From: bruce-usenet@noreplybicycle.synonet.comnoreply   
      
   annalissa wrote:   
      
   > Hi all,   
   >   
   > why NAT is not considered as a proxy server ?   
   >   
   > the reasons i know are   
   >   
   > 1 NAT is transparent to both source and destination hosts, but proxy   
   > server is not.   
   >   
   > 2 NAT is layer 3 protocol in contrast, a proxy server works at layer 4   
   > or higher   
   >   
   > is there any other reasons that i have missed????   
      
   It's in the definition really.   
      
   NAT is simply Network address translation.   It's purpose is simply to map   
   IP sources from one network to another.  This is done through a variety of   
   tricks, including port range mapping, connection tracking & mapping, but it   
   is strictly a network layer service.   
      
   A proxy is a layer-4 protocol that has it's own handshaking & controls and   
   specific behavior with particular protocols. Proxys were originally created   
   before NAT became the norm to allow private networks to request specific   
   protocols (HTTP, HTTPS, WAIS, GOPHER and others) onto a public IP.  They   
   were all implemented at the application layer, if your applications didn't   
   have proxy support often it wouldn't work on someone's network.   
      
   Later a generic application supported protocol wrapper came on the scene,   
   which didn't know the details of the layer 4 protocols, but would allow an   
   application to establish a connection with a proxy to tunnel it's requests   
   to the outside world.  this was called SOCKS.   
      
   More recently Layer-3 forwarding devices have grown smarts to transparently   
   take http requests off the wire and force them through a proxy (transparent   
   proxy). For example WCCP & Linux iptables.   
      
   If you want to see the actual difference configure a client application to   
   point directly to a proxy and packet dump the comms.  You'll see that the   
   requests are encapsulated in the proxy protocol.   
      
   One of the other distinguishing features of a proxy is that because it's   
   generally a layer-4+ protocol it has the ability to do many things with the   
   requests as they pass through, including filtering, caching, redirection   
   etc.   
      
   Again these abilities are now growing into network devices with the   
   popularity of DPI, which allow network administrators to manage traffic   
   without the costly maintenance of the client systems.   
      
   Bruce   
      
   --- SoupGate-Win32 v1.05   
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