Network devices, such as routers, switches, and PCs, hold a form of an ARP cache table, which is a storage area to store IP to MAC address pairings. To see your own ARP cache on a Windows machine, open Command Prompt. Then, enter arp -a to see entries in the ARP table, as shown here:
As shown, the ARP cache table lists the following:
- Internet Address: The IP address
- Physical Address: The MAC address
- Type: Either static or dynamic
The ARP cache values will time out after a period of time. Once the timeout limit is reached, the entry will go away. If the OS needs the MAC address and there is no entry in the ARP table, it will need to issue a new ARP request.
The ARP table timeout values are specific to the system. For example, a Cisco switch has a default timeout timer of 4 hours.
In a Windows OS, you can determine the timeout value by going to the Command-Line Interface (CLI) and running the netsh interface ipv4 show interface NNN command, where NNN is the name of the interface you want to check.
As shown in the screenshot, we see the output of running netsh interface ipv4 show interface Wi-Fi, which provides information on that interface:
netsh show interface
Within the output you will see Base Reachable Time is 30000 ms or 30 seconds, which is how long ARP can live in the cache before going away.
That means the ARP cache will out after 30 seconds. After that, if a MAC address is needed for a specific IP address, the system must send an ARP request out on the network.
We can see how ARP works in an IPv4 network, but what about an IPv6 network? The following section outlines how the Neighbor Discovery Protocol (NDP) takes the place of ARP in an IPv6 network.