Add hostmask() function:

+       <entry><function>hostmask</function>(<type>inet</type>)</entry>
+       <entry><type>inet</type></entry>
+       <entry>construct hostmask for network</entry>
+       <entry><literal>hostmask('192.168.23.20/30')</literal></entry>
+       <entry><literal>0.0.0.3</literal></entry>

Greg Wickham

doc/src/sgml/func.sgml

 <!--
-$Header: /cvsroot/pgsql/doc/src/sgml/func.sgml,v 1.145 2003/03/20 18:58:02 momjian Exp $
+$Header: /cvsroot/pgsql/doc/src/sgml/func.sgml,v 1.146 2003/03/21 21:54:29 momjian Exp $
 PostgreSQL documentation
 -->
 
@@ -5899,6 +5899,13 @@ SELECT TIMESTAMP 'now';
 	<entry><literal>netmask('192.168.1.5/24')</literal></entry>
 	<entry><literal>255.255.255.0</literal></entry>
        </row>
+       <row>
+	<entry><function>hostmask</function>(<type>inet</type>)</entry>
+	<entry><type>inet</type></entry>
+	<entry>construct hostmask for network</entry>
+	<entry><literal>hostmask('192.168.23.20/30')</literal></entry>
+	<entry><literal>0.0.0.3</literal></entry>
+       </row>
        <row>
 	<entry><literal><function>network</function>(<type>inet</type>)</literal></entry>
 	<entry><type>cidr</type></entry>

src/backend/utils/adt/network.c

@@ -3,7 +3,7 @@
  *	is for IP V4 CIDR notation, but prepared for V6: just
  *	add the necessary bits where the comments indicate.
  *
- *	$Header: /cvsroot/pgsql/src/backend/utils/adt/network.c,v 1.38 2002/11/13 00:39:47 momjian Exp $
+ *	$Header: /cvsroot/pgsql/src/backend/utils/adt/network.c,v 1.39 2003/03/21 21:54:29 momjian Exp $
  *
  *	Jon Postel RIP 16 Oct 1998
  */
@@ -605,6 +605,46 @@ network_netmask(PG_FUNCTION_ARGS)
 	PG_RETURN_INET_P(dst);
 }
 
+Datum
+network_hostmask(PG_FUNCTION_ARGS)
+{
+	inet	   *ip = PG_GETARG_INET_P(0);
+	inet	   *dst;
+
+	dst = (inet *) palloc(VARHDRSZ + sizeof(inet_struct));
+	/* make sure any unused bits are zeroed */
+	MemSet(dst, 0, VARHDRSZ + sizeof(inet_struct));
+
+	if (ip_family(ip) == AF_INET)
+	{
+		/* It's an IP V4 address: */
+		unsigned long mask = 0xffffffff;
+
+		/*
+		 * Only shift if the mask len is < 32 bits ..
+		 */
+
+		if (ip_bits(ip) < 32)
+			mask >>= ip_bits(ip);
+		else
+			mask = 0;
+
+		ip_v4addr(dst) = htonl(mask);
+
+		ip_bits(dst) = 32;
+	}
+	else
+		/* Go for an IPV6 address here, before faulting out: */
+		elog(ERROR, "unknown address family (%d)", ip_family(ip));
+
+	ip_family(dst) = ip_family(ip);
+	ip_type(dst) = 0;
+	VARATT_SIZEP(dst) = VARHDRSZ
+		+ ((char *) &ip_v4addr(dst) - (char *) VARDATA(dst))
+		+ ip_addrsize(dst);
+
+	PG_RETURN_INET_P(dst);
+}
 
 /*
  * Convert a value of a network datatype to an approximate scalar value.

src/include/catalog/pg_proc.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2002, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: pg_proc.h,v 1.289 2003/03/20 18:58:02 momjian Exp $
+ * $Id: pg_proc.h,v 1.290 2003/03/21 21:54:29 momjian Exp $
  *
  * NOTES
  *	  The script catalog/genbki.sh reads this file and generates .bki
@@ -2358,6 +2358,8 @@ DATA(insert OID = 699 (  host				PGNSP PGUID 12 f f t f i 1 25 "869"  network_ho
 DESCR("show address octets only");
 DATA(insert OID = 730 (  text				PGNSP PGUID 12 f f t f i 1 25 "869"  network_show - _null_ ));
 DESCR("show all parts of inet/cidr value");
+DATA(insert OID = 1362 (  hostmask			PGNSP PGUID 12 f f t f i 1 869 "869"	network_hostmask - _null_ ));
+DESCR("hostmask of address");
 DATA(insert OID = 1713 (  inet				PGNSP PGUID 12 f f t f i 1 869 "25"  text_inet - _null_ ));
 DESCR("text to inet");
 DATA(insert OID = 1714 (  cidr				PGNSP PGUID 12 f f t f i 1 650 "25"  text_cidr - _null_ ));