diff -up opensm-3.3.13/man/opensm.8.in.prefix opensm-3.3.13/man/opensm.8.in
--- opensm-3.3.13/man/opensm.8.in.prefix	2012-02-28 18:27:33.297714661 -0500
+++ opensm-3.3.13/man/opensm.8.in	2012-02-28 18:31:00.957696942 -0500
@@ -11,6 +11,7 @@ opensm \- InfiniBand subnet manager and 
 [\-g(uid) <GUID in hex>]
 [\-l(mc) <LMC>]
 [\-p(riority) <PRIORITY>]
+[\-\-subnet_prefix <PREFIX in hex>]
 [\-smkey <SM_Key>]
 [\-\-sm_sl <SL number>]
 [\-r(eassign_lids)]
@@ -130,6 +131,13 @@ This will effect the handover cases, whe
 is chosen by priority and GUID.  Range goes from 0
 (default and lowest priority) to 15 (highest).
 .TP
+\fB\-\-subnet_prefix\fR <PREFIX in hex>
+This option specifies the subnet prefix to use in
+on the fabric.  The default prefix is
+0xfe80000000000000.  OpenMPI in particular requires
+separate fabrics plugged into different ports to
+have different prefixes or else it won't run.
+.TP
 \fB\-smkey\fR <SM_Key value>
 This option specifies the SM\'s SM_Key (64 bits).
 This will effect SM authentication.
diff -up opensm-3.3.13/opensm/main.c.prefix opensm-3.3.13/opensm/main.c
--- opensm-3.3.13/opensm/main.c.prefix	2012-01-17 08:22:40.000000000 -0500
+++ opensm-3.3.13/opensm/main.c	2012-02-28 18:31:34.224694111 -0500
@@ -156,6 +156,9 @@ static void show_usage(void)
 	       "          This will effect the handover cases, where master\n"
 	       "          is chosen by priority and GUID.  Range goes\n"
 	       "          from 0 (lowest priority) to 15 (highest).\n\n");
+	printf("--subnet_prefix <prefix>\n"
+	       "          Set the subnet prefix to something other than the\n"
+	       "          default value of 0xfe80000000000000\n\n");
 	printf("--smkey, -k <SM_Key>\n"
 	       "          This option specifies the SM's SM_Key (64 bits).\n"
 	       "          This will effect SM authentication.\n"
@@ -607,6 +610,7 @@ int main(int argc, char *argv[])
 		{"once", 0, NULL, 'o'},
 		{"reassign_lids", 0, NULL, 'r'},
 		{"priority", 1, NULL, 'p'},
+		{"subnet_prefix", 1, NULL, 13},
 		{"smkey", 1, NULL, 'k'},
 		{"routing_engine", 1, NULL, 'R'},
 		{"ucast_cache", 0, NULL, 'A'},
@@ -911,6 +915,11 @@ int main(int argc, char *argv[])
 			printf(" Priority = %d\n", temp);
 			break;
 
+		case 13:
+			opt.subnet_prefix = cl_hton64(strtoull(optarg, NULL, 16));
+			printf(" Subnet_Prefix = <0x%" PRIx64 ">\n", cl_hton64(opt.subnet_prefix));
+			break;
+
 		case 'k':
 			sm_key = cl_hton64(strtoull(optarg, NULL, 16));
 			printf(" SM Key <0x%" PRIx64 ">\n", cl_hton64(sm_key));