ARP stands for Address resolution protocol, it is used to convert the IP address (Logical address in network layer ) to MAC address ( Physical address in the data link layer) that means it converts the 32 bit address of the IP address to 48 bit mac address. Some enthusiasts treat ARP operates on both layer 2 and layer 3, its like 2.5 protocol. ARP is a crucial protocol to successfully send the traffic to the end host.
Let's assume there is an 24 ports L3 capability switch configured with SVI interface IP 192.168.1.1 and the subnet 255.255.255.0, four users are connected to the switch from port 2 to 5, port 2 connected user is user 2 and port 3 connected user is user 3 and so... Port 2 endhost has been configured with the IP 192.168.1.2, port 3 was configured with IP 192.168.1.3 and so..If user connected to port 2 require to send the data to the user on port 3, the user 2 endhost require both the IP and the mac addresses of user 3 without lacking any of these, there would be no communication. Here the user 2 had the IP address of user 3 but no mac address so user 2 PC would check arp cache to find the respective mac, in case if there is no mac address exist for the user 3 IP address then it would generate the broadcast IP with source 192.168.1.2 and his own mac address at the source port , destination is 192.168.1.3 and destination mac is ff:ff:ff:ff:ff:ff
Note: in case if you find any mac with the above format (ff:ff) then it is a broadcast address.
Now the switch receives the frame and adds the mac address of the user 2 in the arp cache table again it will check in the switch table to find the destination mac for IP 192.168.1.3. If there is no mac address binded in the table for the user 3 then switch forward the broadcast packet to the same broadcast domain and waits for the reply from the user 3 and adds the user 3 mac in the arp cache table once it receives the frame from user 3.
By default the arp cache will timeout every 25 minutes to get the updates from scratch which helps to protect from the outdated information.
Given below are the types of ARP:
Reserve Address Resolution Protocol (RARP)
RARP protocol is used by a device that knows its MAC address but does not know its IP address in a local area network (LAN).
A network administrator creates a table in a local area network (LAN) gateway router that maps the physical machine addresses to equivalent IP addresses.
When a new machine is set up, then its RARP client requests the IP address from the gateway router.
Assuming that an entry has been set up in the router table, the reserve ARP server will return the IP address to the machine which can store it for future use.
The problem of getting an IP address from a MAC address can be handled by the reserved address resolution protocol (RARP) which works similar to ARP.To obtain the IP address, the host first broadcasts a RARP request packet containing its MAC address on the network. All the hosts in the network will receive the packets but only the server responds to the host by sending a RARP response packet containing the host MAC and IP address.
Inverse Address Resolution Protocol (InARP)
Inverse ARP is the opposite of ARP. Instead of using a layer-3 IP address to find a layer-2 MAC address, Inverse ARP uses layer-2 MAC addresses to find a layer-3 IP address.
Inverse ARP is mostly used by Frame relay and ATM Networks to map the DLCI to IP Address. So the router asks for the IP Address of the destination or other ends of PVC by listing DLCI for that router.
Proxy ARP
Proxy ARP is a technique using which a proxy server on a given network answers the ARP queries for an IP address that is not on that network.
For example, suppose the host is connected to the router over the Ethernet and the host has the address 10.10.1.1/16 and the router has 10.10.10.0/24.
The host wants to resolve the ARP for 10.10.1.110 and thinks that the router is in the same subnet, so it will be able to get the MAC-address.
However, as routers, by design, limit broadcast domains, so it will not send the ARP reply back and it will reject the request.
But if another router has any other interface connected to the 10.10.1.1/16 network and proxy ARP is enabled, then the router will send the ARP reply to the host by listing its own MAC address.It will act as a proxy for the destination network.
Gratuitous ARP
A gratuitous ARP is an unprompted address resolution protocol response.
When a new device joins the local area network (LAN), it broadcasts its MAC address to the whole network immediately after its network interfaces boot up.
Gratuitous ARP packet has both source and destination IP set to the IP address of the device issuing the packet, and the destination MAC is the broadcast address ff:ff:ff:ff:ff:ff or 00:00:00:00:00:00 based on the address resolution protocol (ARP) implementation.
No continuation packets are sent in response to a gratuitous ARP packet.