diff --git a/lib/spack/docs/packaging_guide.rst b/lib/spack/docs/packaging_guide.rst
index efd4c459a204cc236e843f69d60e3a5f3d2fe08b..bf5763f4f8cdaa91f76d4a987811a8e91d596a1d 100644
--- a/lib/spack/docs/packaging_guide.rst
+++ b/lib/spack/docs/packaging_guide.rst
@@ -526,32 +526,57 @@ in the package.  For example, Spack is smart enough to download
 version ``8.2.1.`` of the ``Foo`` package above from
 ``http://example.com/foo-8.2.1.tar.gz``.
 
-If spack *cannot* extrapolate the URL from the ``url`` field by
-default, you can write your own URL generation algorithm in place of
-the ``url`` declaration.  For example:
+If the URL is particularly complicated or changes based on the release,
+you can override the default URL generation algorithm by defining your
+own ``url_for_version()`` function. For example, the developers of HDF5
+keep changing the archive layout, so the ``url_for_version()`` function
+looks like:
+
+.. literalinclude:: ../../../var/spack/repos/builtin/packages/hdf5/package.py
+   :pyobject: Hdf5.url_for_version
+
+With the use of this ``url_for_version()``, Spack knows to download HDF5 ``1.8.16``
+from ``http://www.hdfgroup.org/ftp/HDF5/releases/hdf5-1.8.16/src/hdf5-1.8.16.tar.gz``
+but download HDF5 ``1.10.0`` from ``http://www.hdfgroup.org/ftp/HDF5/releases/hdf5-1.10/hdf5-1.10.0/src/hdf5-1.10.0.tar.gz``.
+
+You'll notice that HDF5's ``url_for_version()`` function makes use of a special
+``Version`` function called ``up_to()``. When you call ``version.up_to(2)`` on a
+version like ``1.10.0``, it returns ``1.10``. ``version.up_to(1)`` would return
+``1``. This can be very useful for packages that place all ``X.Y.*`` versions in
+a single directory and then places all ``X.Y.Z`` versions in a subdirectory.
+
+There are a few ``Version`` properties you should be aware of. We generally
+prefer numeric versions to be separated by dots for uniformity, but not all
+tarballs are named that way. For example, ``icu4c`` separates its major and minor
+versions with underscores, like ``icu4c-57_1-src.tgz``. The value ``57_1`` can be
+obtained with the use of the ``version.underscored`` property. Note that Python
+properties don't need parentheses. There are other separator properties as well:
+
+===================  ======
+Property             Result
+===================  ======
+version.dotted       1.2.3
+version.dashed       1-2-3
+version.underscored  1_2_3
+version.joined       123
+===================  ======
 
-.. code-block:: python
-   :linenos:
+.. note::
 
-   class Foo(Package):
-       version('8.2.1', '4136d7b4c04df68b686570afa26988ac')
-       ...
-       def url_for_version(self, version):
-           return 'http://example.com/version_%s/foo-%s.tar.gz' \
-               % (version, version)
-       ...
+   Python properties don't need parentheses. ``version.dashed`` is correct.
+   ``version.dashed()`` is incorrect.
 
-If a URL cannot be derived systematically, you can add an explicit URL
-for a particular version:
+If a URL cannot be derived systematically, or there is a special URL for one
+of its versions, you can add an explicit URL for a particular version:
 
 .. code-block:: python
 
    version('8.2.1', '4136d7b4c04df68b686570afa26988ac',
            url='http://example.com/foo-8.2.1-special-version.tar.gz')
 
-For the URL above, you might have to add an explicit URL because the
-version can't simply be substituted in the original ``url`` to
-construct the new one for ``8.2.1``.
+This is common for Python packages that download from PyPi. Since newer
+download URLs often contain a unique hash for each version, there is no
+way to guess the URL systematically.
 
 When you supply a custom URL for a version, Spack uses that URL
 *verbatim* and does not perform extrapolation.
diff --git a/lib/spack/spack/test/versions.py b/lib/spack/spack/test/versions.py
index 9b4dc29f356080bec4952bd679529ae61cfeafd2..c1d427783c777c2da0ba9a40cfbfb6f72571ac88 100644
--- a/lib/spack/spack/test/versions.py
+++ b/lib/spack/spack/test/versions.py
@@ -392,6 +392,7 @@ def test_formatted_strings(self):
             self.assertEqual(v.dotted, '1.2.3')
             self.assertEqual(v.dashed, '1-2-3')
             self.assertEqual(v.underscored, '1_2_3')
+            self.assertEqual(v.joined, '123')
 
     def test_repr_and_str(self):
 
diff --git a/lib/spack/spack/version.py b/lib/spack/spack/version.py
index 67a22f4660abe07d3da7100e8046d1d57495f2b2..0d68a709e8615186911861db17a6eb9333534cd0 100644
--- a/lib/spack/spack/version.py
+++ b/lib/spack/spack/version.py
@@ -146,6 +146,10 @@ def underscored(self):
     def dashed(self):
         return '-'.join(str(x) for x in self.version)
 
+    @property
+    def joined(self):
+        return ''.join(str(x) for x in self.version)
+
     def up_to(self, index):
         """Return a version string up to the specified component, exclusive.
            e.g., if this is 10.8.2, self.up_to(2) will return '10.8'.
diff --git a/var/spack/repos/builtin/packages/cdd/package.py b/var/spack/repos/builtin/packages/cdd/package.py
index bff942af2560b453f4a25fb3ad0751f17861ea43..4a0a0aefef751a84852229bd8d8e83083652b2b6 100644
--- a/var/spack/repos/builtin/packages/cdd/package.py
+++ b/var/spack/repos/builtin/packages/cdd/package.py
@@ -35,16 +35,16 @@ class Cdd(Package):
     homepage = "https://www.inf.ethz.ch/personal/fukudak/cdd_home/cdd.html"
     url      = "ftp://ftp.ifor.math.ethz.ch/pub/fukuda/cdd/cdd-061a.tar.gz"
 
-    def url_for_version(self, version):
-        return ("ftp://ftp.ifor.math.ethz.ch/pub/fukuda/cdd/cdd-%s.tar.gz" %
-                str(version.dotted).replace('.', ''))
-
     version('0.61a', '22c24a7a9349dd7ec0e24531925a02d9')
 
     depends_on("libtool", type="build")
 
     patch("Makefile.spack.patch")
 
+    def url_for_version(self, version):
+        url = "ftp://ftp.ifor.math.ethz.ch/pub/fukuda/cdd/cdd-{0}.tar.gz"
+        return url.format(version.joined)
+
     def install(self, spec, prefix):
         # The Makefile isn't portable; use our own instead
         makeargs = ["-f", "Makefile.spack", "PREFIX=%s" % prefix]
diff --git a/var/spack/repos/builtin/packages/cfitsio/package.py b/var/spack/repos/builtin/packages/cfitsio/package.py
index 35d9662f6f66ae2f1bef9ca028ee40f4108cd588..79af31ae216cb947fc94ff5dd36558e8a984fbd6 100644
--- a/var/spack/repos/builtin/packages/cfitsio/package.py
+++ b/var/spack/repos/builtin/packages/cfitsio/package.py
@@ -34,9 +34,9 @@ class Cfitsio(Package):
 
     version('3.370', 'abebd2d02ba5b0503c633581e3bfa116')
 
-    def url_for_version(self, v):
+    def url_for_version(self, version):
         url = 'ftp://heasarc.gsfc.nasa.gov/software/fitsio/c/cfitsio{0}.tar.gz'
-        return url.format(str(v).replace('.', ''))
+        return url.format(version.joined)
 
     def install(self, spec, prefix):
         configure('--prefix=' + prefix)
diff --git a/var/spack/repos/builtin/packages/cryptopp/package.py b/var/spack/repos/builtin/packages/cryptopp/package.py
index e9294a14a6d45b57d1e79395ab1adea911dde090..c92f262a9a4aef47c8e0a00dc17981b667f4e982 100644
--- a/var/spack/repos/builtin/packages/cryptopp/package.py
+++ b/var/spack/repos/builtin/packages/cryptopp/package.py
@@ -36,12 +36,15 @@ class Cryptopp(Package):
     """
 
     homepage = "http://www.cryptopp.com"
-    base_url = "http://www.cryptopp.com"
 
     version('5.6.3', '3c5b70e2ec98b7a24988734446242d07')
     version('5.6.2', '7ed022585698df48e65ce9218f6c6a67')
     version('5.6.1', '96cbeba0907562b077e26bcffb483828')
 
+    def url_for_version(self, version):
+        url = "{0}/{1}{2}.zip"
+        return url.format(self.homepage, self.name, version.joined)
+
     def install(self, spec, prefix):
         make()
 
@@ -51,7 +54,3 @@ def install(self, spec, prefix):
 
         mkdirp(prefix.lib)
         install('libcryptopp.a', prefix.lib)
-
-    def url_for_version(self, version):
-        version_string = str(version).replace('.', '')
-        return '%s/cryptopp%s.zip' % (Cryptopp.base_url, version_string)
diff --git a/var/spack/repos/builtin/packages/lrslib/package.py b/var/spack/repos/builtin/packages/lrslib/package.py
index 68a907ea59a43d35606e731c68a669620f24f44f..3825867bb6603e6b7caa3668b438e081de053377 100644
--- a/var/spack/repos/builtin/packages/lrslib/package.py
+++ b/var/spack/repos/builtin/packages/lrslib/package.py
@@ -33,10 +33,6 @@ class Lrslib(Package):
     homepage = "http://cgm.cs.mcgill.ca/~avis/C/lrs.html"
     url      = "http://cgm.cs.mcgill.ca/~avis/C/lrslib/archive/lrslib-062.tar.gz"
 
-    def url_for_version(self, version):
-        return ("http://cgm.cs.mcgill.ca/~avis/C/lrslib/archive/lrslib-%s.tar.gz" %
-                ('0' + str(version).replace('.', '')))
-
     version('6.2', 'be5da7b3b90cc2be628dcade90c5d1b9')
     version('6.1', '0b3687c8693cd7d1f234a3f65e147551')
     version('6.0', 'd600a2e62969ad03f7ab2f85f1b3709c')
@@ -51,6 +47,10 @@ def url_for_version(self, version):
 
     patch("Makefile.spack.patch")
 
+    def url_for_version(self, version):
+        url = "http://cgm.cs.mcgill.ca/~avis/C/lrslib/archive/lrslib-0{0}.tar.gz"
+        return url.format(version.joined)
+
     def install(self, spec, prefix):
         # The Makefile isn't portable; use our own instead
         makeargs = ["-f", "Makefile.spack",
diff --git a/var/spack/repos/builtin/packages/nag/package.py b/var/spack/repos/builtin/packages/nag/package.py
index 792e3fe3c7ad27e28511dc3cf5eed8df781ed3b7..66cb2a6a54d8e3ef5d2038921a8520a8d4deb2da 100644
--- a/var/spack/repos/builtin/packages/nag/package.py
+++ b/var/spack/repos/builtin/packages/nag/package.py
@@ -30,7 +30,7 @@ class Nag(Package):
     """The NAG Fortran Compiler."""
     homepage = "http://www.nag.com/nagware/np.asp"
 
-    version('6.1', '1e29d9d435b7ccc2842a320150b28ba4')
+    version('6.1', 'f49bd548e0d5e2458b2dabb3ee01341a')
     version('6.0', '3fa1e7f7b51ef8a23e6c687cdcad9f96')
 
     # Licensing
@@ -43,8 +43,8 @@ class Nag(Package):
     def url_for_version(self, version):
         # TODO: url and checksum are architecture dependent
         # TODO: We currently only support x86_64
-        return 'http://www.nag.com/downloads/impl/npl6a%sna_amd64.tgz' % str(
-            version).replace('.', '')
+        url = 'http://www.nag.com/downloads/impl/npl6a{0}na_amd64.tgz'
+        return url.format(version.joined)
 
     def install(self, spec, prefix):
         # Set installation directories
diff --git a/var/spack/repos/builtin/packages/nauty/package.py b/var/spack/repos/builtin/packages/nauty/package.py
index 167edfe8965d612b2fa7ddeadfdb1d47b1147c76..0d5eed251bea4f117188ec5d4713b770baaabe96 100644
--- a/var/spack/repos/builtin/packages/nauty/package.py
+++ b/var/spack/repos/builtin/packages/nauty/package.py
@@ -33,14 +33,14 @@ class Nauty(Package):
     homepage = "http://pallini.di.uniroma1.it/index.html"
     url      = "http://pallini.di.uniroma1.it/nauty26r7.tar.gz"
 
-    def url_for_version(self, version):
-        return ("http://pallini.di.uniroma1.it/nauty%s.tar.gz" %
-                str(version).replace('.', ''))
-
     version('2.6r7', 'b2b18e03ea7698db3fbe06c5d76ad8fe')
     version('2.6r5', '91b03a7b069962e94fc9aac8831ce8d2')
     version('2.5r9', 'e8ecd08b0892a1fb13329c147f08de6d')
 
+    def url_for_version(self, version):
+        url = "http://pallini.di.uniroma1.it/nauty{0}.tar.gz"
+        return url.format(version.joined)
+
     def install(self, spec, prefix):
         configure('--prefix=%s' % prefix)
         make()