diff --git a/Android.mk b/Android.mk
index a640e06..b3b9fa3 100644
--- a/Android.mk
+++ b/Android.mk
@@ -30,7 +30,7 @@
 
 LOCAL_MODULE := clatd_test
 LOCAL_CFLAGS := -Wall -Werror -Wunused-parameter
-LOCAL_SRC_FILES := clatd_test.cpp dump.c checksum.c translate.c icmp.c ipv4.c ipv6.c logging.c
+LOCAL_SRC_FILES := clatd_test.cpp checksum.c translate.c icmp.c ipv4.c ipv6.c logging.c config.c
 LOCAL_MODULE_TAGS := eng tests
 LOCAL_SHARED_LIBRARIES := liblog
 
diff --git a/checksum.h b/checksum.h
index 6195810..d0af88e 100644
--- a/checksum.h
+++ b/checksum.h
@@ -18,6 +18,10 @@
 #ifndef __CHECKSUM_H__
 #define __CHECKSUM_H__
 
+#include <stdint.h>
+#include <netinet/ip.h>
+#include <netinet/ip6.h>
+
 uint32_t ip_checksum_add(uint32_t current, const void *data, int len);
 uint16_t ip_checksum_finish(uint32_t temp_sum);
 uint16_t ip_checksum(const void *data, int len);
diff --git a/clatd.c b/clatd.c
index dbf725b..4b6b8cc 100644
--- a/clatd.c
+++ b/clatd.c
@@ -164,16 +164,16 @@
     return;
   }
 
-  config_generate_local_ipv6_subnet(&interface_ip->ip6);
+  if(!ipv6_prefix_equal(&interface_ip->ip6, &Global_Clatd_Config.ipv6_local_subnet)) {
+    config_generate_local_ipv6_subnet(&interface_ip->ip6);
 
-  if(!IN6_ARE_ADDR_EQUAL(&interface_ip->ip6, &Global_Clatd_Config.ipv6_local_subnet)) {
     char from_addr[INET6_ADDRSTRLEN], to_addr[INET6_ADDRSTRLEN];
     inet_ntop(AF_INET6, &Global_Clatd_Config.ipv6_local_subnet, from_addr, sizeof(from_addr));
     inet_ntop(AF_INET6, &interface_ip->ip6, to_addr, sizeof(to_addr));
-    logmsg(ANDROID_LOG_WARN, "clat subnet changed from %s to %s", from_addr, to_addr);
+    logmsg(ANDROID_LOG_WARN, "clat IPv6 address changed from %s to %s", from_addr, to_addr);
 
     // Start translating packets to the new prefix.
-    memcpy(&Global_Clatd_Config.ipv6_local_subnet, &interface_ip->ip6, sizeof(struct in6_addr));
+    Global_Clatd_Config.ipv6_local_subnet = interface_ip->ip6;
 
     // Update our packet socket filter to reflect the new 464xlat IP address.
     if (!configure_packet_socket(tunnel->read_fd6)) {
diff --git a/clatd.conf b/clatd.conf
index b17b862..3805c6d 100644
--- a/clatd.conf
+++ b/clatd.conf
@@ -1,6 +1,7 @@
-# host ID to use as the source of CLAT traffic
-# this is a /128 taken out of the /64 routed to the phone
-ipv6_host_id ::464
+# Host IID to use as the source of CLAT traffic.
+# This is a /128 taken out of the /64 on the parent interface.
+# A host IID of :: means to generate a checksum-neutral, random IID.
+ipv6_host_id ::
 
 # ipv4 subnet for the local traffic to use.  This is a /32 host address
 ipv4_local_subnet 192.0.0.4
diff --git a/clatd_test.cpp b/clatd_test.cpp
index b35bf70..171aecb 100644
--- a/clatd_test.cpp
+++ b/clatd_test.cpp
@@ -20,6 +20,7 @@
 
 #include <stdio.h>
 #include <arpa/inet.h>
+#include <netinet/in6.h>
 #include <sys/uio.h>
 
 #include <gtest/gtest.h>
@@ -370,7 +371,7 @@
   *reassembled_len = total_length;
 }
 
-void check_data_matches(const uint8_t *expected, const uint8_t *actual, size_t len, const char *msg) {
+void check_data_matches(const void *expected, const void *actual, size_t len, const char *msg) {
   if (memcmp(expected, actual, len)) {
     // Hex dump, 20 bytes per line, one space between bytes (1 byte = 3 chars), indented by 4.
     int hexdump_len = len * 3 + (len / 20 + 1) * 5;
@@ -382,11 +383,11 @@
         sprintf(actual_hexdump + pos, "\n   ");
         pos += 4;
       }
-      sprintf(expected_hexdump + pos, " %02x", expected[i]);
-      sprintf(actual_hexdump + pos, " %02x", actual[i]);
+      sprintf(expected_hexdump + pos, " %02x", ((uint8_t *) expected)[i]);
+      sprintf(actual_hexdump + pos, " %02x", ((uint8_t *) actual)[i]);
       pos += 3;
     }
-    FAIL() << msg << ": Translated packet doesn't match"
+    FAIL() << msg << ": Data doesn't match"
            << "\n  Expected:" << (char *) expected_hexdump
            << "\n  Actual:" << (char *) actual_hexdump << "\n";
   }
@@ -460,6 +461,7 @@
 
   translate_packet(write_fd, (version == 4), original, original_len);
 
+  snprintf(foo, sizeof(foo), "%s: Invalid translated packet", msg);
   if (version == 6) {
     // Translating to IPv4. Expect a tun header.
     struct tun_pi new_tun_header;
@@ -472,13 +474,15 @@
       ASSERT_LT((size_t) len, *outlen) << msg << ": Translated packet buffer too small\n";
       EXPECT_EQ(expected_proto, new_tun_header.proto) << msg << "Unexpected tun proto\n";
       *outlen = len - sizeof(new_tun_header);
+      check_packet(out, *outlen, msg);
     } else {
-      FAIL() << msg << ": Packet was not translated";
+      FAIL() << msg << ": Packet was not translated: len=" << len;
       *outlen = 0;
     }
   } else {
     // Translating to IPv6. Expect raw packet.
     *outlen = read(read_fd, out, *outlen);
+    check_packet(out, *outlen, msg);
   }
 }
 
@@ -514,6 +518,44 @@
   check_packet(translated, translated_len, msg);
 }
 
+int get_transport_checksum(const uint8_t *packet) {
+  struct iphdr *ip;
+  struct ip6_hdr *ip6;
+  uint8_t protocol;
+  const void *payload;
+
+  int version = ip_version(packet);
+  switch (version) {
+    case 4:
+      ip = (struct iphdr *) packet;
+      if (is_ipv4_fragment(ip)) {
+          return -1;
+      }
+      protocol = ip->protocol;
+      payload = ip + 1;
+      break;
+    case 6:
+      ip6 = (struct ip6_hdr *) packet;
+      protocol = ip6->ip6_nxt;
+      payload = ip6 + 1;
+      break;
+    default:
+      return -1;
+  }
+
+  switch (protocol) {
+    case IPPROTO_UDP:
+      return ((struct udphdr *) payload)->check;
+
+    case IPPROTO_TCP:
+      return ((struct tcphdr *) payload)->check;
+
+    case IPPROTO_FRAGMENT:
+    default:
+      return -1;
+  }
+}
+
 struct clat_config Global_Clatd_Config;
 
 class ClatdTest : public ::testing::Test {
@@ -522,10 +564,126 @@
     inet_pton(AF_INET, kIPv4LocalAddr, &Global_Clatd_Config.ipv4_local_subnet);
     inet_pton(AF_INET6, kIPv6PlatSubnet, &Global_Clatd_Config.plat_subnet);
     inet_pton(AF_INET6, kIPv6LocalAddr, &Global_Clatd_Config.ipv6_local_subnet);
+    Global_Clatd_Config.ipv6_host_id = in6addr_any;
   }
 };
 
-TEST_F(ClatdTest, Sanitycheck) {
+void expect_ipv6_addr_equal(struct in6_addr *expected, struct in6_addr *actual) {
+  if (!IN6_ARE_ADDR_EQUAL(expected, actual)) {
+    char expected_str[INET6_ADDRSTRLEN], actual_str[INET6_ADDRSTRLEN];
+    inet_ntop(AF_INET6, expected, expected_str, sizeof(expected_str));
+    inet_ntop(AF_INET6, actual, actual_str, sizeof(actual_str));
+    FAIL()
+        << "Unexpected IPv6 address:: "
+        << "\n  Expected: " << expected_str
+        << "\n  Actual:   " << actual_str
+        << "\n";
+  }
+}
+
+TEST_F(ClatdTest, TestIPv6PrefixEqual) {
+  EXPECT_TRUE(ipv6_prefix_equal(&Global_Clatd_Config.plat_subnet,
+                                &Global_Clatd_Config.plat_subnet));
+  EXPECT_FALSE(ipv6_prefix_equal(&Global_Clatd_Config.plat_subnet,
+                                 &Global_Clatd_Config.ipv6_local_subnet));
+
+  struct in6_addr subnet2 = Global_Clatd_Config.ipv6_local_subnet;
+  EXPECT_TRUE(ipv6_prefix_equal(&Global_Clatd_Config.ipv6_local_subnet, &subnet2));
+  EXPECT_TRUE(ipv6_prefix_equal(&subnet2, &Global_Clatd_Config.ipv6_local_subnet));
+
+  subnet2.s6_addr[6] = 0xff;
+  EXPECT_FALSE(ipv6_prefix_equal(&Global_Clatd_Config.ipv6_local_subnet, &subnet2));
+  EXPECT_FALSE(ipv6_prefix_equal(&subnet2, &Global_Clatd_Config.ipv6_local_subnet));
+}
+
+int count_onebits(const void *data, size_t size) {
+  int onebits = 0;
+  for (size_t pos = 0; pos < size; pos++) {
+    uint8_t *byte = ((uint8_t*) data) + pos;
+    for (int shift = 0; shift < 8; shift++) {
+      onebits += (*byte >> shift) & 1;
+    }
+  }
+  return onebits;
+}
+
+TEST_F(ClatdTest, TestCountOnebits) {
+  uint64_t i;
+  i = 1;
+  ASSERT_EQ(1, count_onebits(&i, sizeof(i)));
+  i <<= 61;
+  ASSERT_EQ(1, count_onebits(&i, sizeof(i)));
+  i |= ((uint64_t) 1 << 33);
+  ASSERT_EQ(2, count_onebits(&i, sizeof(i)));
+  i = 0xf1000202020000f0;
+  ASSERT_EQ(5 + 1 + 1 + 1 + 4, count_onebits(&i, sizeof(i)));
+}
+
+TEST_F(ClatdTest, TestGenIIDConfigured) {
+  struct in6_addr myaddr, expected;
+  ASSERT_TRUE(inet_pton(AF_INET6, "::bad:ace:d00d", &Global_Clatd_Config.ipv6_host_id));
+  ASSERT_TRUE(inet_pton(AF_INET6, "2001:db8:1:2:0:bad:ace:d00d", &expected));
+  ASSERT_TRUE(inet_pton(AF_INET6, "2001:db8:1:2:f076:ae99:124e:aa54", &myaddr));
+  config_generate_local_ipv6_subnet(&myaddr);
+  expect_ipv6_addr_equal(&expected, &myaddr);
+}
+
+TEST_F(ClatdTest, TestGenIIDRandom) {
+  struct in6_addr interface_ipv6;
+  ASSERT_TRUE(inet_pton(AF_INET6, "2001:db8:1:2:f076:ae99:124e:aa54", &interface_ipv6));
+  Global_Clatd_Config.ipv6_host_id = in6addr_any;
+
+  // Generate a boatload of random IIDs.
+  int onebits = 0;
+  uint64_t prev_iid = 0;
+  for (int i = 0; i < 100000; i++) {
+    struct in6_addr myaddr =  interface_ipv6;
+
+    config_generate_local_ipv6_subnet(&myaddr);
+
+    // Check the generated IP address is in the same prefix as the interface IPv6 address.
+    EXPECT_TRUE(ipv6_prefix_equal(&interface_ipv6, &myaddr));
+
+    // Check that consecutive IIDs are not the same.
+    uint64_t iid = * (uint64_t*) (&myaddr.s6_addr[8]);
+    ASSERT_TRUE(iid != prev_iid)
+        << "Two consecutive random IIDs are the same: "
+        << std::showbase << std::hex
+        << iid << "\n";
+    prev_iid = iid;
+
+    // Check that the IID is checksum-neutral with the NAT64 prefix and the
+    // local prefix.
+    struct in_addr *ipv4addr = &Global_Clatd_Config.ipv4_local_subnet;
+    struct in6_addr *plat_subnet = &Global_Clatd_Config.plat_subnet;
+
+    uint16_t c1 = ip_checksum_finish(ip_checksum_add(0, ipv4addr, sizeof(*ipv4addr)));
+    uint16_t c2 = ip_checksum_finish(ip_checksum_add(0, plat_subnet, sizeof(*plat_subnet)) +
+                                     ip_checksum_add(0, &myaddr, sizeof(myaddr)));
+
+    if (c1 != c2) {
+      char myaddr_str[INET6_ADDRSTRLEN], plat_str[INET6_ADDRSTRLEN], ipv4_str[INET6_ADDRSTRLEN];
+      inet_ntop(AF_INET6, &myaddr, myaddr_str, sizeof(myaddr_str));
+      inet_ntop(AF_INET6, plat_subnet, plat_str, sizeof(plat_str));
+      inet_ntop(AF_INET, ipv4addr, ipv4_str, sizeof(ipv4_str));
+      FAIL()
+          << "Bad IID: " << myaddr_str
+          << " not checksum-neutral with " << ipv4_str << " and " << plat_str
+          << std::showbase << std::hex
+          << "\n  IPv4 checksum: " << c1
+          << "\n  IPv6 checksum: " << c2
+          << "\n";
+    }
+
+    // Check that IIDs are roughly random and use all the bits by counting the
+    // total number of bits set to 1 in a random sample of 100000 generated IIDs.
+    onebits += count_onebits(&iid, sizeof(iid));
+  }
+  EXPECT_LE(3190000, onebits);
+  EXPECT_GE(3210000, onebits);
+}
+
+TEST_F(ClatdTest, DataSanitycheck) {
   // Sanity checks the data.
   uint8_t v4_header[] = { IPV4_UDP_HEADER };
   ASSERT_EQ(sizeof(struct iphdr), sizeof(v4_header)) << "Test IPv4 header: incorrect length\n";
@@ -681,3 +839,43 @@
                              kIPv4Fragments, kIPv4FragLengths,
                              ARRAYSIZE(kIPv6Fragments), "IPv6->IPv4 fragment translation");
 }
+
+void check_translate_checksum_neutral(const uint8_t *original, size_t original_len,
+                                      size_t expected_len, const char *msg) {
+  uint8_t translated[MAXMTU];
+  size_t translated_len = sizeof(translated);
+  do_translate_packet(original, original_len, translated, &translated_len, msg);
+  EXPECT_EQ(expected_len, translated_len) << msg << ": Translated packet length incorrect\n";
+  // do_translate_packet already checks packets for validity and verifies the checksum.
+  int original_check = get_transport_checksum(original);
+  int translated_check = get_transport_checksum(translated);
+  ASSERT_NE(-1, original_check);
+  ASSERT_NE(-1, translated_check);
+  ASSERT_EQ(original_check, translated_check)
+      << "Not checksum neutral: original and translated checksums differ\n";
+}
+
+TEST_F(ClatdTest, TranslateChecksumNeutral) {
+  // Generate a random clat IPv6 address and check that translation is checksum-neutral.
+  Global_Clatd_Config.ipv6_host_id = in6addr_any;
+  ASSERT_TRUE(inet_pton(AF_INET6, "2001:db8:1:2:f076:ae99:124e:aa54",
+                        &Global_Clatd_Config.ipv6_local_subnet));
+  config_generate_local_ipv6_subnet(&Global_Clatd_Config.ipv6_local_subnet);
+  ASSERT_NE((uint32_t) 0x00000464, Global_Clatd_Config.ipv6_local_subnet.s6_addr32[3]);
+  ASSERT_NE((uint32_t) 0, Global_Clatd_Config.ipv6_local_subnet.s6_addr32[3]);
+
+  // Check that translating UDP packets is checksum-neutral. First, IPv4.
+  uint8_t udp_ipv4[] = { IPV4_UDP_HEADER UDP_HEADER PAYLOAD };
+  fix_udp_checksum(udp_ipv4);
+  check_translate_checksum_neutral(udp_ipv4, sizeof(udp_ipv4), sizeof(udp_ipv4) + 20,
+                                   "UDP/IPv4 -> UDP/IPv6 checksum neutral");
+
+  // Now try IPv6.
+  uint8_t udp_ipv6[] = { IPV6_UDP_HEADER UDP_HEADER PAYLOAD };
+  // The test packet uses the static IID, not the random IID. Fix up the source address.
+  struct ip6_hdr *ip6 = (struct ip6_hdr *) udp_ipv6;
+  memcpy(&ip6->ip6_src, &Global_Clatd_Config.ipv6_local_subnet, sizeof(ip6->ip6_src));
+  fix_udp_checksum(udp_ipv6);
+  check_translate_checksum_neutral(udp_ipv4, sizeof(udp_ipv4), sizeof(udp_ipv4) + 20,
+                                   "UDP/IPv4 -> UDP/IPv6 checksum neutral");
+}
diff --git a/config.c b/config.c
index 623a1b0..09d3df0 100644
--- a/config.c
+++ b/config.c
@@ -31,6 +31,7 @@
 #include "logging.h"
 #include "getaddr.h"
 #include "clatd.h"
+#include "checksum.h"
 
 struct clat_config Global_Clatd_Config;
 
@@ -149,6 +150,16 @@
   }
 }
 
+/* function: ipv6_prefix_equal
+ * compares the prefixes two ipv6 addresses. assumes the prefix lengths are both /64.
+ * a1 - first address
+ * a2 - second address
+ * returns: 0 if the subnets are different, 1 if they are the same.
+ */
+int ipv6_prefix_equal(struct in6_addr *a1, struct in6_addr *a2) {
+    return !memcmp(a1, a2, 8);
+}
+
 /* function: dns64_detection
  * does dns lookups to set the plat subnet or exits on failure, waits forever for a dns response with a query backoff timer
  * net_id - (optional) netId to use, NETID_UNSET indicates use of default network
@@ -175,6 +186,28 @@
 }
 
 
+void gen_random_iid(struct in6_addr *myaddr, struct in_addr *ipv4_local_subnet,
+                    struct in6_addr *plat_subnet) {
+  // Fill last 8 bytes of IPv6 address with random bits.
+  arc4random_buf(&myaddr->s6_addr[8], 8);
+
+  // Make the IID checksum-neutral. That is, make it so that:
+  //   checksum(Local IPv4 | Remote IPv4) = checksum(Local IPv6 | Remote IPv6)
+  // in other words (because remote IPv6 = NAT64 prefix | Remote IPv4):
+  //   checksum(Local IPv4) = checksum(Local IPv6 | NAT64 prefix)
+  // Do this by adjusting the two bytes in the middle of the IID.
+
+  uint16_t middlebytes = (myaddr->s6_addr[11] << 8) + myaddr->s6_addr[12];
+
+  uint32_t c1 = ip_checksum_add(0, ipv4_local_subnet, sizeof(*ipv4_local_subnet));
+  uint32_t c2 = ip_checksum_add(0, plat_subnet, sizeof(*plat_subnet)) +
+                ip_checksum_add(0, myaddr, sizeof(*myaddr));
+
+  uint16_t delta = ip_checksum_adjust(middlebytes, c1, c2);
+  myaddr->s6_addr[11] = delta >> 8;
+  myaddr->s6_addr[12] = delta & 0xff;
+}
+
 /* function: config_generate_local_ipv6_subnet
  * generates the local ipv6 subnet when given the interface ip
  * requires config.ipv6_host_id
@@ -183,8 +216,16 @@
 void config_generate_local_ipv6_subnet(struct in6_addr *interface_ip) {
   int i;
 
-  for(i = 2; i < 4; i++) {
-    interface_ip->s6_addr32[i] = Global_Clatd_Config.ipv6_host_id.s6_addr32[i];
+  if (IN6_IS_ADDR_UNSPECIFIED(&Global_Clatd_Config.ipv6_host_id)) {
+    /* Generate a random interface ID. */
+    gen_random_iid(interface_ip,
+                   &Global_Clatd_Config.ipv4_local_subnet,
+                   &Global_Clatd_Config.plat_subnet);
+  } else {
+    /* Use the specified interface ID. */
+    for(i = 2; i < 4; i++) {
+      interface_ip->s6_addr32[i] = Global_Clatd_Config.ipv6_host_id.s6_addr32[i];
+    }
   }
 }
 
@@ -195,10 +236,12 @@
  */
 int subnet_from_interface(cnode *root, const char *interface) {
   union anyip *interface_ip;
+  char addrstr[INET6_ADDRSTRLEN];
 
-  if(!config_item_ip6(root, "ipv6_host_id", "::464", &Global_Clatd_Config.ipv6_host_id))
+  if(!config_item_ip6(root, "ipv6_host_id", "::", &Global_Clatd_Config.ipv6_host_id))
     return 0;
 
+  // TODO: check that the prefix length is /64.
   interface_ip = getinterface_ip(interface, AF_INET6);
   if(!interface_ip) {
     logmsg(ANDROID_LOG_FATAL,"unable to find an ipv6 ip on interface %s",interface);
@@ -210,6 +253,9 @@
 
   config_generate_local_ipv6_subnet(&Global_Clatd_Config.ipv6_local_subnet);
 
+  inet_ntop(AF_INET6, &Global_Clatd_Config.ipv6_local_subnet, addrstr, sizeof(addrstr));
+  logmsg(ANDROID_LOG_INFO, "Using %s on %s", addrstr, interface);
+
   return 1;
 }
 
@@ -240,9 +286,6 @@
 
   strncpy(Global_Clatd_Config.default_pdp_interface, uplink_interface, sizeof(Global_Clatd_Config.default_pdp_interface));
 
-  if(!subnet_from_interface(root,Global_Clatd_Config.default_pdp_interface))
-    goto failed;
-
   if(!config_item_int16_t(root, "mtu", "-1", &Global_Clatd_Config.mtu))
     goto failed;
 
@@ -275,6 +318,9 @@
     }
   }
 
+  if(!subnet_from_interface(root,Global_Clatd_Config.default_pdp_interface))
+    goto failed;
+
 
   return 1;
 
diff --git a/config.h b/config.h
index da7446e..654e909 100644
--- a/config.h
+++ b/config.h
@@ -40,5 +40,6 @@
 int read_config(const char *file, const char *uplink_interface, const char *plat_prefix,
         unsigned net_id);
 void config_generate_local_ipv6_subnet(struct in6_addr *interface_ip);
+int ipv6_prefix_equal(struct in6_addr *a1, struct in6_addr *a2);
 
 #endif /* __CONFIG_H__ */