cf775747   
   XPost: comp.arch.embedded   
   From: nobody@nowhere.com   
      
   On Mon, 31 Jan 2011 09:53:11 -0800, Didi wrote:   
      
   >> I'm not sure why you need a static ARP entry.   
   >   
   > Well that is straight forward enough. On my DPS machines, outbound packets   
   > will route according to an entry (or "subnet", ARP table entries are   
   > maskable)   
   > in the ARP table; if no routing entry is found in the ARP table, the   
   > packet   
   > will go down its respective subnet gateway if it belongs there; if it is   
   > on the same subnet and there was no ARP entry for it, a network ARP query   
   > will be initiated.   
   > I expected similar behaviour from windows; but no, the manually set ARP   
   > entry is ignored.   
      
   ARP and routing are quite separate issues.   
      
   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.   
   You can view the routing table (in XP at least) with "route print" from a   
   command prompt, and can modify it via "route add" etc.   
      
   ARP provides hardware-layer (Ethernet etc) addresses for a given IP   
   address.   
      
   If your routing table specifies that a given IP address should be routed   
   via the default gateway, then any ARP entries for that IP address won't   
   make the slightest bit of difference, as the computer will never request   
   an ARP entry for the IP address, it will request one for the gateway.   
      
   An ARP entry only matters for a host which is, according to the routing   
   table, on the same network segment.   
      
   A typical routing table for a desktop PC will have a network route (no   
   gateway) for each interface, corresponding to the interface's IP address   
   and subnet mask, a default route via a gateway, a route for 127.0.0.0/8   
   via the loopback interface, plus broadcast/multicast routes.   
      
   If you have a device on a different subnet (either on the other side of a   
   router, or on the same segment but with an IP address outside of the range   
   specified by the local interface's address and subnet mask), you need to   
   add a route to it, otherwise packets will get sent to the default gateway.   
   ARP won't change this.   
      
   On Windows XP, go to Control Panel, Network Connections, Local Area   
   Connection 1 (or whatever number corresponds to the appropriate ethernet   
   interface) > Properties, Internet Protocol (TCP/IP), Properties, Advanced,   
   IP Settings (tab), Add..., and add an IP address and subnet mask for the   
   network (i.e. IP address range) corresponding to the device with which you   
   wish to communicate.   
      
   Where ARP /will/ work is if the local system doesn't know about the   
   gateway but has a default route which simply refers to the local network.   
   In that situation, a router can perform proxy ARP for any number of other   
   networks, or even the entire internet, with the effect that every host on   
   the internet appears to be on the same ethernet segment so far as routing   
   is concerned. A host sending a packet to a given IP address notes that the   
   IP address is part of the local network, requests its ethernet address via   
   ARP, and (hopefully) the gateway replies with its own ethernet address.   
      
   This practice is generally avoided because it can result in large ARP   
   tables (there'll be an entry for every IP address) and it's easy to get   
   into ARP fights if there is more than one router. However, it does provide   
   a simple means of "zero configuration" networking: only the routers need   
   to know any routing information; other hosts just spit packets onto the   
   network.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)