diff --git a/src/arch/i386/firmware/pcbios/smbios_settings.c b/src/arch/i386/firmware/pcbios/smbios_settings.c
index b088e51dc..3238fb195 100644
--- a/src/arch/i386/firmware/pcbios/smbios_settings.c
+++ b/src/arch/i386/firmware/pcbios/smbios_settings.c
@@ -24,6 +24,16 @@
 #include <gpxe/uuid.h>
 #include <smbios.h>
 
+/** SMBIOS settings tag magic number */
+#define SMBIOS_TAG_MAGIC 0x5B /* "SmBios" */
+
+/**
+ * Construct SMBIOS empty tag
+ *
+ * @ret tag		SMBIOS setting tag
+ */
+#define SMBIOS_EMPTY_TAG ( SMBIOS_TAG_MAGIC << 24 )
+
 /**
  * Construct SMBIOS raw-data tag
  *
@@ -33,7 +43,8 @@
  * @ret tag		SMBIOS setting tag
  */
 #define SMBIOS_RAW_TAG( _type, _structure, _field )		\
-	( ( (_type) << 16 ) |					\
+	( ( SMBIOS_TAG_MAGIC << 24 ) |				\
+	  ( (_type) << 16 ) |					\
 	  ( offsetof ( _structure, _field ) << 8 ) |		\
 	  ( sizeof ( ( ( _structure * ) 0 )->_field ) ) )
 
@@ -46,7 +57,8 @@
  * @ret tag		SMBIOS setting tag
  */
 #define SMBIOS_STRING_TAG( _type, _structure, _field )		\
-	( ( (_type) << 16 ) |					\
+	( ( SMBIOS_TAG_MAGIC << 24 ) |				\
+	  ( (_type) << 16 ) |					\
 	  ( offsetof ( _structure, _field ) << 8 ) )
 
 /**
@@ -78,16 +90,18 @@ static int smbios_fetch ( struct settings *settings __unused,
 			  struct setting *setting,
 			  void *data, size_t len ) {
 	struct smbios_structure structure;
+	unsigned int tag_magic;
 	unsigned int tag_type;
 	unsigned int tag_offset;
 	unsigned int tag_len;
 	int rc;
 
 	/* Split tag into type, offset and length */
-	tag_type = ( setting->tag >> 16 );
+	tag_magic = ( setting->tag >> 24 );
+	tag_type = ( ( setting->tag >> 16 ) & 0xff );
 	tag_offset = ( ( setting->tag >> 8 ) & 0xff );
 	tag_len = ( setting->tag & 0xff );
-	if ( ! tag_type )
+	if ( tag_magic != SMBIOS_TAG_MAGIC )
 		return -ENOENT;
 
 	/* Find SMBIOS structure */
@@ -127,6 +141,7 @@ static struct settings_operations smbios_settings_operations = {
 static struct settings smbios_settings = {
 	.refcnt = NULL,
 	.name = "smbios",
+	.tag_magic = SMBIOS_EMPTY_TAG,
 	.siblings = LIST_HEAD_INIT ( smbios_settings.siblings ),
 	.children = LIST_HEAD_INIT ( smbios_settings.children ),
 	.op = &smbios_settings_operations,
diff --git a/src/core/nvo.c b/src/core/nvo.c
index 13078022b..e5c07d986 100644
--- a/src/core/nvo.c
+++ b/src/core/nvo.c
@@ -203,7 +203,7 @@ void nvo_init ( struct nvo_block *nvo, struct nvs_device *nvs,
 	nvo->nvs = nvs;
 	nvo->fragments = fragments;
 	settings_init ( &nvo->settings, &nvo_settings_operations, refcnt,
-			"nvo" );
+			"nvo", 0 );
 }
 
 /**
diff --git a/src/core/settings.c b/src/core/settings.c
index f97842f4b..9ad2ced26 100644
--- a/src/core/settings.c
+++ b/src/core/settings.c
@@ -665,6 +665,7 @@ static int parse_setting_name ( const char *name, struct settings **settings,
 			}
 			tmp++;
 		}
+		setting->tag |= (*settings)->tag_magic;
 	}
 
 	/* Identify setting type, if specified */
diff --git a/src/drivers/net/phantom/phantom.c b/src/drivers/net/phantom/phantom.c
index 659bd2c1e..47bbcb9a4 100644
--- a/src/drivers/net/phantom/phantom.c
+++ b/src/drivers/net/phantom/phantom.c
@@ -1589,6 +1589,12 @@ static struct net_device_operations phantom_operations = {
  *
  */
 
+/** Phantom CLP settings tag magic */
+#define PHN_CLP_TAG_MAGIC 0xc19c1900UL
+
+/** Phantom CLP settings tag magic mask */
+#define PHN_CLP_TAG_MAGIC_MASK 0xffffff00UL
+
 /** Phantom CLP data
  *
  */
@@ -1790,7 +1796,7 @@ struct phantom_clp_setting {
 	/** gPXE setting */
 	struct setting *setting;
 	/** Setting number */
-	unsigned int number;
+	unsigned int clp_setting;
 };
 
 /** Phantom CLP settings */
@@ -1802,25 +1808,29 @@ static struct phantom_clp_setting clp_settings[] = {
  * Find Phantom CLP setting
  *
  * @v setting		gPXE setting
- * @v clp_setting	Equivalent Phantom CLP setting, or NULL
+ * @v clp_setting	Setting number, or 0 if not found
  */
-static struct phantom_clp_setting *
-phantom_find_clp_setting ( struct phantom_nic *phantom,
-			   struct setting *setting ) {
+static unsigned int
+phantom_clp_setting ( struct phantom_nic *phantom, struct setting *setting ) {
 	struct phantom_clp_setting *clp_setting;
 	unsigned int i;
 
+	/* Search the list of explicitly-defined settings */
 	for ( i = 0 ; i < ( sizeof ( clp_settings ) /
 			    sizeof ( clp_settings[0] ) ) ; i++ ) {
 		clp_setting = &clp_settings[i];
 		if ( setting_cmp ( setting, clp_setting->setting ) == 0 )
-			return clp_setting;
+			return clp_setting->clp_setting;
 	}
 
+	/* Allow for use of numbered settings */
+	if ( ( setting->tag & PHN_CLP_TAG_MAGIC_MASK ) == PHN_CLP_TAG_MAGIC )
+		return ( setting->tag & ~PHN_CLP_TAG_MAGIC_MASK );
+
 	DBGC2 ( phantom, "Phantom %p has no \"%s\" setting\n",
 		phantom, setting->name );
 
-	return NULL;
+	return 0;
 }
 
 /**
@@ -1838,18 +1848,17 @@ static int phantom_store_setting ( struct settings *settings,
 	struct phantom_nic_port *phantom_port =
 		container_of ( settings, struct phantom_nic_port, settings );
 	struct phantom_nic *phantom = phantom_port->phantom;
-	struct phantom_clp_setting *clp_setting;
+	unsigned int clp_setting;
 	int rc;
 
 	/* Find Phantom setting equivalent to gPXE setting */
-	clp_setting = phantom_find_clp_setting ( phantom, setting );
+	clp_setting = phantom_clp_setting ( phantom, setting );
 	if ( ! clp_setting )
 		return -ENOTSUP;
 
 	/* Store setting */
 	if ( ( rc = phantom_clp_store ( phantom, phantom_port->port,
-					clp_setting->number,
-					data, len ) ) != 0 ) {
+					clp_setting, data, len ) ) != 0 ) {
 		DBGC ( phantom, "Phantom %p could not store setting \"%s\": "
 		       "%s\n", phantom, setting->name, strerror ( rc ) );
 		return rc;
@@ -1873,19 +1882,18 @@ static int phantom_fetch_setting ( struct settings *settings,
 	struct phantom_nic_port *phantom_port =
 		container_of ( settings, struct phantom_nic_port, settings );
 	struct phantom_nic *phantom = phantom_port->phantom;
-	struct phantom_clp_setting *clp_setting;
+	unsigned int clp_setting;
 	int read_len;
 	int rc;
 
 	/* Find Phantom setting equivalent to gPXE setting */
-	clp_setting = phantom_find_clp_setting ( phantom, setting );
+	clp_setting = phantom_clp_setting ( phantom, setting );
 	if ( ! clp_setting )
 		return -ENOTSUP;
 
 	/* Fetch setting */
 	if ( ( read_len = phantom_clp_fetch ( phantom, phantom_port->port,
-					      clp_setting->number,
-					      data, len ) ) < 0 ) {
+					      clp_setting, data, len ) ) < 0 ){
 		rc = read_len;
 		DBGC ( phantom, "Phantom %p could not fetch setting \"%s\": "
 		       "%s\n", phantom, setting->name, strerror ( rc ) );
@@ -2269,7 +2277,7 @@ static int phantom_probe ( struct pci_device *pci,
 		phantom_port->port = i;
 		settings_init ( &phantom_port->settings,
 				&phantom_settings_operations,
-				&netdev->refcnt, "clp" );
+				&netdev->refcnt, "clp", PHN_CLP_TAG_MAGIC );
 	}
 
 	/* BUG5945 - need to hack PCI config space on P3 B1 silicon.
diff --git a/src/include/gpxe/settings.h b/src/include/gpxe/settings.h
index ee7dacee3..37c01b058 100644
--- a/src/include/gpxe/settings.h
+++ b/src/include/gpxe/settings.h
@@ -72,6 +72,14 @@ struct settings {
 	struct refcnt *refcnt;
 	/** Name */
 	const char *name;
+	/** Tag magic
+	 *
+	 * This value will be ORed in to any numerical tags
+	 * constructed by parse_setting_name(), and can be used to
+	 * avoid e.g. attempting to retrieve the subnet mask from
+	 * SMBIOS, or the system UUID from DHCP.
+	 */
+	unsigned int tag_magic;
 	/** Parent settings block */
 	struct settings *parent;
 	/** Sibling settings blocks */
@@ -225,16 +233,19 @@ extern struct setting mac_setting __setting;
  * @v op		Settings block operations
  * @v refcnt		Containing object reference counter, or NULL
  * @v name		Settings block name
+ * @v tag_magic		Tag magic
  */
 static inline void settings_init ( struct settings *settings,
 				   struct settings_operations *op,
 				   struct refcnt *refcnt,
-				   const char *name ) {
+				   const char *name,
+				   unsigned int tag_magic ) {
 	INIT_LIST_HEAD ( &settings->siblings );
 	INIT_LIST_HEAD ( &settings->children );
 	settings->op = op;
 	settings->refcnt = refcnt;
 	settings->name = name;
+	settings->tag_magic = tag_magic;
 }
 
 /**
@@ -248,7 +259,7 @@ static inline void simple_settings_init ( struct simple_settings *simple,
 					  struct refcnt *refcnt,
 					  const char *name ) {
 	settings_init ( &simple->settings, &simple_settings_operations,
-			refcnt, name );
+			refcnt, name, 0 );
 }
 
 /**
diff --git a/src/net/netdevice.c b/src/net/netdevice.c
index 6ea90b41b..81c4ae886 100644
--- a/src/net/netdevice.c
+++ b/src/net/netdevice.c
@@ -268,7 +268,7 @@ struct net_device * alloc_netdev ( size_t priv_size ) {
 		INIT_LIST_HEAD ( &netdev->rx_queue );
 		settings_init ( netdev_settings ( netdev ),
 				&netdev_settings_operations, &netdev->refcnt,
-				netdev->name );
+				netdev->name, 0 );
 		netdev->priv = ( ( ( void * ) netdev ) + sizeof ( *netdev ) );
 	}
 	return netdev;
diff --git a/src/net/udp/dhcp.c b/src/net/udp/dhcp.c
index ab751cd53..213478323 100644
--- a/src/net/udp/dhcp.c
+++ b/src/net/udp/dhcp.c
@@ -256,7 +256,7 @@ static struct dhcp_settings * dhcpset_create ( const struct dhcphdr *dhcphdr,
 		dhcppkt_init ( &dhcpset->dhcppkt, data, len );
 		settings_init ( &dhcpset->settings,
 				&dhcpset_settings_operations, &dhcpset->refcnt,
-				DHCP_SETTINGS_NAME );
+				DHCP_SETTINGS_NAME, 0 );
 	}
 	return dhcpset;
 }