Added some UNITY defines; fix by aeronaut84. Untested.

Lidgren.Network/Encryption/NetAESEncryption.cs

@@ -19,7 +19,7 @@ namespace Lidgren.Network
 
 		static NetAESEncryption()
 		{
-#if !IOS && !__ANDROID__
+#if !IOS && !__ANDROID__ && !UNITY_4_5
 			AesCryptoServiceProvider aes = new AesCryptoServiceProvider();
 			List<int> temp = new List<int>();
 			foreach (KeySizes keysize in aes.LegalKeySizes)
@@ -103,7 +103,7 @@ namespace Lidgren.Network
 		/// </summary>
 		public bool Encrypt(NetOutgoingMessage msg)
 		{
-#if !IOS && !__ANDROID__
+#if !IOS && !__ANDROID__ && !UNITY_4_5
 			try
 			{
 				// nested usings are fun!
@@ -138,7 +138,7 @@ namespace Lidgren.Network
 		/// </summary>
 		public bool Decrypt(NetIncomingMessage msg)
 		{
-#if !IOS && !__ANDROID__
+#if !IOS && !__ANDROID__ && !UNITY_4_5
 			try
 			{
 				// nested usings are fun!

Lidgren.Network/NetPeer.Internal.cs

@@ -1,4 +1,4 @@
-#if !__ANDROID__ && !IOS
+#if !__ANDROID__ && !IOS && !UNITY_WEBPLAYER
 #define IS_MAC_AVAILABLE
 #endif
 

Lidgren.Network/NetPeerConfiguration.cs

@@ -35,6 +35,10 @@ namespace Lidgren.Network
 		//  -4 bytes to be on the safe side and align to 8-byte boundary
 		// Total 1408 bytes
 		// Note that lidgren headers (5 bytes) are not included here; since it's part of the "mtu payload"
+		
+		/// <summary>
+		/// Default MTU value in bytes
+		/// </summary>
 		public const int kDefaultMTU = 1408;
 		
 		private const string c_isLockedMessage = "You may not modify the NetPeerConfiguration after it has been used to initialize a NetPeer";

Lidgren.Network/NetUtility.cs

@@ -16,13 +16,17 @@ LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRA
 TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
 USE OR OTHER DEALINGS IN THE SOFTWARE.
 */
-#if !__ANDROID__ && !IOS
+#if !__ANDROID__ && !IOS && !UNITY_WEBPLAYER
 #define IS_FULL_NET_AVAILABLE
 #endif
 
 using System;
 using System.Net;
+
+#if IS_FULL_NET_AVAILABLE
 using System.Net.NetworkInformation;
+#endif
+
 using System.Net.Sockets;
 using System.Text;
 using System.Text.RegularExpressions;