What is an ARP?
ARP (Address Resolution Protocol) is a protocol for mapping an IP address to a physical machine address on a network. It is used by devices such as routers and switches to keep track of which MAC address is associated with which IP address.
When a device wants to send a packet to another device on the network, it looks up the destination IP address in its ARP table to find the corresponding MAC address. It then sends the packet directly to that MAC address.
ARP is a key part of how IP works, and it is a critical component of any network. Without ARP, devices would not be able to communicate with each other using IP addresses.
What is ARP Table?
An ARP table is a database that maps IP addresses to MAC addresses. Every device on a network has its own ARP table, which it uses to keep track of which MAC address is associated with which IP address. When a device wants to send a packet to another device on the network, it looks up the destination IP address in its ARP table to find the corresponding MAC address. It then sends the packet directly to that MAC address.
An ARP table is populated by a process called ARP resolution. When a device wants to communicate with another device on the network, it first checks its ARP table to see if there is an entry for the destination IP address.
How Can I Get an ARP Table of a Switch?
To get the ARP table of a switch, you can use a command line utility called arp-scan. This tool will scan the network and return the MAC address and IP addresses of each device connected to the switch. This is a handy way to see which devices are connected to the switch, their respective IPs, and their corresponding MAC addresses.
The ARP table is useful in many ways.
- For example, it can be used to determine whether a particular machine or user is connected to the network and if so, what IP address they may be using. It can also provide insights into the behavior of a user or device that might otherwise remain hidden.
- For example, a malicious actor might try to access a system by contenting different MAC addresses until one works, which can be detected through the ARP table.
How Does ARP Work?
The flow of ARP between two cisco Switches
Here are the steps:
- The source switch sends an ARP request packet to the destination IP address.
- The destination switch responds with an ARP reply packet, which includes its MAC address.
- The source switch updates its ARP table with the MAC address of the destination switch.
- The source switch sends the packet to the destination switch using the MAC address.
- The destination switch receives the packet and processes it.
What are the Uses of ARP?
ARP is used for a variety of purposes, including the following:
- To resolve IP addresses to MAC addresses
- To maintain a mapping of IP addresses to MAC addresses
- To update ARP tables when changes occur on the network
- To send packets directly to another device on the network using its MAC address
- ARP can be used for both IPv4 and IPv6 addresses.
Differentiate the Normal ARP, Proxy ARP, and Reverse ARP
Normal ARP: It is the most common type of ARP. In this, the host sends an ARP request to the router asking for the MAC address of the destination host. The router replies with its own MAC address. This type of ARP is used when the destination host is on a different network than the source host.
Proxy ARP: In this type of ARP, the router responds to the ARP request with its own MAC address even if the destination host is on the same network as the source host. This is done to avoid broadcasting the ARP request on the network.
Reverse ARP: It is used to map an IP address to a MAC address. In this, the host sends an ARP request with its own IP address as the destination IP address. The router replies with the MAC address of the host. This type of ARP is used when the host does not know its own MAC address.