diff --git a/service/native/libs/libclat/clatutils.cpp b/service/native/libs/libclat/clatutils.cpp
index 69076c5..9ceed75 100644
--- a/service/native/libs/libclat/clatutils.cpp
+++ b/service/native/libs/libclat/clatutils.cpp
@@ -27,6 +27,68 @@
 namespace net {
 namespace clat {
 
+bool isIpv4AddressFree(in_addr_t addr) {
+    int s = socket(AF_INET, SOCK_DGRAM | SOCK_CLOEXEC, 0);
+    if (s == -1) {
+        return 0;
+    }
+
+    // Attempt to connect to the address. If the connection succeeds and getsockname returns the
+    // same then the address is already assigned to the system and we can't use it.
+    struct sockaddr_in sin = {
+            .sin_family = AF_INET,
+            .sin_port = htons(53),
+            .sin_addr = {addr},
+    };
+    socklen_t len = sizeof(sin);
+    bool inuse = connect(s, (struct sockaddr*)&sin, sizeof(sin)) == 0 &&
+                 getsockname(s, (struct sockaddr*)&sin, &len) == 0 && (size_t)len >= sizeof(sin) &&
+                 sin.sin_addr.s_addr == addr;
+
+    close(s);
+    return !inuse;
+}
+
+// Picks a free IPv4 address, starting from ip and trying all addresses in the prefix in order.
+//   ip        - the IP address from the configuration file
+//   prefixlen - the length of the prefix from which addresses may be selected.
+//   returns: the IPv4 address, or INADDR_NONE if no addresses were available
+in_addr_t selectIpv4Address(const in_addr ip, int16_t prefixlen) {
+    return selectIpv4AddressInternal(ip, prefixlen, isIpv4AddressFree);
+}
+
+// Only allow testing to use this function directly. Otherwise call selectIpv4Address(ip, pfxlen)
+// which has applied valid isIpv4AddressFree function pointer.
+in_addr_t selectIpv4AddressInternal(const in_addr ip, int16_t prefixlen,
+                                    isIpv4AddrFreeFn isIpv4AddressFreeFunc) {
+    // Impossible! Only test allows to apply fn.
+    if (isIpv4AddressFreeFunc == nullptr) {
+        return INADDR_NONE;
+    }
+
+    // Don't accept prefixes that are too large because we scan addresses one by one.
+    if (prefixlen < 16 || prefixlen > 32) {
+        return INADDR_NONE;
+    }
+
+    // All these are in host byte order.
+    in_addr_t mask = 0xffffffff >> (32 - prefixlen) << (32 - prefixlen);
+    in_addr_t ipv4 = ntohl(ip.s_addr);
+    in_addr_t first_ipv4 = ipv4;
+    in_addr_t prefix = ipv4 & mask;
+
+    // Pick the first IPv4 address in the pool, wrapping around if necessary.
+    // So, for example, 192.0.0.4 -> 192.0.0.5 -> 192.0.0.6 -> 192.0.0.7 -> 192.0.0.0.
+    do {
+        if (isIpv4AddressFreeFunc(htonl(ipv4))) {
+            return htonl(ipv4);
+        }
+        ipv4 = prefix | ((ipv4 + 1) & ~mask);
+    } while (ipv4 != first_ipv4);
+
+    return INADDR_NONE;
+}
+
 // Alters the bits in the IPv6 address to make them checksum neutral with v4 and nat64Prefix.
 void makeChecksumNeutral(in6_addr* v6, const in_addr v4, const in6_addr& nat64Prefix) {
     // Fill last 8 bytes of IPv6 address with random bits.
diff --git a/service/native/libs/libclat/clatutils_test.cpp b/service/native/libs/libclat/clatutils_test.cpp
index 1900abf..0b38bce 100644
--- a/service/native/libs/libclat/clatutils_test.cpp
+++ b/service/native/libs/libclat/clatutils_test.cpp
@@ -33,6 +33,68 @@
 
 class ClatUtils : public ::testing::Test {};
 
+// Mock functions for isIpv4AddressFree.
+bool neverFree(in_addr_t /* addr */) {
+    return 0;
+}
+bool alwaysFree(in_addr_t /* addr */) {
+    return 1;
+}
+bool only2Free(in_addr_t addr) {
+    return (ntohl(addr) & 0xff) == 2;
+}
+bool over6Free(in_addr_t addr) {
+    return (ntohl(addr) & 0xff) >= 6;
+}
+bool only10Free(in_addr_t addr) {
+    return (ntohl(addr) & 0xff) == 10;
+}
+
+// Apply mocked isIpv4AddressFree function for selectIpv4Address test.
+in_addr_t selectIpv4Address(const in_addr ip, int16_t prefixlen,
+                            isIpv4AddrFreeFn fn /* mocked function */) {
+    // Call internal function to replace isIpv4AddressFreeFn for testing.
+    return selectIpv4AddressInternal(ip, prefixlen, fn);
+}
+
+TEST_F(ClatUtils, SelectIpv4Address) {
+    struct in_addr addr;
+
+    inet_pton(AF_INET, kIPv4LocalAddr, &addr);
+
+    // If no addresses are free, return INADDR_NONE.
+    EXPECT_EQ(INADDR_NONE, selectIpv4Address(addr, 29, neverFree));
+    EXPECT_EQ(INADDR_NONE, selectIpv4Address(addr, 16, neverFree));
+
+    // If the configured address is free, pick that. But a prefix that's too big is invalid.
+    EXPECT_EQ(inet_addr(kIPv4LocalAddr), selectIpv4Address(addr, 29, alwaysFree));
+    EXPECT_EQ(inet_addr(kIPv4LocalAddr), selectIpv4Address(addr, 20, alwaysFree));
+    EXPECT_EQ(INADDR_NONE, selectIpv4Address(addr, 15, alwaysFree));
+
+    // A prefix length of 32 works, but anything above it is invalid.
+    EXPECT_EQ(inet_addr(kIPv4LocalAddr), selectIpv4Address(addr, 32, alwaysFree));
+    EXPECT_EQ(INADDR_NONE, selectIpv4Address(addr, 33, alwaysFree));
+
+    // If another address is free, pick it.
+    EXPECT_EQ(inet_addr("192.0.0.6"), selectIpv4Address(addr, 29, over6Free));
+
+    // Check that we wrap around to addresses that are lower than the first address.
+    EXPECT_EQ(inet_addr("192.0.0.2"), selectIpv4Address(addr, 29, only2Free));
+    EXPECT_EQ(INADDR_NONE, selectIpv4Address(addr, 30, only2Free));
+
+    // If a free address exists outside the prefix, we don't pick it.
+    EXPECT_EQ(INADDR_NONE, selectIpv4Address(addr, 29, only10Free));
+    EXPECT_EQ(inet_addr("192.0.0.10"), selectIpv4Address(addr, 24, only10Free));
+
+    // Now try using the real function which sees if IP addresses are free using bind().
+    // Assume that the machine running the test has the address 127.0.0.1, but not 8.8.8.8.
+    addr.s_addr = inet_addr("8.8.8.8");
+    EXPECT_EQ(inet_addr("8.8.8.8"), selectIpv4Address(addr, 29));
+
+    addr.s_addr = inet_addr("127.0.0.1");
+    EXPECT_EQ(inet_addr("127.0.0.2"), selectIpv4Address(addr, 29));
+}
+
 TEST_F(ClatUtils, MakeChecksumNeutral) {
     // We can't test generateIPv6Address here since it requires manipulating routing, which we can't
     // do without talking to the real netd on the system.
diff --git a/service/native/libs/libclat/include/libclat/clatutils.h b/service/native/libs/libclat/include/libclat/clatutils.h
index b86ee88..d1e2277 100644
--- a/service/native/libs/libclat/include/libclat/clatutils.h
+++ b/service/native/libs/libclat/include/libclat/clatutils.h
@@ -20,10 +20,16 @@
 namespace net {
 namespace clat {
 
+bool isIpv4AddressFree(in_addr_t addr);
+in_addr_t selectIpv4Address(const in_addr ip, int16_t prefixlen);
 void makeChecksumNeutral(in6_addr* v6, const in_addr v4, const in6_addr& nat64Prefix);
 int generateIpv6Address(const char* iface, const in_addr v4, const in6_addr& nat64Prefix,
                         in6_addr* v6);
 
+// For testing
+typedef bool (*isIpv4AddrFreeFn)(in_addr_t);
+in_addr_t selectIpv4AddressInternal(const in_addr ip, int16_t prefixlen, isIpv4AddrFreeFn fn);
+
 }  // namespace clat
 }  // namespace net
 }  // namespace android