Gratuitous Address Resolution Protocol(G-ARP)
- 2. Address Resolution Protocol(ARP) It is protocol for mapping an Internet Protocol address to a physical machine address that is recognized in the local network. It is a low-level network protocol for translating network layer addresses into link layer addresses. A table called the ARP cache, is used to maintain a correlation between each MAC address and its corresponding IP address.
- 3. ARP Header
- 4. Hardware Type - this is 1 for Ethernet. Protocol Type - the protocol used at the network layer. Hardware Address Length - this is the length in bytes, so it would be 6 for Ethernet. Protocol Address Length - For TCP/IP th value is 4 bytes. Operation Code - this code indicates whether the packet is an ARP Request (1) or an ARP Response (2).
- 5. Senders Hardware Address - hardware address of the source node. Senders Protocol Address - layer 3 address of the source node. Target Hardware Address - used in a RARP request, the response carries both the destination's hardware and layer 3 addresses. Target Protocol Address - used in an ARP request, the response carries both the destination's hardware and layer 3 addresses.
- 6. What is Gratuitous ARP It occurs when a host sends an ARP request looking for its own IP address. It is an Address Resolution Protocol packet where the source and destination IP are both set to the IP of the machine issuing the packet and the destination MAC is the broadcast address ff:ff:ff:ff:ff:ff. The Gratuitous ARP is sent as a broadcast, as a way for a node to announce or update its IP to MAC mapping to the entire network.
- 7. What is use of Gratuitous ARP It helps to detect IP conflicts. When a machine receives an ARP request containing a source IP that matches its own, then it knows there is an IP conflict. They inform switches of the MAC address of the machine on a given switch port, so that the switch knows that it should transmit packets sent to that MAC address on that switch port.
- 8. Example
- 9. Scenario 1- Case of Duplicate IP address PC1: IP configured with 172.16.10.10/16 PC2: Trying to use 172.16.10.10/16 PC2 is trying to use an IP 172.16.10.10 that is being utilized by PC1, Before PC2 assigns the IP address to itself, it will generate a G-ARP to check for duplicate IP
- 10. Scenario 1- Case of Duplicate IP address PC2 received the reply from PC1 as Duplicate IP address Detected. PC2 will not assigned the IP address on it’s system.
- 11. Scenario 2- No Duplicate IP address Assigning an IP address to an end-user, G-ARP protocol checks for duplicate IP address and generates the below header If there is no IP conflict , no reply will be generated and IP will be assigned to the system.