Your computer has an IP address it wants to contact. It determines, using the subnet, whether the IP is on the local network or not. - If it's not on the local network then it forwards the message on to the gateway, and it never knows or cares what the eventual MAC address is that is associated with that IP - in this case the ARP is ONLY used to find the MAC of the gateway device if it is not known. - If it's on the local network, then it uses ARP to find the MAC address if an antry does not exist in its routing table already. On 8/20/08, Justin Richards wrote: > my understanding of arp tables is that when a layer 3 packet is > destined for a given ip an arp request is sent out the adapter. Only when the desitnation IP address is on the local subnet, as defined by something similar to 255.255.255.0, where the first 24 bits of the destination and host IP must match to be considered on the local network, where the MAC address is required for proper routing. > I am experincing conflicting results on what appears to be identicle > setups as follows:- > > 1. Any destination ip address handled by the gateway device does NOT > result in an arp entry but all local ip address are added. This is what should be happening. > 2. Any destination ip address handled by the gateway device does > result in an arp entry but all local ip address are added. This should not be happening. The ARP is used only to associate a MAC address with an IP address so that you don't have to have a router between your computer and your local LAN. you should not be getting ARP listings for IP addresses that are outside your subnet. Check the subnet and all IP settings on each computer and router, and verify that they all agree on the subnet, gateway, etc. -Adam -- EARTH DAY 2008 Tuesday April 22 Save Money * Save Oil * Save Lives * Save the Planet http://www.driveslowly.org -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist