieValidation.domains_match('.com', 'x.y.z.foo.com')).to be_nil
       expect(CookieValidation.domains_match('foo.com', 'com')).to be_nil
     end
     it 'should not match domains with and without a dot suffix together' do
@@ -211,6 +211,9 @@
     it 'should return nil for a root domain' do
       expect(CookieValidation.hostname_reach('github.com')).to be_nil
     end
+    it 'should return nil for a two part tld' do
+      expect(CookieValidation.hostname_reach('co.nz')).to be_nil
+    end
     it "should return 'local' for a local domain" do
       ['foo.local', 'foo.local.'].each do |hostname|
         expect(CookieValidation.hostname_reach(hostname)).to eq 'local'