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ARP

ARP, the Address resolution Protocol, has the job to resolve an IP address to a MAC (Media Access Control) address. Let's have a look why this is necessary and how ARP works!

If a node wants to send an IP datagram, the adapter adds a MAC layer header (e.g. Ethernet) with a MAC address. In order to find a corresponding MAC address, each node owns an ARP table which contains mappings of IP to MAC addresses. If there is no entry in the table, the sender node broadcasts an ARP request on the subnet, in which it asks for the MAC address to a given IP address. The node, which owns the requested IP address, sends in an ARP response its MAC address back. The mapping is added to the ARP table of the requesting node.

Please notice, that ARP resolves IP addresses only for nodes on the same subnet. If a packet has to travel over one or more routers in order to reach the destination, each node uses in a first step its routing table to find the IP address of the next hop. In a next step it looks in the ARP table for the MAC address corresponding to this IP address.

Please have a look at the following scenario and see how MAC addresses are determined by ARP. You are able to select different scenarios, have a look at the routing and ARP tables of selected nodes and delete entries of the ARP tables.