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| ARP request and ARP reply |
The ARP protocol is, strictly speaking a layer 2 protocol, but it carries information to facilitate layer 3 functions. As a layer 2 broadcast it is not routable, but by means of a proxy it can function between clients on different subnets.
Proxy ARP
Proxy ARP can facilitate communication between clients in a scenario where you wish to segment a larger IPv4 scope, but wish to change the addresses of hosts on the subnet gradually.
For example: you have a subnet of 192.168.0.0 /21. This network allows for more than 2000 hosts on the same broadcast domain. Say you would like to segment this /21 subnet into smaller /24 subnets, but some of the hosts include printers that have statically configure IP addresses and subnet masks on the devices. You configure your subnets so that clients are located on the subnet 192.168.1.0 /24 and the printers are in the subnet 192.168.2.0 /24. The clients acquire their address from a DHCP server so they will get the new /24 subnet mask automatically, but the printers are all statically configured with a subnet mask of /21. This means that the clients trying to contact a printer knows that the printers are on another subnet and therefore sends the traffic to their default gateway, but the printers will think that the clients are on the same layer 2 segment as them and will attempt to look them up using an ARP request.
This is where proxy ARP comes into play. It is configured on the interface of the router connected to the printer network so that if it receives an ARP request on the printer interface, requesting a MAC address of a client, it will go ahead and reply to the ARP request with its own MAC address in the reply. The result: the client routes traffic as it was supposed to and the printer sends the traffic to the MAC address of the ARP proxy router, thinking it is the MAC address of the client machine.
Now, the administrator in charge of changing the IP address (or at least the subnet mask) on the printers can do so at his or her own pace.
I am not saying this is a recommended scenario - just that it is possible given the proper circumstances.
There are many other uses for Proxy ARP, which is turned on by default on Cisco devices. You can turn it off on a per interface basis with the command no ip proxy-arp.
Gratuitous ARP
The gratuitous ARP is a special form of ARP request that functions like a reply, but a reply that no one asked for - hence the gratuitous part. It is an ARP request and not an ARP reply mainly because it would be the type of ARP message most likely to be processed by the largest audience of endpoints. More can be read about the specific reasoning here.
The following is true about a Gratuitous ARP packet.
- The destination MAC address is the layer 2 broadcast (ff
: ff: ff: ff: ff: ff) - The source and destination IP address
is that of the host sending the gratuitous ARP packet - No reply is expected
ARP Probes
A feature in IPv4 uses ARP probes as a means of Address Conflict Detection (defined in RFC 5227) by using ARP requests to check if an IP address is already in use in the particular broadcast domain.
Reverse ARP
Reverse ARP is where an ARP request is sent out on a subnet with the source and destination MAC address being that of the sender and the source and destination IP address being all zeros (0.0.0.0). This allows a Reverse ARP server to reply to this request with a reply to the MAC address of the sender with an IP address, which needs to be manually defined on the Reverse ARP server beforehand. Essentially making an entry stating which IP address is associated with which MAC address. This is a means of dealing out IP addresses to hosts without statically configuring them on the hosts themselves. This is rarely used today (I have never encountered a network using Reverse ARP for addressing purposes) because of the cumbersome act of manually configuring the database with IP to MAC mappings and the need for the Reverse ARP server to be reachable on the clients layer 2 subnet.
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