I was recently contacted by a HappyRouter member who posed a great question about the difference between ARP, RARP, and inverse ARP. I felt that the answer to this question might help others and, especially, those studying for the Cisco CCNA exam. So what is the different between these three? Let’s find out…
The most common use of ARP is what I discussed in this article on ARP caching. That is just “regular ARP”. With ARP, your PC, let’s say, is trying to get an Ethernet MAC address for an IP address.
If you use a packet sniffer like Ethereal / Wireshark, you will see ARP requests just streaming across a production network (they are all broadcasts and go to every station).
Then, there is RARP (reverse ARP) where your PC has a MAC address and is trying to get an IP address for it. Today, RARP is pretty much obsolete because every one uses DHCP.
Then, you have inverse ARP (called InARP). Now, Happy Router member tuongdq posted some great information on how inverse ARP is used with Frame-Relay. This information is “spot on”. He is correct that inverse ARP can be used with frame relay. Inverse ARP can also be used with ATM. InARP is used to find the Layer 3 address from a Layer 2 address (the DLCI in frame relay). With frame, when using
inverse ARP, you know the DLCI of the remote router but you don’t know its IP address. Inverse ARP sends a request to obtain that IP address and map it to the Layer 2 frame-relay DLCI. However, it is always recommended to disable & not use inverse ARP because your frame end points could get different IP’s if the network were to be ‘bounced’.
If you are in for some “techie” reading on these different types of ARP, you can always check out the RFC’s:
- RFC 826 – Address Resolution Protocol
- RFC 903 – Reverse Address Resolution Protocol
- RFC 2390 – Inverse Address Resolution Protocol
Also, if you are still looking for MORE information on ARP, you might want to learn what a Gratuitous ARP is…