refactor: use packed struct for IpAddr

This is more intuitive than using u32 directly, and follows the same pattern as the new MAC addr handling

src/EthIpUdp.zig

@@ -14,6 +14,28 @@
 // You should have received a copy of the GNU General Public License along with
 // Zaprus. If not, see <https://www.gnu.org/licenses/>.
 
+pub const IpAddr = packed struct {
+    int: I,
+
+    const V = @Vector(4, u8);
+    const I = u32;
+
+    pub fn fromBytes(s: V) IpAddr {
+        return .{ .int = @bitCast(s) };
+    }
+};
+
+pub const MacAddr = packed struct {
+    int: I,
+
+    const V = @Vector(6, u8);
+    const I = @Int(.unsigned, @bitSizeOf(V));
+
+    pub fn fromBytes(s: V) MacAddr {
+        return .{ .int = @bitCast(s) };
+    }
+};
+
 pub const EthIpUdp = packed struct(u336) { // 42 bytes * 8 bits = 336
     // --- UDP (Last in memory, defined first for LSB->MSB) ---
     udp: packed struct {
@@ -25,8 +47,8 @@ pub const EthIpUdp = packed struct(u336) { // 42 bytes * 8 bits = 336
 
     // --- IP ---
     ip: packed struct {
-        dst_addr: u32,
-        src_addr: u32,
+        dst_addr: IpAddr,
+        src_addr: IpAddr,
         header_checksum: u16 = 0,
         protocol: u8 = 17, // udp
         ttl: u8 = 0x40,
@@ -54,18 +76,7 @@ pub const EthIpUdp = packed struct(u336) { // 42 bytes * 8 bits = 336
     // --- Ethernet ---
     eth_type: u16 = std.os.linux.ETH.P.IP,
     src_mac: MacAddr,
-    dst_mac: MacAddr = .fromSlice(@splat(0xff)),
-
-    pub const MacAddr = packed struct {
-        int: I,
-
-        pub const V = @Vector(6, u8);
-        pub const I = @Int(.unsigned, @bitSizeOf(V));
-
-        pub fn fromSlice(s: V) MacAddr {
-            return .{ .int = @bitCast(s) };
-        }
-    };
+    dst_mac: MacAddr = .fromBytes(@splat(0xff)),
 
     pub fn toBytes(self: @This()) [336 / 8]u8 {
         var res: [336 / 8]u8 = undefined;