diff --git a/binaries/cuprated/src/rpc/request/address_book.rs b/binaries/cuprated/src/rpc/request/address_book.rs
index 3bde5c6..f129dbe 100644
--- a/binaries/cuprated/src/rpc/request/address_book.rs
+++ b/binaries/cuprated/src/rpc/request/address_book.rs
@@ -65,7 +65,7 @@ pub(crate) async fn connection_info<Z: NetworkZone>(
                 address_type: info.address_type,
                 avg_download: info.avg_download,
                 avg_upload: info.avg_upload,
-                connection_id: String::from(FIELD_NOT_SUPPORTED),
+                connection_id: String::from(ConnectionId::DEFAULT_STR),
                 current_download: info.current_download,
                 current_upload: info.current_upload,
                 height: info.height,
diff --git a/p2p/p2p-core/src/types.rs b/p2p/p2p-core/src/types.rs
index 3b7c3cb..ca56055 100644
--- a/p2p/p2p-core/src/types.rs
+++ b/p2p/p2p-core/src/types.rs
@@ -53,12 +53,32 @@ pub struct ConnectionInfo<A: NetZoneAddress> {
     pub support_flags: u32,
 
     // The following fields are slightly different than `monerod`.
+
     //
     /// [`None`] if Tor/i2p or unknown.
     pub socket_addr: Option<std::net::SocketAddr>,
-    // This field does not exist for Cuprate:
-    // <https://github.com/Cuprate/cuprate/pull/320#discussion_r1811335020>
-    // pub connection_id: u128,
+
+    /// This field does not exist for `cuprated`'s RPC, this is just a marker type:
+    /// - <https://github.com/Cuprate/cuprate/pull/320#discussion_r1811335020>
+    /// - <https://github.com/Cuprate/cuprate/pull/320#discussion_r1819826080>
+    ///
+    /// [`ConnectionId::DEFAULT_STR`] is used when mapping to the RPC type.
+    pub connection_id: ConnectionId,
+}
+
+/// Marker type for `monerod`'s connection ID.
+///
+/// `connection_id` is a 128-bit `uuid` in `monerod`.
+/// `cuprated` does not support this field so it returns
+/// the default value in the RPC interface, an all 0-bit UUID.
+///
+/// This default value in string form is [`ConnectionId::DEFAULT_STR`].
+#[derive(Copy, Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct ConnectionId;
+
+impl ConnectionId {
+    /// [`str`] representation of a default connection ID.
+    pub const DEFAULT_STR: &str = "00000000000000000000000000000000";
 }
 
 /// Used in RPC's `sync_info`.