diff --git a/tuic/src/protocol/address.rs b/tuic/src/protocol/address.rs
index da3a7f4..717629b 100644
--- a/tuic/src/protocol/address.rs
+++ b/tuic/src/protocol/address.rs
@@ -15,10 +15,10 @@ use std::{
 ///
 /// The address type can be one of the following:
 ///
-/// 0x00: None
-/// 0x01: Fully-qualified domain name (the first byte indicates the length of the domain name)
-/// 0x02: IPv4 address
-/// 0x03: IPv6 address
+/// 0xff: None
+/// 0x00: Fully-qualified domain name (the first byte indicates the length of the domain name)
+/// 0x01: IPv4 address
+/// 0x02: IPv6 address
 ///
 /// The port number is encoded in 2 bytes after the Domain name / IP address.
 #[derive(Clone, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
@@ -29,10 +29,10 @@ pub enum Address {
 }
 
 impl Address {
-    pub const TYPE_CODE_NONE: u8 = 0x00;
-    pub const TYPE_CODE_DOMAIN: u8 = 0x01;
-    pub const TYPE_CODE_IPV4: u8 = 0x02;
-    pub const TYPE_CODE_IPV6: u8 = 0x03;
+    pub const TYPE_CODE_NONE: u8 = 0xff;
+    pub const TYPE_CODE_DOMAIN: u8 = 0x00;
+    pub const TYPE_CODE_IPV4: u8 = 0x01;
+    pub const TYPE_CODE_IPV6: u8 = 0x02;
 
     pub fn type_code(&self) -> u8 {
         match self {
diff --git a/tuic/src/protocol/authenticate.rs b/tuic/src/protocol/authenticate.rs
index 9f1a781..988230d 100644
--- a/tuic/src/protocol/authenticate.rs
+++ b/tuic/src/protocol/authenticate.rs
@@ -13,7 +13,7 @@ pub struct Authenticate {
 impl Authenticate {
     pub(super) const TYPE_CODE: u8 = 0x00;
 
-    pub fn new(token: [u8; 8]) -> Self {
+    pub const fn new(token: [u8; 8]) -> Self {
         Self { token }
     }
 }
diff --git a/tuic/src/protocol/connect.rs b/tuic/src/protocol/connect.rs
index 286eaf7..db57013 100644
--- a/tuic/src/protocol/connect.rs
+++ b/tuic/src/protocol/connect.rs
@@ -13,7 +13,7 @@ pub struct Connect {
 impl Connect {
     pub(super) const TYPE_CODE: u8 = 0x01;
 
-    pub fn new(addr: Address) -> Self {
+    pub const fn new(addr: Address) -> Self {
         Self { addr }
     }
 }
diff --git a/tuic/src/protocol/dissociate.rs b/tuic/src/protocol/dissociate.rs
index f443f06..d931d13 100644
--- a/tuic/src/protocol/dissociate.rs
+++ b/tuic/src/protocol/dissociate.rs
@@ -11,9 +11,9 @@ pub struct Dissociate {
 }
 
 impl Dissociate {
-    pub const TYPE_CODE: u8 = 0x03;
+    pub(super) const TYPE_CODE: u8 = 0x03;
 
-    pub fn new(assoc_id: u16) -> Self {
+    pub const fn new(assoc_id: u16) -> Self {
         Self { assoc_id }
     }
 }
diff --git a/tuic/src/protocol/heartbeat.rs b/tuic/src/protocol/heartbeat.rs
index efb7e52..7b03ad1 100644
--- a/tuic/src/protocol/heartbeat.rs
+++ b/tuic/src/protocol/heartbeat.rs
@@ -9,9 +9,9 @@ use super::Command;
 pub struct Heartbeat;
 
 impl Heartbeat {
-    pub const TYPE_CODE: u8 = 0x04;
+    pub(super) const TYPE_CODE: u8 = 0x04;
 
-    pub fn new() -> Self {
+    pub const fn new() -> Self {
         Self
     }
 }
diff --git a/tuic/src/protocol/packet.rs b/tuic/src/protocol/packet.rs
index 68af7a9..83e4504 100644
--- a/tuic/src/protocol/packet.rs
+++ b/tuic/src/protocol/packet.rs
@@ -18,7 +18,7 @@ pub struct Packet {
 impl Packet {
     pub(super) const TYPE_CODE: u8 = 0x02;
 
-    pub fn new(
+    pub const fn new(
         assoc_id: u16,
         pkt_id: u16,
         frag_total: u8,