Address Resolution Protocol (ARP) | Computer Networks - Computer Science Engineering (CSE) PDF Download

Address Resolution Protocol (ARP):
IP data grams contain IP addresses, but the physical interface hardware on the host or router can only understands the addressing scheme of that particular network. So the IP address should be translated to a link level address.

One simplest way to map an IP address in to a physical network address is to encode a host‟s physical address in the host part of its IP address. For example, a host with physical address 00100001 01001001 (which has the decimal value 33 in the upper byte and 81 in the lower byte) might be given the IP address 128.96.33.81. But in class C only 8 bits for host part. It is not enough for 48 bit Ethernet address.

A more general solution would be for each host to maintain a table of address pairs, i.e, and the table would map IP addresses into physical address. While this table could be centrally managed by a system administrator and then be copied to each host ion the network, a better approach would be for each host to dynamically learn the contents of the table using the network. This can be accomplished by Address Resolution Protocol (ARP). The goal of ARP is to enable each host on a network to build up a table of mappings between IP address and link level addresses.

Since these mappings may change over time, the entries are timed out periodically and removed. This happens on the order of every 15 minutes. The set of mappings currently stored in a host is known as ARP cache or ARP table.

Address Resolution Protocol (ARP) | Computer Networks - Computer Science Engineering (CSE)

 

The above figure shows the ARP packet format for IP to Ethernet address mappings. ARP can be used for lots of other kinds of mappings the major difference is their address size. In addition to the IP and link level addresses of both sender and target, the packet contains

  • a HardwareType fiels, which specifies the type of the physical network (ex., Ethernet)
  • a ProtocolType field, which specifies the higher layer protocol (ex., IP)
  • HLen (hardware address length) and PLen (protocol address length) fields, which specifies the length of the link layer address and higher layer protocol address, respectively
  • An Operation field, which specifies whether this is a request or a response 
  • The source and target hardware (Ethernet) and protocol (IP) address.

The results of the ARP process can be added as an extra column in a forwarding table.

The document Address Resolution Protocol (ARP) | Computer Networks - Computer Science Engineering (CSE) is a part of the Computer Science Engineering (CSE) Course Computer Networks.
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FAQs on Address Resolution Protocol (ARP) - Computer Networks - Computer Science Engineering (CSE)

1. What is ARP and how does it work?
Ans. ARP, or Address Resolution Protocol, is a communication protocol used to resolve IP addresses to physical MAC addresses on a local network. When a device wants to send data to another device within the same network, it needs to know the MAC address of the destination. ARP helps in this process by broadcasting an ARP request message to all devices in the network, asking for the MAC address of a specific IP address. The device with the matching IP address then responds with its MAC address, allowing the sender to establish a direct connection.
2. Why is ARP important in computer networks?
Ans. ARP is essential in computer networks as it enables communication between devices within the same network. Without ARP, devices would not be able to determine the MAC addresses of other devices and establish direct connections. It helps in the efficient transmission of data packets by mapping the IP addresses to the corresponding MAC addresses, ensuring that the data reaches the intended recipient.
3. What is an ARP cache and how does it work?
Ans. An ARP cache, also known as an ARP table or ARP cache table, is a temporary database stored in a device's memory. It contains the mappings of IP addresses to MAC addresses that the device has recently resolved through ARP requests. When a device needs to communicate with another device, it first checks its ARP cache to see if it already has the MAC address for that particular IP address. If the mapping is found, the device can directly send the data without needing to send an ARP request. However, if the mapping is not present or has expired, the device must send an ARP request to resolve the MAC address again.
4. Can ARP be used for communication between devices in different networks?
Ans. No, ARP is specifically designed for resolving IP addresses to MAC addresses within the same network. It operates at the data link layer of the networking stack and relies on the devices being on the same local network segment. For communication between devices in different networks, the devices use different protocols such as the Address Resolution Protocol over IP (ARPIP) or the Proxy ARP. These protocols involve additional layers of routing and forwarding to enable communication across different networks.
5. What are the security risks associated with ARP?
Ans. ARP can be susceptible to various security risks, including ARP spoofing or ARP poisoning attacks. In these attacks, malicious actors manipulate the ARP cache of devices to redirect traffic or intercept sensitive information. By sending false ARP responses, attackers can associate their MAC addresses with legitimate IP addresses, leading to potential unauthorized access or monitoring of network traffic. To mitigate these risks, network administrators can implement security measures such as ARP inspection, ARP spoofing detection, and using secure protocols like ARPSEC to protect against ARP-based attacks.
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