322e5b05   
   XPost: comp.arch.embedded   
   From: andrews@sdf.lonestar.org   
      
   On 2011-02-02, Didi  wrote:   
   > On Feb 2, 9:38?pm, Nobody  wrote:   
   >   
   >> Routing is an IP-layer feature. The routing table tells the computer the   
   >> next "hop" (the interface and possibly the gateway) for a given IP address.   
   >   
   > This is how IP routing works indeed. It only involves IP addresses,   
   > however; there is more routing to take place at the lower levels.   
   >   
   >> ARP provides hardware-layer (Ethernet etc) addresses for a given IP   
   >> address.   
   >   
   > Yes, calling it routing - as I did - may be unusual but in essence it   
   > is that. It does work in conjunction with the IP address and the   
   > separating line between the two can depend on the context.   
      
   Yes, it is definitely slack use of terminology - the IEEE 802   
   standards make no mention of routing except in the context of laying   
   cables.  They do mention routers but they are treated as simply   
   another ethernet node for all that it matters.   
      
   I wouldn't characterise it as routing-like either - the destination   
   MAC address is more of a tag than anything else.  At its heart   
   ethernet is still based on the idea of a common access network   
   medium, conceptually at least.  The transmitting station simply   
   places the frame on the wire and expects it to arrive at the   
   destination without any real consideration as to how it is to   
   actually get there.  It may well arrive at many other nodes as well   
   but that is neither here nor there since it will be discarded in   
   short order, typically within the NIC.  Anything that breaks the   
   common access topology - that's bridges or switches for the most   
   part - must do so in a way that is transparent to the network nodes.   
      
   The only time I can think of you would usually want ARP for something   
   not on the local network would be for proxy ARP, where the router   
   will respond with _it's_ MAC when there is an ARP for a remote IP   
   address.  From then on packets to that host get sent to the router   
   which forwards them on as normal.  Proxy ARP simplifies the nodes   
   (which need no routing tables) but at the cost of more processing,   
   network broadcasts and a lack of redundancy.  It used to be fairly   
   common but I haven't seen it for probably a decade at least.   
      
   --   
   Andrew Smallshaw   
   andrews@sdf.lonestar.org   
      
   --- SoupGate-Win32 v1.05   
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