add :hostmask()

Signed-off-by: Asbjørn Sloth Tønnesen <ast@2e8.dk>

2 files changed, 25 insertions, 0 deletions

diff --git a/README.rst b/README.rst
index df955e2..186fca3 100644
--- a/README.rst
+++ b/README.rst
@@ -80,6 +80,7 @@ Operator          Description
 ``:ipstring()``   ip as string without prefix
 ``:cidrstring()`` format CIDR notation
 ``:netmask()``    generate netmask as an address
+``:hostmask()``   generate hostmask as an address
 ``:flip()``       flip the least significant network bit
 ================= ======================================
 
@@ -239,6 +240,19 @@ Build an IP address mask with the netmask of ``foo``.
   inet('2001:db8::/64'):netmask() -- returns inet('ffff:ffff:ffff:ffff::')
 
 
+``foo:hostmask()``
+~~~~~~~~~~~~~~~~~
+
+Build an IP address mask with the netmask of ``foo``.
+
+::
+
+  inet('192.0.2.0/24'):hostmask()   -- returns inet('0.0.0.255')
+  inet('2001:db8::/64'):hostmask()  -- returns inet('::ffff:ffff:ffff:ffff')
+  inet('2001:db8::/116'):hostmask() -- returns inet('::fff')
+  inet('2001:db8::/112'):hostmask() -- returns inet('::ffff')
+
+
 ``foo:flip()``
 ~~~~~~~~~~~~~~
 
diff --git a/lua/inet/core.lua b/lua/inet/core.lua
index 0b062e7..f8e17cc 100644
--- a/lua/inet/core.lua
+++ b/lua/inet/core.lua
@@ -385,6 +385,12 @@ function inet4:netmask()
 	return new_inet4(replace(0xffffffff, 0, 0, hostbits), 32)
 end
 
+function inet4:hostmask()
+	local mask = self.mask
+	local hostbits = 32 - mask
+	return new_inet4(replace(0xffffffff, 0, hostbits, mask), 32)
+end
+
 function inet4:flip()
 	-- find twin by flipping the last network bit
 	local mask = self.mask
@@ -716,6 +722,11 @@ function inet6:netmask()
 	return build_inet6_mask(0, mask, 128 - mask)
 end
 
+function inet6:hostmask()
+	local mask = self.mask
+	return build_inet6_mask(mask, 128 - mask, 0)
+end
+
 function inet6:flip()
 	-- find twin by flipping the last network bit
 	local mask = self.mask