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arp

What does mean the "permanent" word in the "arp -a" output?

I'm trying to learn the ARP protocol. My idea about ARP is that it is a protocol that help addressing the IP to the exact MAC Address.

But when I try to do:

arp -a

Which I think it will show the list of IP addresses and the matching MAC addresses. I understand everything until I see the word permanent. The other lines are marked ifscope, and one is marked permanent.

Can someone tell me what mean permanent? Why is this line different?

Thanks in advance.

Arp table

Solution

  • According to the TCP/IP Guide:

    There are two different ways that cache entries can be put into the ARP cache:

    • Static ARP Cache Entries: These are address resolutions that are manually added to the cache table for a device and are kept in the cache on a permanent basis. Static entries are typically managed using a tool such as the arp software utility.

    • Dynamic ARP Cache Entries: These are hardware/IP address pairs that are added to the cache by the software itself as a result of successfully-completed past ARP resolutions. They are kept in the cache only for a period of time and are then removed.

    So the word permanent in the arp -a output means that the entry is manually added to the cache (static).