![]() ![]() ![]() If the first request fails, then the IP routing algorithm takes over. The local Ethernet device with the correct IP address responds, and then the device stores this new IP/MAC address mapping in it's ARP table. If it can't find one, then it sends out a broadcast packet using the ARP protocol. Whenever a packet is ready to be sent out across the Ethernet, the corresponding "link layer protocol" (software responsible for routing the packet over Ethernet) first consults the local ARP cache to see if it can directly find a corresponding MAC address. There are also some preconfigured addresses in there and some "static" (manually programmed) MAC addresses if you do this (most people never use this, but there are some special preprogrammed entries). Inside the device, it has an ARP table, which is a cache of the IP/MAC map. ARP (Address Resolution Protocol) is an Ethernet protocol which is used to map IP addresses to MAC addresses. Each device in your Ethernet network has a MAC address. Understanding how the routing works is key to understanding why ARP -A may or may not work, and also generally helps to understand why MAC addresses matter, and why routing may or may not work at all when you expect it to. ![]() The IP network is an "overlay" over the top of the underlying Ethernet network. We're talking about a layer 2 system here (Ethernet) and a layer 3 system (IP network). You are on to something here and no, it definitely does not "have" to work. ![]()
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